NO346411B1

NO346411B1 - Method and apparatus for performing a marine CSEM survey

Info

Publication number: NO346411B1
Application number: NO20210285A
Authority: NO
Inventors: Jostein Kåre Kjerstad; Marina Persova; Yuri G Soloveichik
Original assignee: Captrol As
Priority date: 2021-03-03
Filing date: 2021-03-03
Publication date: 2022-07-11
Also published as: WO2022186701A1; NO20210285A1

Description

METHOD AND APPARATUS FOR PERFORMING A CSEM SURVEY

Technical field

The present invention relates in particular to controlled source electromagnetic (CSEM) surveys for detecting resistive targets in the Earth’s subsurface.

Background

Controlled source electromagnetic (CSEM) surveys have been performed to facilitate remotely detecting resistive hydrocarbon targets in the Earth’s subsurface in subsea locations. In such a survey, an electromagnetic field is transmitted by an electric source located in the sea. The source transmission is controlled actively. The electromagnetic field propagates in subsurface rock formations, and receiver sensors located in the sea are configured to detect components of the electromagnetic field. Electromagnetic field data from the sensors are processed to determine the resistivity of different regions of the subsurface.

If a resistive feature is present in the subsurface strata, such as fluid hydrocarbon oil or gas reservoir, then a response may be obtained that differs significantly from that which may be obtained if the resistive structure were not present. The response depends upon the resistivity and depth of the structure.

CSEM surveys have been used in the oil and gas exploration and production industry. The technique is sensitive to the resistive properties of the fluid content in the rock formation, allowing discrimination between rocks that contain saline water of low resistivity and rocks that contain oil and/or gas of high resistivity.

Various controlled source marine electromagnetic (CSEM) techniques are currently known for prospecting hydrocarbon deposits.

US2010148784 discloses a method of processing CSEM data comprising the steps of providing inline and broadside marine controlled source electromagnetic data and forming a linear combination of the inline and broadside data to reduce the airwave content. US2012123683 discloses a method of controlled source electromagnetic survey, that have been carried out using horizontal or vertical electric dipole sources for transmitting an EM field, with receivers positioned a distance away from the source to measure the EM field components. US2009184715 discloses an apparatus and method for estimating the 3D orientation angles for remotely deployed devices with flexible arms such as dipole antennas of receivers used in marine controlled-source electromagnetic surveys to explore for hydrocarbons. GB2507536 discloses a seabed receiver of the kind comprising a central unit, and a plurality of sensor arms extending outwardly from the central unit is provided in which an arm mount on which the associated arm is pivotally mounted on the central unit. US2008091356 discloses a method for computing values for surveying a subterranean structure underneath a seabed in subsea or marine environment.

Although various CSEM survey techniques are known, there remains a need in the industry for greater sensitivity and/or detectability of resistive targets in offshore areas.

At least one aim of the invention is to obviate, or at least mitigate, one or more drawbacks associated with prior art.

Summary

According to a first aspect of the invention, there is provided a method of performing a marine CSEM survey to detect a resistive target located in a subsurface of the Earth, the method comprising the steps of: providing survey apparatus which is supported upon the seabed, the survey apparatus comprising at least one receiver and at least one electromagnetic field transmitter, the electromagnetic field transmitter comprising at least one horizontal electric dipole; transmitting an electromagnetic field from the electromagnetic field transmitter through at least one region of the subsurface; detecting the electromagnetic field by combining components in at least one pair of opposite directions along an axis on at least one channel of the receiver; and positioning the receiver in spaced apart relationship from the transmitter, on a transmitter-receiver axis which is perpendicular to a dipole axis of the horizontal electric dipole, either forward or rearward with respect to the dipole centre.

The components of the field in opposite directions may in this way combine advantageously to permit or facilitate detection of three-dimensional (3D) structure of the subsurface, such as a lateral edge of a resistive target such as a hydrocarbon reservoir in the subsurface.

The method may include providing sensors. More specifically, the method may include providing at least one receiver on the seabed which receiver may include sensors, e.g. at least one pair of sensors. The sensors may be disposed in positions along the axis to detect the field. The sensors may be coupled sensors and may detect the field simultaneously for obtaining the components along the axis in opposite directions. The method may include combining the components in the opposite directions on at least one signal channel of the receiver.

The detection step may comprise detecting the electromagnetic field by combining components in opposite directions along a first axis and components in opposite directions along a second axis. The first axis may intersect with or be arranged across the second axis. The first axis may be orthogonal to the second axis.

The detection step may comprise detecting the electromagnetic field by combining: components in opposite directions along a first axis; components in opposite directions along a second axis which may be orthogonal to the first axis; and components in opposite directions along at least one or more further axes. The further axes may comprise any number of axes. For example, third and/or fourth axes, which may each form an angle with respect to the first or second axis that may be in the range of 30 to 60 degrees. Preferably one of the first, second, or further axis may be parallel to a dipole axis of the transmitter.

The components may combine additively. Thus, the data may be obtained from the channel in the form representing a field component sum of the components in the opposite directions.

The sensors may comprise sensor elements, e.g. electrodes. The sensors may comprise sensor elements arranged along the axis to sense the field respectively in opposite directions. The sensor elements may be arranged to define electric dipoles. The sensor elements may detect or obtain an electric field component along the axis.

The sensor elements may define at least one pair of sensors wherein the pair has one sensor, e.g. dipole, to obtain the component in one direction and another sensor, e.g. dipole, to obtain the component in an opposite direction.

The sensors may comprise a first sensor element and one or more further sensor elements positioned about the first sensor element, which may be an inner or central sensor element, to detect the at least one pair of components of the field in opposite directions along the axis. Thus, the sensors may be oppositely directed dipole sensors.

Preferably, the transmitter may be located on the seabed, in stationary configuration. Preferably, the receiver and/or sensors may be located on the seabed, in stationary configuration. Further preferably, the receiver and/or sensor may be located in a predefined position relative to the transmitter. This configuration may facilitate repeat measurement, and increase of data quality, e.g. through stacking signals.

The transmitter may comprise transmitter elements, e.g. electrodes, which may define a dipole having a dipole axis and the dipole axis having a dipole centre point.

In a first configuration, a receiver comprising the sensors may be located in the forward location relative to the transmitter, whereby the receiver may be positioned apart from the transmitter on a perpendicular axis from the dipole axis and forward with respect to the dipole centre. In a second configuration, the receiver comprising the sensors may be located in the rearward location relative to the transmitting transmitter, whereby the receiver may be positioned apart from the transmitter on a perpendicular axis from the dipole axis and rearward with respect to the dipole centre. The method may further comprise moving the transmitter from one position to another position in order to move between the first configuration and the second configuration. Alternatively, the method may further comprise providing one transmitter in one position to obtain the first configuration and providing another transmitter in another position to obtain the second configuration. In another variant, the method may comprise providing a transmitter device with at least two pairs of transmitter elements, e.g. electrodes, and selectively activating the pairs of transmitter elements to move from the first configuration to the second configuration, whereby one pair of transmitter elements may be selected to provide a first transmitter dipole to transmit the electromagnetic field into the subsurface in the first configuration, and another pair of transmitter elements may be selected to provide a second transmitter dipole in the second configuration.

Conversely, the method may comprise providing one receiver arranged in the forward location relative to the transmitter, thereby obtaining a first configuration, and another receiver arranged in the rearward location relative to the transmitter, thereby obtaining a second configuration.

The method may further comprise: detecting the electromagnetic field using the receiver in forward location; detecting the electromagnetic using the receiver in the rearward location; and combining at least one signal, data, or at least one data components from the signal channel of the receiver in the forward location and at least one signal, data, or at least one data components from the signal channel of the receiver in the rearward. More specifically, the combination may comprise adding the signals, data or data components of the forward and rearward locations. Thus, a combination sum signal may be obtained. Alternatively or in addition, the combination may comprise subtracting the signals, data or data components of the forward and rearward locations. Thus, a combination difference signal may be obtained. The combination sum signal and the combination difference signals may facilitate determination of particular characteristics of the subsurface. The combination sum signal may facilitate in particular detection of 3-D structure such as an edge of a reservoir. The combination signals from the forward and rearward locations may be obtained by performing data processing. The data processing may typically be carried out by computer apparatus.

The method may further comprise detecting at least one calibrating component of the field. The method may comprise detecting the electromagnetic field by combining main components in at least one pair of opposite directions along an axis, whereby the main components may be combined on at least one signal channel of the receiver. The method may further comprise further combining the calibrating component with the main components on at least one calibrated channel. Thus, in the presence of topography and subsurface 3D inhomogeneities, the relative response from the target using calibration can be obtained at about the same level as in a medium without topography and without subsurface 3D inhomogeneities and this may facilitate detection of the target.

The method may include using any one or more of the signals on the signal channel of a receiver to detect or determine one or more characteristics of subsurface resistivity structure, e.g. a hydrocarbon reservoir target. The method may include using the combination sum and/or the combination difference signal to detect or determine one or more characteristics of the subsurface resistivity structure. The method may include using the combination signals or signals from channels of the receiver individually at one or more offsets relative to the transmitter to facilitate detection. In particular, the signals which may be utilized either for the receiver individually in either the forward or rearward location or in combination between the forward and rearward locations may be calibrated or non-calibrated signals from the appropriate corresponding calibrated or non-calibrated signal channel.

The survey apparatus may comprise one or more sets of at least one transmitter and at least one receiver. The method may further comprise locating the one or more sets of at least one transmitter and at least one receiver on the seabed in different locations to survey different regions of the seafloor and subsurface. The method may therefore include moving the transmitter to different transmitter locations and/or receiver to different receiver locations. Accordingly, the method may further include obtaining responses from the receivers in the different locations; processing and/or analysing the data obtained on one or more of the receiver channels from the different locations.

The method may include arranging one or more transmitter-receiver sets of at least one receiver and at least one transmitter, away from the target. Placement away from the target can allow obtaining a signal that may be free from the influence of the subsurface resistive target and thus may allow the background to be defined or better defined.

Placing the receiver at far distances away from the transmitter may facilitate or allow removal of IP effects.

The method may include obtaining data from signals from receivers in various locations, including the far away location, and using the data from the various locations together to facilitate detecting the target. In particular, the method may include processing and analysing acquired data to determine data points which may be best representative of the target anomaly unaffected by induced polarization effects. Alternatively or in addition, the method may include using data expected to be unaffected by induced polarization and data that may be affected by induced polarization in one or more inversion models to detect one or more properties of the subsurface, which properties may be indicative of presence or extent of a subsurface reservoir.

The method may further comprise operating the transmitter to emit an electromagnetic field in the form of one or more pulses. The electromagnetic field may be detected by the sensors over time. The method includes using the signal from the receiver(s) over time following transmitter switch off time. This may facilitate time domain processing and analysis of signals, whereby amplitudes of the signal and decay of amplitudes after transmitter switch off time may be usefully employed to detect anomalies associated with the subsurface resistive targets. The processing of the transient signal may comprise inverse modelling to evaluate and determine parameters, e.g. resistivity and dimensions, of different subsurface layers of the Earth.

The method may include deploying the transmitter and or the at least one receiver on the seabed, e.g. from a ship.

The method may be performed to obtain 4D data from the survey area. The 4D data may allow detecting at least one change of condition of the resistive target structure. To this end, the steps of the method may be performed to provide a first data vintage and later repeated to provide a second data vintage with one or more weeks, months, or years between vintages.

The method may then include comparing the data obtained by the receiver between different vintages to determine changes in the subsurface target. The survey transmitter and or the receiver may remain deployed in position between the data vintages. In the case of the target being a hydrocarbon reservoir, the comparison may facilitate monitoring of the size and/or reduction in size of reservoir in the time between the acquisition of the different data vintages.

According to a second aspect of the invention, there is provided a receiver for detecting an electromagnetic field from the Earth’s subsurface in a marine CSEM survey, the receiver comprising sensors to detect the field along at least one axis, so that at least one pair of components of the field in opposite horizontal directions along the axis are combined on at least one signal channel. The electromagnetic field in use is typically produced using at least one transmitter with the receiver being positioned in spaced apart relationship from the transmitter, on a transmitter-receiver axis perpendicular to a horizontal dipole axis of the transmitter, either forward or rearward with respect to the dipole centre.

The components of the field in opposite directions may in this way combine advantageously to permit or facilitate detection of three-dimensional (3D) structure of the subsurface, such as a lateral edge of a resistive target such as a hydrocarbon reservoir in the subsurface. In use, preferably, the sensors are arranged to obtain horizontal components of the field in the opposite directions along the axis, and preferably the sensor may detect the field simultaneously. By way of combining the components in the opposite directions on the signal channel, an output from the channel may be provided which may comprise a result of the combination. The components in opposite directions are typically combined additively. The result of combination may thus be a sum of the components.

The sensors may comprise sensor elements, such as electrodes, with sensor positions respectively along the axis. The sensors may define electric dipoles defining dipole axes therebetween and/or along the axis. Accordingly, the sensors may comprise at least two pairs of sensor elements along the axis, wherein the sensor elements of the one pair may be configured to detect a component of the field in one direction along the axis and the sensor elements of the other pair may be configured to detect a component of the field in an opposite direction along the axis.

The receiver may have at least two pairs of sensors to detect at least two pairs of opposite components of the field along the axis, wherein sensor element spacings between sensor elements of the one of the pairs may be greater than between the sensor elements of the other of the pairs. In this way, per axis, one may have several receiver sensors with different spacings between sensor elements, e.g. between electrodes. This may facilitate obtaining an improved signal. In some variants, the sensors may comprise a first group of sensor elements comprising outer sensor elements to be arranged about one or more inner sensor elements to define at least one pair of sensors for detecting components of the field in opposite directions, and may further comprise a second group of sensor elements comprising outer sensor elements to be arranged about one or more inner sensor elements to define at least one other pair of sensors for detecting components of the field in opposite directions. The sensors may each define a sensor element spacing between the inner and the outer sensor element, and sensor element spacings in the second group may be greater than in the first group.

The receiver may comprise electrical combining circuitry for coupling the sensors and combining the components from the sensors onto the signal channel. The sensing elements of a respective sensor may detect an electrical potential of the field and may operate to obtain a potential difference between the sensing elements. The sensing elements of the sensors for respective detections in opposite directions may thus conveniently be positioned apart by substantially the same distance. The sensor elements of each of the sensors of an opposite pair may be configured to be positioned apart by a distance in the region of 100 to 500 m. The receiver apparatus may further comprise at least one support member configured to extend radially away from a body of the receiver (e.g. horizontally in use) for supporting one or more sensor elements of sensors and/or for positioning or aligning the sensor elements along the axis.

The sensors may each comprise an inner sensor element and an outer sensor element with respect to central part of the apparatus. The sensors may comprise outer sensor elements arranged about an inner sensor element which may for example be a central sensor element. In this way, one sensor may be formed by the inner sensor and the outer sensor being positioned along the axis for detecting the component in one direction, and another sensor may be formed by the inner sensor and the other outer sensor being positioned along the axis for detecting the component in the opposite direction. The inner sensor element, e.g. the central sensor element, may thus be a common sensor element, e.g. electrode, for oppositely detecting sensors. The common sensor element may thus conveniently detect therefore a common potential of the field for the different sensors. This may facilitate combining components on the signal channel.

The receiver apparatus may comprise sensors for obtaining pairs of components in opposite directions along any number of axis. For example, along four axes with the sensors detecting the electromagnetic field so that pairs of components of the field in opposite directions radially from a central part of the receiver apparatus on each of the four axes are combined on the signal channel. In this example, the sensors detect the field in a total of eight directions.

The sensors may comprise main sensor elements for detecting main components of the field along the opposite directions which combine on the signal channel. The sensors may comprise further sensor elements for detecting one or more calibration components. The calibration components may combine with the combined main components on at least one signal channel. The main components may combine on a main, first signal channel which may be an uncalibrated channel. The calibration components may combine with the combined main components on one or more calibrated signal channels. The further, calibration sensor elements may be arranged apart by a distance, in use horizontal distance, in the range of 1 to 10 m.

The calibration component may allow in practice to eliminate the contribution of the seabed topography and subsurface 3D inhomogeneities in the signal using 3D modelling and 3D inversion. By obtaining the calibration component on sensors with significantly shorter distance between sensor elements, e.g. electrodes, than used for the main sensors, the relative contribution of near surface subsurface structure (above a deep target reservoir) compared to a deep target, may typically be greater. The calibration component may thus be combined with the main components additively, e.g. summed on the calibration channel which can then allow using 3D modelling and 3D inversion to exclude the effects of near-surface topography and subsurface 3D inhomogeneities, which may be present in an uncalibrated channel. The calibrated channel can therefore provide a signal that may have a greater response and anomaly from the target than otherwise, which thus can make the target structure more easily detectable. The preferred channel may typically be the calibrated channel with the minimum absolute value of signals.

According to a third aspect of the invention, there is provided apparatus for performing a marine CSEM survey, the apparatus comprising: at least one receiver apparatus in accordance with the second aspect of the invention; and at least one electromagnetic field transmitter for propagating an electromagnetic field through the subsurface. The apparatus may comprise at least one processor configured to process data obtained from the receiver to facilitate detecting the resistive target, wherein the receiver in use may typically be configured to obtain opposite horizontal components and be positioned in spaced apart relationship from the transmitter, on a transmitter-receiver axis perpendicular to a horizontal dipole axis of the transmitter, either forward or rearward with respect to the dipole centre

The apparatus may be deployed on the seabed. In use, the apparatus may have a first configuration in which the receiver apparatus is positioned sideward of the transmitter dipole and forward with respect to the dipole centre, and a second configuration in which the receiver apparatus is positioned sideward of the transmitter dipole and rearward with respect to the dipole centre.

The apparatus may comprise means, e.g. a computer device, including the processor for processing data from signal channels of the receiver in the respective forward and rearward positions.

According to a fourth aspect of the invention, there is provided computer apparatus for receiving and processing the data in the method of the first aspect.

According to a fifth aspect of the invention, there is provided a computer program which when executed causes the computer of the fourth aspect to perform one or more steps in the method of the first aspect.

Any of the above aspects may have further features as defined above as set out in relation to any of other of the aspects or anywhere else herein.

Embodiments of the invention may be advantageous in various ways as will be apparent from throughout herein.

Brief description of the drawings

There will now be described, by way of example only, embodiments of the invention with reference to the accompanying drawings, in which:

Figure 1 is a perspective overview of a survey region in the course of performing a CSEM survey;

Figure 2 is a perspective representation of apparatus for performing the survey of Figure 1 in larger scale;

Figure 3 is a top-view of apparatus for performing the CSEM survey in another example;

Figure 4 is a schematic representation of apparatus including a computer device for use in performing the CSEM survey;

Figures 5A and 5B are plan views of survey apparatus with the transmitter in forward position and in rearward position respectively;

Figure 6 is a plan view of calibrating electrode positions of the apparatus of Figures 5A and 5B;

Figure 7 is a table of signal channels including calibrated signal channels from the receiver of the apparatus of Figures 5A and 5B;

Figure 8A is a representation of an earth model, the upper part providing a map view, the lower part providing a sectional view along the profile Pr of the map view;

Figures 8B to 8E are plots of modelled responses from the earth model of Figure 8A; Figures 9A to 9D are plots of modelled responses for the earth model of Figure 8A without and with induced polarization for different transmitter-receiver offsets;

Figures 10A to 10E are plots of modelled responses of different signal channels from an earth model including bathymetry with and without target;

Figures 11A and 11B are plan views of transmitters and receivers provided in an array over a survey region; and

Figures 12A and 12B are plan views of alternative sensor element arrangements of a receiver for use in the apparatus.

Specific description of embodiments of the invention

With reference first to Figure 1, marine CSEM survey apparatus 10 is seen disposed offshore in a survey region 1 for performing a controlled source electromagnetic survey of the Earth’s subsurface 8. The subsurface 8 comprises geological formations and in the region shown includes a hydrocarbon layer 7 which is sought to be detected. The hydrocarbon layer 7 is relatively resistive compared with the general background structure 5 and overburden.

The apparatus 10 includes a horizontal electric dipole transmitter 12. The transmitter 12 is located on the seabed 2 and emits an electromagnetic field 9 that propagates into and through a region of the subsurface 8.

The apparatus 10 also includes a receiver 16. The receiver 16 is located underwater on the seabed 2 and is arranged to detect the electromagnetic field that results in response to the transmission. The electromagnetic field 9 penetrates and interacts with the subsurface, and where this includes a hydrocarbon-bearing structure 7, an anomalous response may be detected by the receiver 16 compared with that if no such hydrocarbon structure were present.

A survey vessel 4 is provided on the sea surface 6 to assist with the deployment of the receiver 16 and the transmitter 12. An umbilical 13 extending from the vessel 4 to the transmitter 12 is used to supply electrical power for operating the transmitter.

The arrangement of the transmitter 12 and receiver 16 are configured and arranged in ways that will be described further below to obtain data that can facilitate detection of the target hydrocarbon structure. In particular, it is sought to conduct the survey to facilitate time domain data analysis and processing. Accordingly, the transient electromagnetic field following a period of transmission (i.e. in pauses, after transmitter switch off) is of particular interest, and it is sought to obtain data that allows the transient behaviour after switch off to be detected.

Although only one transmitter and one receiver are shown in Figure 1, this is only an example, and in practice several transmitters or transmitter positions and several receivers and/or receiver positions would typically be utilized in the survey, not least in the interest of efficient surveying of larger areas and/or where the horizontal extent of a target is of interest.

Turning now to Figure 2, examples of the transmitter 12 and the receiver 16 are depicted, located on the seabed 2. The transmitter 12 has electrodes 22a, 22b defining a horizontal dipole axis A extending from the first electrode 22a to the second electrode 22b. The dipole axis A between the electrodes 22a, 22b has a mid-point C.

The receiver 16 has multiple electrodes 26a-26i to allow the electromagnetic field to be detected simultaneously in multiple directions at the receiver location. More specifically, the eight outer electrodes 26a-26h are arranged around an inner, centre electrode 26i. Each of the eight outer electrodes 26a-26h forms a directional detecting pair together with the inner electrode 26i so as to allow detecting the electric field in an azimuthal direction away from the centre electrode 26i. The outer electrodes 26a-26h are arranged relative to the inner electrode 26i so that opposite detecting pairs are created. For example, the outer electrode 26a and inner electrode 26i form a pair that detects the electric field in one direction, and the outer electrode 26e and inner electrode 26i form another pair that detects the electric field in the 180-degree opposite direction. This receiver arrangement is referred to below as a STAR-receiver 16.

The receiver 16 has hydroacoustic devices 21a-21h at or near the location of the electrodes to obtain the actual positions of the electrodes 26a-26h during the survey for helping to position the electrodes accurately during deployment or for taking account any positioning errors.

The receiver 16 is arranged for detecting the electromagnetic field by combining components in at least one pair of opposite directions along an axis. The components of n-number of opposite detecting pairs in the various directions can thus combine directly onto a signal channel of the receiver 16, representing a sum of the opposite directed components.

It can then be appreciated for example in a theoretical 1-D earth that the signal from the receiver by the detecting pairs in opposite directions becomes zero if the receiver is positioned 90 degrees side-side on to the dipole (i.e. on line Cx in Figure 2). However, for a 3-D earth – as is the case in a real survey situation – the response is not zero. The non-zeroness of the signal obtained from the receiver 16 can thus be attributable to the 3-D structure and the response from the receiver 16 may therefore be particularly effective in detecting 3-D aspects of the subsurface resistivity structure. Indeed, hydrocarbon layers are 3-D in nature, that is they have a limited lateral extent, so the presence of the hydrocarbon layer, in particular the edge of the layer, can produce a prominent effect in the data obtained from this receiver 16.

The receiver 16 is actually preferably not arranged exactly at 90 degrees side-on to the dipole centre, on the dipole line Cx. Rather, as can be seen in Figure 2, the receiver 16 is positioned forward of the centre C, Cx. As a result, the response from the receiver would not be zero even if the earth were theoretically 1D. However, as will be seen in the following, the response from the receiver 16 in this forward position, can be beneficially combined e.g. by summation with the response from the receiver 16 in a corresponding reverse position with respect to the dipole (on the other side of the liner Cx in Figure 2), so that the contribution in the response from background can be suppressed whilst the 3-D response due to reservoir can be relative enhanced.

In Figure 3, apparatus 10 has a transmitter 12 defining a dipole axis A on the seabed 2, similar to Figure 2. In this example, two receivers 16 are provided. The receiver 16 has just two opposite detecting pairs of electrodes for detecting components of the field in opposite directions about the inner electrode 26i. The one pair is formed by the outer electrode 26a and the inner electrode 26i, and the other opposite pair is formed by the outer electrode 26e and the inner electrode 26i.

In performing the survey in this configuration, the signal, data or one or more components thereof from the receiver 16 in the forward configuration is combined with the signal, data or one or more components thereof from the receiver in the rearward configuration. The signals, data or components thereof from respective receivers 16 are combined by steps of data processing. In one example, the signals can be combined by subtracting the response from the one receiver to the other receiver, thus providing a difference component (e.g. from a signal difference). Using the difference component can significantly reduce the influence of topography and subsurface 3D inhomogeneities, thereby determining the background in general, and for large targets to obtain additional information about their characteristics.

In other examples, the signal can be combined by summing the responses from the one receiver and the other receiver 26, thus providing a sum component (e.g. from a signal sum). Using the sum component can increase the abnormal response associated with a resistive target structure.

The signal provides combined data is thus pre-processed to reduce percentage contributions of background relative to target. The combined data are inverted using an inversion algorithm to obtain parameters of the subsurface structure such as resistivity, depth, position, etc.

In this example, the inner electrode 26i is a common electrode for both pairs, although in other variants separate inner electrodes are provided, one for each pair. The combined response from both pairs may be easier technically to implement for the common electrode variant.

The example of Figure 3 shows an arrangement using two receivers, i.e. with their detection centres C, forward and rearward with respect to the transmitter. The converse is possible, for one receiver having instead two transmitters one shifted in the dipole axis A direction relative to the other. Such examples are provided below.

Figures 12A and 12B provide examples of receivers 416, 516 with different configurations of electrodes. These can be used instead of the receiver 16 described in other examples herein.

In Figure 12A, corresponding components to those of Figure 2 are denoted with the same reference numerals but are incremented by four hundred, except using different letter indices where applicable. In Figure 12A, the receiver 416 has multiple electrodes 426a-426i to allow the electromagnetic field to be detected simultaneously in multiple directions at the receiver location. More specifically, four intermediate electrodes 426a-426d are arranged around an inner centre at a first distance from a common centre electrode 426i in the detection centre C of the receiver, and four outer electrodes 426e-426h are arranged at a second, greater distance from a common centre electrode 426i in the detection centre C. Each of the four outer electrodes 426e-426h forms a directional detecting pair together with the inner electrode 426i so as to allow detecting the electric field in an azimuthal direction away from the centre electrode 426i. The outer electrodes 426e-426h are arranged relative to the inner electrode 426i so that opposite detecting pairs are created. For example, the outer electrode 426e and inner electrode 426i form a pair that detects the electric field in one direction, and the outer electrode 426g and inner electrode 426i form another pair that detects the electric field in the 180-degree opposite direction. The intermediate electrodes 426a-426d are arranged similarly but form pairs with inner electrode 426i with smaller distances between. More specifically, the intermediate electrodes 426a-426d are arranged relative to the inner electrode 426i so that opposite detecting pairs are created. For example, the outer electrode 426a and inner electrode 426i form a pair that detects the electric field in one direction, and the outer electrode 426c and inner electrode 426i form another pair that detects the electric field in the 180-degree opposite direction. In this case therefore, one then obtains two sets of opposite directed pairs on each of the axes x and y, one nested within the other. This provides for the acquisition of further data from the receiver that may improve signal and facilitate detection of the target.

In Figure 12B, corresponding components to those of Figure 2 are denoted with the same reference numerals but are incremented by five hundred, except using different letter indices where applicable. In Figure 12B, the receiver 516 has multiple electrodes 526i-526k to allow the electromagnetic field to be detected simultaneously in multiple directions at the receiver location. More specifically, eight inner electrodes are distributed at equal angles around a detection centre C of the receiver 516 by a first distance away from the centre C. Eight further electrodes 526j are distributed arranged around the centre C by a second, greater distance, and eight yet further electrodes 526k are distributed arranged around the centre C by a third, yet greater distance from the centre C. Each axis has an opposite first directed pair formed by the inner electrode 526i and the intermediate electrode 526j in the direction away from the centre in one direction along the axis and by the inner electrode 526i and the intermediate electrode 526j in the opposite direction. Each axis has an opposite second directed pair formed by the inner electrode 526i and the outer electrode 526k in the direction away from the centre in one direction along the axis and by the inner electrode 526i and the outer electrode 526k in the opposite direction. In this way, there are four axes, where each has at least two sets of opposite directed pairs on each, one nested within the other.

Clearly, the axes may have one or more further electrodes if desired to form further opposite pairs. Some axes may have more opposite pairs than others. For example, in Figure 12B, selected axes have further electrodes 526l which are positioned at a yet further distances from the centre for providing further pairs with the respective inner electrodes 526i in opposite directions along the axis.

In Figure 4, computer apparatus 100 is depicted for processing and analysing data from the receivers 16. The computer apparatus 100 has an In/Out unit 101, a processor 102 and memory 103. Response signals comprising data from the receivers 16 are received by the computer apparatus 100 through the In/Out unit 101. The data are processed by the processor 102. The data e.g. processed and/or raw data are stored on the memory 103. One or more computer programs are also stored on the memory 103. The computer program(s) include for example data processing program comprising machine-readable instructions which when executed by a computer causes the computer to perform the combination of signals from respective forward and reverse receivers 16. The computer program(s) includes a program for inverting the data to determine one or more parameters of the subsurface structure. The processor 102 is used for executing the one or more computer programs. The computer apparatus 100 is in certain variants distributed apparatus, e.g. provided over several locations and/or units.

Turning now to Figures 5A and 5B, another example of the survey apparatus 10 is depicted on the seabed 2 for acquiring data in the CSEM survey. Two horizontal electric dipole transmitters 112a, 112b are provided instead of transmitter 12. In Figure 5A, the transmitter 112a is arranged with electrode 122a, 122b defining a dipole with dipole axis A- for emitting an electromagnetic field. The receiver 16 is positioned on a line perpendicular to the diploe axis A- forward of the dipole centre. In Figure 5B, the transmitter 112b is arranged with electrodes 122c, 122d defining a dipole with dipole axis A+ for emitting an electromagnetic field. The receiver 16 is then positioned on the line perpendicular to the dipole axis A+ rearward of the dipole centre. This illustrates that for example the transmitter 12 can be moved or selected to activate a different set of electrodes to provide in effect a second transmitter in the other position.

Figures 5A and 5B are annotated further where T- and T are first and second transmitter lines, Line A is the line on which the transmitters are located, D is the offset between the projection of the СSTAR centre of the STAR detection system onto line A and the centres C- and C+ of the transmitters 112a, 112b, Ec is the electrode 26i in the centre of the detection system of the receiver 16, {Ei} denotes the outer electrodes 26a-26h of the detection system of the receiver 16, {Lsi} denotes the electrical lines that extend between the centre electrode Ec and the respective outer electrodes {Ei}, On is the offset between the centre of the detection system of the receiver 16 and the transmitter lines A, {Mk} denotes calibration electrodes, Line A1 is the line along which the calibration electrodes are located (Line A1 is parallel to Line A.

Two horizontal electric dipoles 112a, 112b (T-, T+) used as transmitters are typically located on the seabed 2 along some line (Line A). The centres (C-, C+) of the dipoles are located apart relative to each other by some small distance, e.g. up to 200 m, along the transmitter line (Line A).

The two transmitters 112a, 112b are configured to introduce sign-alternating pulses of sequentially into the medium of subsurface 8. The sign-alternating pulses typically have an almost rectangular shape, separated by pauses. In variants of this example, pulses can be simultaneously supplied from the two transmitters, switched on oppositely.

The receiver 16 comprises a multi-channel system for detecting the electromagnetic field response. The change of the response signal over time is recorded by the multi-channel system of the receiver 26. The centre of the detection system (CSTAR) is positioned so that the offset (D) between the centres of the two transmitters (112a, 112b) and the CSTAR projection on each of the transmitter lines is equal and ranges from 10 to 100 m.

The electrode 26i (Ec) is positioned in the centre of the detection system and the outer electrodes 26a-26h ({Ei}) are placed around the centre electrode 26i forming azimuthal lines ({Lsi}) of length in the range of 100 to 500 m. Each of these lines connects the central electrode 26i to one of the outer electrodes 26a-26h. Thus, in practice the centre electrode 26i is electrically coupled to the respective outer electrodes 26a-26h. Eight dipole receiver strands are provided which each are arranged to detect the electric field component horizontally in the direction from the centre toward the outer electrode. Opposite pairs are defined as specified above in relation to Figure 2.

It can be noted that two of the lines ({Lsi}) are parallel to the transmitter lines, and two of lines are perpendicular to the transmitter lines. Four more lines are located between the first four lines. Every two “opposite” outer electrodes ({Ei}) connected to the central electrode (Ec) by lines located on the same axis must be equidistant from the central electrode. The calibration electrodes ({Mk}) are positioned along an axis parallel to the transmitter lines 112a, 112b, and will now be described further.

The result of this configuration of the receiver electrodes is that the response is obtained from the receiver 16 so that the components simultaneously measured on the different lines in opposite directions combine into one signal, in effect by summing.

Referring additionally now to Figure 6, the calibrating means 30 includes calibrating electrodes 32a-32d. The calibrating electrodes 32a-32d are located at respective positions spaced apart from one another along an axis of the sensor (Line A1) that is parallel to the dipole axis (Line A) of the transmitter 112a, 112b. In this way, various calibration responses can be obtained for different detecting pairs for detecting the electric field formed by one of the calibration electrodes and the centre electrode 26i.

In Figure 7, the various signal channels {Сhj} of the multi-channel system of the receiver 16 are set out. Сh1 provides the signal Pstar, which is the sum of the electric field detected in pairwise opposite directions on the eight lines of the STAR receiver 26. The remaining channels {Сhj} can be considered calibrated channels that provide the sums of the signals {Pstar Sj}. The signals {Sj} are obtained using various combinations of calibration electrodes {Mk}. The signal is applied to the Pstar signal and in effect provides a “calibrating correction” that may then allow local changes in the background structure to be eliminated. The calibration electrodes in fact detect the electric field on the receiving lines (Line A1) of the calibration electrode detecting pairs having lengths as indicated in brackets in Figure 7. The receiving lines of the calibration electrode pairs, i.e. the distance between electrode pairs, which have different lengths ranging from 0.1 to 20 m with a step of no more than 1.5 m. This is one or more orders of magnitude smaller than the distances between the main electrodes. The use of data on these channels makes it possible to increase the relative response from the target in the case of bathymetry and interfering 3D inhomogeneities.

More specifically, there are 46 calibrated signals in total in this example obtained for each STAR receiver. Each of these calibrated signals is provided on its own channel, i.e. we get 46 channels. That is, each channel provides the sum combination of the signal from the main receiver electrodes 26a-26h and one or more signals from the calibrating electrodes 32a-32d. For each position of the receiver (STAR), there are one to three channels with the maximum relative target response.

Additional channels (not shown) can be provided to provide the electric field (not the full star signal) on at least four lines of the STAR ({Lsi}), which are located parallel and perpendicular to the transmitter lines. The electric field can be detected on the line connecting the central electrode 26i (Ec) and one of the calibration electrodes 32a-32d (Mk). Such additional channels of the electric field may serve to assess the adequacy of measurements on calibrated channels and can provide additional information for the selection of the background medium, e.g. for facilitating in constraining forward/inverse modelling.

Depending upon the position of the receiver 16 relative to the transmitter, the receiver 16 provides data from the signal channels associated with either the forward (Star+) position or the rearward (Star-) position relative to the transmitter dipole centre. As mentioned above more generally, the signals from the forward and rearward positions can be advantageously combined through data processing.

In this regard, each channel of the STAR measuring system of the receiver 16 records a sequence of signals from alternating impulses, excited initially by the first transmitter 112a ("Star-"), and then by the second transmitter 112b ("Star+"). In addition, first combined signals "Star-" minus "Star+" and second combined signals "Star-" plus "Star+" are obtained for each channel. These signals are obtained through data processing. These signals are calculated using the "Star-" and "Star+" signals. Additionally, or alternatively, the signal "Star-" minus "Star+" can be determined from two opposite transmitter lines that are turned on simultaneously. Thus, for each channel four types of signals can then be obtained.

In Figures 8A to 8E, results of forward modelling are shown. Figure 8A shows a horizontally layered geoelectric earth model in plan and section view including a resistive target Tr. The target layer Tr has a resistivity of ρ=50 Ωm and is located at a depth of 2450-2470 m below seabed. The target is located in a horizontally layered background medium with wherein lw is the water layer, followed downward below the seabed Sb by successive first to fifth layers l1 to l5, wherein the water layer has resistivity ρlw=0.28 Ωm and thickness Δhlw= 300 m, the first layer l1 has resistivity ρl1= 3 Ωm and thickness Δhl1=150 m, the second layer l2 has resistivity ρl2 = 2.2 Ωm and thickness Δhl2= 150 m, the third layer l3 has resistivity ρl3= 1.7 Ωm and thickness Δhl3=800m, the fourth layer l4 has resistivity ρl4=1 Ωm and thickness Δhl4= 1600 m, and the fifth layer l5 has resistivity ρl5=3 Ωm and extends to infinite extent downward. In Figure 8A, the positions of the star receiver 16 are indicated STAR0 to STAR3. In each position, the receiver 16 is associated with two transmitters 112a, 112b arranged as indicated above in relation to Figures 5A and 5B, thus providing the full set of channels from both the forward and the rearward positions. The points of intersection P1, P2 of the profile and projection of the target boundaries on the seabed are indicated.

Figures 8B and 8C show the "Star-" and "Star+" response (over time), i.e. amplitude of the signal associated with the transmission from the first transmitter line 112a (T-) and the second transmitter line 112b (T ) for the positions STAR0 to STAR3. Figures 8D and 8E show the first combined response "Star-" minus "Star+" and second combined response "Star-" plus "Star+" along the profile (Pr) of Figure 8A at time t = 5 s. The transmitter current is 6000 A.

Advantageously, due to in effect summing opposite directed components on the respective axes of the Star receiver 26, the "Star-" and "Star+" signals of Figures 8B and 8C are relatively highly sensitive to the 3D attributes of the target. Thus, when in proximity to an edge of the target, the relative response due to the presence of the target Tr, as can be seen from the STAR1 curve in Figures 8B and 8C, provides a significant anomaly compared with the STAR3 curve which is sensitive primarily to background structure. In Figure 8E, the combined signal ("Star-" "Star+") by summing of signals of the rearward and forward locations provides a "clean" anomalous response associated with the target. Indeed, the maximum anomalous response in the "Star-", "Star " and ("Star-" "Star+") signals occurs in the vicinity of the target boundary as seen in Figures 8B, 8C, and 8E.

In order to obtain high relative response from the target (relative to background) in the event of significant influence of bathymetry and/or interfering 3D inhomogeneities, the signals "Star-“ and "Star+" are used on the sequence of calibrated channels with the minimum absolute values of the signals. On calibrated channels where "Star-" and "Star+" signals have their lowest absolute values in the late stage, the highest relative target response can be obtained. On this basis, suitable channels are selected for processing that have the lowest levels. The channels with lowest levels are the most suited because these will have the greatest relative effect of the subsurface target.

The calibration electrodes are located preferably in a line parallel to the transmitter dipole axis because this provides for good quality signal. It may be preferable and more convenient for processing when calibration channels are formed using lines parallel to the transmitter. This allows the use of lines of minimum length and always be sure that the calibration will take place (i.e. a signal with a large relative anomaly from the target will be received).

The maximum anomalous response of the target in the first combined signal by subtracting the "Star-" and "Star+" signals between rearward and forward locations is located above the centre of the target as can be seen in Figure 8D. It is higher than the anomalous response from the target in prior art vertical lines as a receiver and transmitter. The anomalous response in the "Star-" and "Star+" signals is even higher. The signal ("Star-" - "Star+") advantageously therefore may have little sensitivity to bathymetry and interfering 3D inhomogeneities and is thus used to determine the background medium and induced polarization (IP) effects. IP signal strength analysis is performed using numerical simulation and inversion. For this, signals ("Star-" - "Star+") are used, which are averaged over the sequence of channels on which the signals "Star-" and "Star+" have their minimum absolute values. Use of the channels on which the “Star-” and “Star ” signals have the minimum absolute values is proposed, since, as a result, one obtains the minimum influence of the measurement error. Other channels can be used. But there will be mutual subtraction of much larger values and, as a result, accuracy may deteriorate.

In example variants, a method of performing STAR-CSEM-TD survey can involve adaptively selecting the number of STAR-CSEM positions, as follows: The ship 4 arrives at a first point, which is close to the boundary of the prospective reservoir target Tr and sets up a STAR measuring system on the seabed such as in the STAR0 position of Figure 8A. Then, transmitter lines 112a, 112b are installed on the seabed at a distance of 500 m from the STAR centre (offset O1) and measurements are made for 0.5-2.5 hours using the first and second transmitter lines for example repeating the sequence of transmission and detection of the field to acquire the response from the receiver 100-200 times for each transmitter position T-, T+. The signals ("Star-" - "Star+") and ("Star-" "Star+") are calculated. Then the transmitter lines are transferred to other distances, for example, 1500 m and 2500 m from the STAR centre (offsets O2 and O3) and measurements are repeated.

In Figures 9A to 9D, synthetic modelling results showing the effects of induced polarization (IP) upon the response signals from different layers and source-receiver offsets are provided. In Figures 9A to 9D, the signal "Star-" minus "Star+" is plotted against time for the receiver position STAR0 for three offsets O1 = 500m, O2 = 1500m, O3 = 2500m for four model options whereby the responses in Figure 9A are without IP, Figure 9B has IP in the l1 layer, Figure 9C has IP in the l2 layer, and Figure 9D has IP in the l3 layer. In all cases, the polarizability η = 2%. The decay function with time t was calculated using the Komarov formula:

where T 0 = 0.2 , and B = 100. The transmitter current is 6000 A.

If the signals for all offsets are close enough as seen in Figure 9A, then the influence of IP is practically absent and for further measurements it is sufficient to use one separation O1,opt. In the case of significant IP influence, the signals in the late stage will differ at different offsets as seen in Figures 9B to 9D. However, with increasing offset, the IP influence weakens (see Figures 9B to 9D), and the signal without IP influence and the signal for the O3 offset are very close in the late stage. This allows the influence of the induction component of the signal and the field of induced polarization to be separated, which, in turn, makes it possible to determine both the conductivity of the medium and the parameters of polarizability. If the polarizability is concentrated in the bottom layer, then the IP signals may have different signs for the O1 and O2 offsets (see Figure 4B). This fact allows us to assess the depth of the location of the polarizing rocks. If influence of IP is significant, further measurements should be taken at multiple offsets, both to reliably separate the effects of the inductive component of the signal and the induced polarization field, and to determine the presence of polarizability anomalies that may indicate the presence of hydrocarbons. Thus, based on the results of measurements at the first point, the optimal set of offsets {On,opt} for further measurements in the survey is determined. Then a series of measurements is made for at least three more STAR-CSEM device locations with optimal offsets {On,opt}. Two STAR-CSEMs are placed near the first point (STAR0) on either side of it (for example, the STAR1 and STAR2 positions in Figure 8A), and the third STAR-CSEM is located at some distant point from the reservoir (for example, STAR3 in Figure 8A).

Data processing is performed as follows. For each location of the measuring system of the receiver 26, a sequence of calibrated channels is selected where the "Star-" and "Star+" signals have their lowest absolute values at a late stage. For these channels, the signals are averaged ("Star-" - "Star+"). The background medium and local inhomogeneities in the nearbottom part of the section located under the receivers are reconstructed using 1D/3D inversion using averaged signals ("Star-" - "Star+"), signals "Star-" and "Star+" at early times, and signals measured on additional channels. Using 3D modelling, in the reconstructed medium, bathymetry effect is calculated for the "Star-" and "Star+" signals on the selected sequence of calibrated channels (an example of "Star-" signals is shown in Figures 10A-10D).

Figures 10A to 10E show plots of modelled “Star-” signals on the first (uncalibrated) channel (Pstar) and two adjacent calibrated channels with the minimum absolute values of the signals for the medium with target and bathymetry, compared to the "Star-" signals calculated with bathymetry in the background medium (without target) in four positions STAR0 (Figure 10A) to STAR3 (Figure 10D). In Figure 10E, residual anomalous signals "Star-" at the four points {STAR0 to 3} are shown compared to the signal ("Star-" - "Star+") calculated for the background medium. The transmitter current is 6000 A. Figures 10A to 10D show that the calibrated channels have a relative response from target above 100%. The "Star-" and "Star+" signals calculated in the reconstructed bathymetric environment are subtracted from the corresponding "Star-" and "Star+" signals measured on the selected sequence of calibrated channels. As a result, residual anomalous signals "Star-" and "Star+" are obtained each one being averaged over a selected sequence of calibrated channels (an example of a residual anomalous signal "Star-" is shown in Figure 10E). In the late stage, the averaged residual anomalous signals "Star-" and "Star+" will correspond to the anomalous response from the deep part of the section, in fact, from the target, if it exists (see Figure 10E). Based on the obtained results, a decision on the following points for the location of the STAR-receiver is made: for the "Star-" and "Star+" signals, there should be an increase in residual anomalies near the reservoir boundary and a decrease in the anomaly at the central and remote point (Figure 10E). If the transverse dimension of the reservoir is large enough, then for the signal "Star-" minus "Star+" there should be an increase in the anomalous response at the centre point and a decrease in the anomaly at a point at the reservoir boundary and at a distant point. Using the averaged residual anomalous signals "Star-" and "Star+", a geometric 3D-inversion (Persova et al., 2020) is carried out in the reconstructed background medium, as a result the presence and boundaries of the hydrocarbon reservoir are determined. The inversion uses the inverse of the signal "Star-" minus "Star+" as weighting functions (an example of this signal is shown in Figure 10E).

It is possible to collect data by moving the STAR receiver with one or more offsets ({On}) along some profiles with a given step or a step decreasing in the area of the prospective reservoir. Data processing is carried out as described above.

Figures 11A and 11B show alternative methods of surveying. Figure 11A is a "wide profile" technique whereas Figure B shows an "area" technique. In the wide profile option, the transmitters (T-, T ) are placed along a predefined profile line, and the receivers 16 (STAR) are placed at different offsets ({On}) on both sides of the transmitters (see Figure 11A). Following on from the wide profile, an “area” survey option can be provided whereby a next set of wide profile measurements are made by moving transmitters keeping some of the receivers 16 (Figure 11B) in place. Data processing is carried out as described above.

With reference again to Figures 5 to 11, a method of surveying using the STAR-CSEM receiver(s) 16 can be performed as follows: 1) Adaptive option: carrying out measurements by the STAR-SCEM device in several stages (series) with a decision on the expediency of carrying out the next series of measurements based on the results of the previous series of measurements; 2) Profile option: carrying out measurements by the STAR-SCEM device with one or more offsets {On} (see Figures 5A and 5B) along one or several profiles with a constant step or a step, decreasing in the area of the prospective reservoir Tr; Figure 8A); and Wideprofile option and area option: carrying out measurements by the STAR-SCEM device, when the transmitters T-, T+ are located along the profile, and the STAR receivers are located at different offsets {On} and both sides of the transmitters (Figure 11A); the following measurements are made with moving of the transmitters and keeping the location of a part of the receivers (Figure 11B).

The adaptive option is performed as follows. The multi-channel measuring system of the receiver 16 is deployed on the seabed Sb in the first position STAR0, which is on the Profile line (Pr) near the edge of the prospective reservoir (Tr). Line A1 is directed along Profile line. The centre electrode Ec is located on Line A1, and the outer electrodes {Ei} are located around the centre electrode Ec and connected to it by lines {Lsi} so that two of these lines lie on Line A1, two lines are perpendicular to Line A1. The remaining four lines are located between the first four lines and are the bisectors of the corresponding right angles. Every two opposite outer electrodes {Ei} connected to the centre electrode Ec via lines {Lsi}, and located on the same line, are equidistant from the central electrode. The preferred distance between the central electrode Ec and the outer electrodes {Ei}, which are located on line A1, is 200 m. The rest of the external electrodes are located so that they form a square with side 2*de and centre at the location point {Ei}. The calibrating electrodes {Mk} are located along Line A1 at distances of 0.5 m, 1 m, 2 m, and 4 m from the centre electrode Ec. At a distance of 4 m, there are two calibration electrodes M4 and M4a. The connection of the measuring electrodes Ec, {Ei}, {Mk} is carried out so as to ensure measurement on the 47 main channels listed in Figure 7, and five additional channels. The first main channel Ch1 measures the Pstar signal, i.e. sum of the electric field on eight lines of the STAR receiver. The remaining main channels {Chj} measure the sum of the Pstar signal and the calibrating signals {Sj}, which are equivalent to the electric field on the receiving lines with lengths from 0.5 m to 11.5 m (see Figure 6). Four additional channels (not shown) record the electric field on four STAR {Lsi} lines, which are located parallel and perpendicular to the transmitter lines. The fifth additional channel (not shown) measures the electric field on the line connecting the centre electrode Ec and the calibrating electrode M4.

Two transmitter lines T- and T+ are located along Line A, which is parallel to Line A1 and is at a distance of {On}. The preferred length of transmitter lines is 500 to 1000 m and the preferred first offset O1 is 500 m. Transmitter lines T- and T+ are displaced relative to each other so that their centres C- and C+ are equidistant from the projection of the central electrode Ec of the STAR measuring system on distance D. The preferred distance D is 40-50 m.

Two sets of measurements are carried out in sequence. First, by means of transmitter line T-alternating pulses are excited with pauses with durations from 3 to 20 s. The preferred current is 6000 A. Measurements with the STAR multi-channel measuring system are performed for 1⁄4 of the pulse duration before switching off and during a pause with a preferred step of 200 μs. Measurements are performed within 0.5-2.5 hours (100-200 stacking repetitions). Based on the measurement results, a set of signals {"Star-"} is obtained on all channels of the STAR measuring system. Then similar measurements are made from transmitter line T+ and a set of signals {"Star "} is obtained on all channels of the STAR measuring system. Then, for each main channel of the measuring system, the difference and the sum of the measured signals are calculated, i.e. sets of signals {"Star-" - "Star+"} and {"Star-" "Star+"} are obtained.

Then transmitter lines T- and T+ are transferred to other distances from the STAR centre and measurements are repeated. It is preferable to use two more offsets: O2 = 1500m and O3 = 2500m.

On the basis of the received sets of signals {"Star-"} and {"Star+"}, it is determined the sequence of channels {Chj,min} on which the signals "Star-" and "Star+" measured from the first T- and the second T+ transmitter lines have minimum absolute values. For the selected channels, the signals "Star-"-"Star+" are averaged. Using simulation and inversion of the averaged signals "Star-"-"Star+" obtained for different offsets, the approximate characteristics of the background medium are determined: resistivity and parameters of the induced polarization. Based on the results obtained, a decision is made on the optimal offsets {On, opt} for further measurements. In the absence of the IP effect, the signals for all offsets will coincide at a later stage, and for further measurements one offset of О1,opt will be enough, otherwise for further measurements it is necessary to use at least two offsets {On, opt}. Measurements are performed for three more STAR-CSEM positions with optimal offsets {On, opt}. STAR1 and STAR2 are located near the first point of STAR0 on either side of it, and STAR3 is located at a point some distance from the prospective reservoir Tr. The preferred distance for STAR3 is 4000-5000 m.

Data processing is performed, including the following steps. For each STAR position, a sequence of channels {Chj, min} is selected, on which the signals "Star-" and "Star+", measured from the first T- and the second T+ transmitter lines, have minimal absolute values, and for these channels signals "Star-"-"Star+" are averaged. Using the “Star-”-“Star+” signals, “Star-” and “Star+” signals at early times and signals measured on additional channels, 1D/3D inversion is performed and the background medium is determined, including local inhomogeneities near seabed. For the resulting medium, 3D modeling is performed taking into account bathymetry and local near-bottom inhomogeneities, and the signals "Star-" and "Star+" on the channels {Chj, min} are calculated, which are subtracted from the corresponding measured signals, thereby the residual abnormal signals are obtained. The analysis of the obtained residual anomalies is performed and a decision is made on the next points of the STAR-SCEM device location, for which the processing is performed according to the above algorithm.

When using alternative options for data collection, including a profile option or a wide profile option and areal option, data processing is also performed according to the above algorithm.

At the final stage, based on the obtained data set, the presence and boundaries of the target object of increased resistance is determined by geometric 3D inversion (See reference [11] in the List of References - Persova et al., 2020).

The survey may be used to provide 4D monitoring of a resistive target such as a hydrocarbon reservoir. This can be done by obtaining performing the method to obtain a first data vintage and then repeat survey of the area one or more weeks, months, or years later, to obtain one or more further data vintages, for example without removing the receiver and transmitter from the area in the meantime.

Advantages

The technique described above can be termed STAR-CSEM-TD and can be beneficial in various ways. In various examples, it may facilitate increased reliability of geophysical forecasting of offshore hydrocarbon deposits, in particular the detection of small deep reservoirs of hydrocarbons. This can be achieved in particular by detecting and combining signals from receivers in suitable manner so as to increase the anomalous response of the target and the ability to determine its boundaries. The acquisition by combining opposite components on channels can enhance detectability of reservoir edge features. Similarly, the use of sum combination of data from the receiver(s) side on to the transmitter in forward and rearward positions can also facilitate the detectability of edge features. Providing calibration components of electric fields from short dipole sensors and combining with the main electrode sets and selecting the one which based on signal levels is likely to provide the best representation of target anomaly can further facilitate detection of response from the target.

Furthermore, the anomalous response of a target can be enhanced by compensating both for the effects of low resistivity background media, e.g. conductive brine-filled rocks and/or overburden, and for the effects of interfering 3D inhomogeneities. Yet further, acquired data at different offsets can allow induced polarization (IP) effects to be identified and determined and used in data selection and analysis to focus upon the appropriate parts data points (e.g. those at later times) or take into account the IP response data in inversion for facilitating detection of reservoir target.

Various documents that may be relevant for an understanding of how the present invention is distinguished from prior art are indicated in the List of References set out below.

Various further examples are set out by the numbered Paragraphs below:

1. STAR-SCEM device for the detection of hydrocarbon deposits, comprising: two horizontal electric lines (T-, T+), used as transmitters, located on the seabed (Sb) along a line (A), the centres of which are offset relative to each other by some distance (D); receiving unit (STAR), including main and calibrating electrodes, where one of the main electrodes (Ec) is placed in the centre of the detection system (CSTAR), which is equidistant from the centres (C-, C ) transmitter lines (T-, T+), calibration electrodes ({Mk}) are placed along a line (A1) parallel to the transmitter lines (T-, T+), and the other main external electrodes ({Ei}) are placed around the first main electrode (Ec) and form the lines ({Lsi}), each of which connects the central electrode (Ec) with one of the external electrodes (Ei); multi-channel system (STAR), which records the change in the signal over time by several channels ({Chj}) using the receiving unit (STAR), the first (Ch1) registers a signal by a multi-electrode device (Pstar) that actually measures the electric field on the lines ({Lsi }) of the receiver (STAR), and the other channels register the sums ({Pstar Sj}) of the signal (Pstar) by the multi-electrode device and the calibrating signals ({Sj}), which are measured using various combinations of calibrating electrodes, and in fact measure the electric fields on receiving lines that have different lengths.

2. The device of Paragraph 1, wherein the receiver (STAR) contains two external electrodes ({Ei}), which form two lines ({Lsi}), which are parallel to the transmitter lines and each of which connects a central electrode (Ec) with one of the external electrodes (Ei).

3. The device of Paragraph 1 or 2, wherein the receiver (STAR) contains four external electrodes ({Ei}), which are placed around the first main electrode (Ec), forming four lines ({Lsi}), each of which connects the centre electrode (Ec) to one of the external electrodes (Ei ) and two of which are parallel to transmitter lines, and two lines are perpendicular to transmitter lines.

4. The device of any of Paragraphs 1 to 3, wherein the receiver (STAR) contains eight external electrodes ({Ei}), which are placed around the first main electrode (Ec), forming eight lines ({Lsi}), each of which connects the centre electrode (Ec) to one of the external electrodes (Ei ) and two of which are parallel to transmitter lines, two lines are perpendicular to transmitter lines, and the remaining four lines are located between the first four.

5. A method for detecting hydrocarbon deposits, comprising: placing of transmitters (T-, T+) and a receiver (STAR) with the first offset (O1) on the seabed (Sb) near the edge of the prospective reservoir (Tr); recording a sequence of signals ("Star-"and "Star+") from bipolar pulses excited first by the first transmitter line (T-) and then by the second transmitter line (T+) by all channels of the measuring system (STAR); calculating of the difference ("Star-"-"Star+") and the sum ("Star-"+"Star+") of signals measured from the first (T-) and second (T ) transmitter lines for each channel of the measuring system; moving transmitters (T-, T ) to other offsets ({On}) and repeating the recording of signals; determining the sequence of channels ({Chj, min}) on which the signals ("Star-" and "Star+") measured from the first (T-) and second (T+) transmitter lines have minimum absolute values; averaging the difference ("Star-"-"Star+") of the signals measured from the first (T-) and second (T+) transmitter lines for the selected sequence of channels ({Chj, min}); analysing of the effect of IP using averaged differences ("Star-"-"Star+") and selection of one or more optimal offsets ({On, opt}); carrying out a series of measurements, including at least three more STAR-CSEM device locations with optimal offsets ({On, opt}), with two STAR-CSEM located on either side of the first position, and the third STAR-CSEM is located at the point, remote from the prospective reservoir; data processing, comprising: selecting of a sequence of channels ({Chj, min}), on which the signals ("Star-" and "Star+"), obtained from the first (T-) and second (T+) transmitter lines, have minimum absolute values; averaging the difference ("Star-"-"Star+") of the signals measured from the first (T-) and second (T ) transmitter lines for the selected sequence of channels ({Chj, min}); recovering of the background medium and local inhomogeneities near seabed by means of 1D/3D inversion using averaged differences ("Star-"-"Star+") signals, signals "Star-" and "Star+" at early times and signals obtained on additional channels; calculating of signals ("Star-" and "Star ") from the first (T-) and second (T+) transmitter lines on the selected sequence of calibrated channels for the medium with bathymetry and near-seabed inhomogeneities; obtaining residual anomalies in signals ("Star-" and "Star+") from the first (T-) and second (T+) transmitter lines by subtracting the calculated signals from measured signals on the selected sequence of the calibrated channels; deciding on next placing points of the STAR-SCEM device; determination of the presence and boundaries of the target object of increased resistance using geometric 3D-inversion using the entire data set;

6. The method of Paragraph 5, wherein the STAR-SCEM device with one or more offsets ({On}) is moved along some profiles with a predetermined step or a step decreasing in the area of the prospective reservoir, and measurements are taken.

7. The method of Paragraph 5 or 6, wherein the transmitters (T-, T+) are located along the profile, and the receiver (STAR) at different offsets ({On}) on both sides of the transmitters.

8. The method of any of Paragraphs 5 to 7, wherein the following measurements are made with the transfer of the transmitters, keeping the location of the part of the receiving units (STAR).

List of References

Published patent documents with publication numbers:

US7834632B2 US20060129322A1

US7834632B2 US8030934B2

US10042073B2 US20120038362A1

US20130221969A1 AU2007326078B2

US9766361B2 US8264230B2

US7529627B2 US7737699B2

US8253418B2 US7728596B2

US7912649B2 US6114855A

Non-patent literature:

[1] Zhdanov M., Endo M., Yoon D., Cuma M., Mattsson J.,Midgley J. Anisotropic 3D inversion of towed-streamer electromagnetic data: Case study from the Troll West Oil Province. Interpretation, 2, p.SH97-SH113. (2014).

[2] Du Z., Namo G., May J., Reiser C., Midgley J. Total hydrocarbon volume in place: Improved reservoir characterization from integration of towed-streamer EM and dual-sensor broadband seismic data. First Break, 35, p.89-96. (2017).

[3] Du Z., and Key K. North Sea case study: Heavy oil reservoir characterization from integrated analysis of Towed Streamer EM and dual-sensor seismic data. ASEG Extended Abstracts, 2015.

[4] Constable S. Review paper: Instrumentation for marine magnetotelluric and controlled source electromagnetic sounding. Geophysical Prospecting, 61, p.505-532. (2013).

[5] Holten T., Flekkøy E., Singer B., Blixt E., Hanssen A., Måløy K. Vertical source, vertical receiver, electromagnetic technique for offshore hydrocarbon exploration. First Break, 27, p.89-93. (2009).

[6] Håland E., Flekkøy E., Måløy K. Vertical and horizontal components of the electric background field at the sea bottom. GEOPHYSICS, 77, p.E1-E8. (2012).

[7] Helwig S., Wood W., Frafjord Ø. First CSEM Surveys with a Newly Designed Receiver.Conference Proceedings, 79th EAGE Conference and Exhibition 2017, Jun 2017, Volume 2017, p.1 – 5. (2017).

[8] Nobes D. The Magnetometric Off-Shore Electrical Sounding (MOSES) Method and Its Application in a Survey of Upper Jervis Inlet, British Columbia. (1984).

[9] Legeido P., Ageenkov E. Differentially-normalized Method of Electroinvestigation (DNME) - An Efficient Instrument for HC Exploration Off- and Onshore (Russian). Society of Petroleum Engineers. doi:10.2118/149910-RU (2011).

[10] V. Mogilatov, M. Goldman, M. G. Persova, Y. G. Soloveichik, Y. I. Koshkina, O. S. Trubacheva, A. Zlobinskiy. Application of the marine circular electric dipole method in high latitude Arctic regions using drifting ice floes // Journal of Applied Geophysics. - 2016. – Vol.

135. – P. 17-31.

[11] Persova, M.G., Soloveichik, Y.G., Vagin, D.V., Kiselev, D.S., Grif, A.M., Koshkina, Y.I. & Sivenkova A.P., 2020. Three-dimensional inversion of airborne data with applications for detecting elongated subvertical bodies overlapped by an inhomogeneous conductive layer with topography. Geophysical Prospecting 68, 2217-2253.

Claims

1. A method of performing a marine controlled source electromagnetic survey to detect a resistive target (7) located in a subsurface (8) of the Earth, the method comprising the steps of:

providing survey apparatus (10) upon the seabed (2), the survey apparatus (10) comprising at least one receiver (16, 416, 516) and at least one electromagnetic field transmitter (12), and the electromagnetic field transmitter (12) comprising at least one horizontal electric dipole (112a, 112b);

transmitting an electromagnetic field (9) from the electromagnetic field transmitter (12) through at least one region of the subsurface (8);

characterised in that the method further comprises:

detecting the electromagnetic field (9) by combining components in at least one pair of opposite directions along an axis on at least one signal channel of the receiver (16, 26, 416, 516); and

positioning the receiver (16, 416, 516) in spaced apart relationship from the transmitter (12), on a transmitter-receiver axis which is perpendicular to a dipole axis (A, A+, A-) of the horizontal electric dipole (112a, 112b), either forward or rearward with respect to the dipole centre.

2. A method as claimed in claim 1, wherein the detection step may comprise detecting the electromagnetic field (9) by sensor elements of the receiver (16, 416, 516) and combining on at least one signal channel: components in opposite directions along a first axis; and components in opposite directions along a second axis; and components in opposite directions along at least one or more further axes; wherein the axes extend in different horizontal directions to one another.

3. A method as claimed in claim 1 or 2, wherein the components combine additively for obtaining a field component sum of the components in the opposite directions.

4. A method as claimed in any preceding claim, which further comprises:

providing a first configuration wherein the receiver (16, 416, 516) is positioned in the forward location relative to the transmitter (12), whereby the receiver (16, 416, 516) is positioned apart from the transmitter (12) on a perpendicular axis from the dipole axis (A, A+, A-) and forward with respect to the dipole centre;

providing a second configuration wherein the receiver (16, 416, 516) is positioned in the rearward location relative to the transmitter (12), whereby the receiver (16, 416, 516) is positioned apart from the transmitter (12) on a perpendicular axis from the dipole axis (A, A+, A-) and rearward with respect to the dipole centre;

detecting the electromagnetic field (9) using the receiver (16, 416, 516) in forward location;

detecting the electromagnetic field (9) using the receiver (16, 416, 516) in the rearward location; and

combining at least one signal, data, or at least one data component from the signal channel of the receiver (16, 416, 516) in the forward location and at least one signal, data, or at least one data component from the signal channel of the receiver in the rearward location.

5. A method as claimed in claim 4, wherein the combination may comprise adding the signals, data or data components of the forward and rearward locations, so that a combination sum signal is obtained.

6. A method as claimed in claim 4 or 5, wherein the combination may comprise subtracting the signals, data or data components of the forward and rearward locations, so that a combination difference signal is obtained.

7. A method as claimed in any preceding claim, which further comprises:

detecting at least one calibrating component of the field;

detecting the electromagnetic field (9) by combining main components in at least one pair of opposite directions along the axis, whereby the main components are combined on at least one signal channel; and

further combining the calibrating component with the main components on at least one calibrated channel.

8. A method as claimed in any preceding claim, which further comprises locating the transmitter (12) and receiver (16, 416, 516) on the seabed (2) in different locations to survey different regions of the seafloor and subsurface (8); moving the transmitter (12) to different transmitter locations and/or the receiver (16, 416, 516) to different receiver locations; obtaining responses from the receivers (16, 416, 516) in the different locations; and processing and/or analysing the data obtained on one or more of the receiver channels from the different locations.

9. A method as claimed in any preceding claim, wherein the apparatus (10) comprises a plurality of transmitter-receiver pairs each of which has a separation between the transmitter (12) from the receiver (16, 416, 516) in the direction perpendicular to the dipole axis (A, A+, A-), wherein the transmitter and receiver separation of at least one pair differs from at least one other pair.

10. A method as claimed in any preceding claim, wherein the apparatus (10) comprises one or more transmitter-receiver sets each comprising at least one transmitter (12) and at least one receiver (16, 416, 516), wherein the method includes:

deploying one transmitter-receiver set in a first arrangement on the seabed (2) above the target structure (7) or above region in lateral proximity to the target structure (7) for obtaining a signal that comprises an effect of the target structure (7); and

either or both of:

further deploying the one transmitter-receiver set in a second arrangement on the seabed (2) away from the target for obtaining a signal that is free of effects from the target structure; and

deploying another transmitter-receiver set in a second arrangement on the seabed (2) away from the target for obtaining a signal that is free of effects from the target structure (7).

11. A method as claimed in any preceding claim, which further comprises operating the transmitter (12) to emit an electromagnetic field (9) in the form of one or more pulses; obtaining and using the signal from the receiver(s) (16, 416, 516) over time following transmitter switch off time.

12. A method as claimed in any preceding claim performed to obtain four-dimensional electromagnetic data from the survey area (1) for detecting at least one change of condition of the resistive target structure (7).

13. A receiver (16, 416, 516) for detecting an electromagnetic field (9) from the Earth’s subsurface (8) in a marine controlled source electromagnetic survey, the receiver (16, 416, 516) comprising sensors to detect the field along at least one axis,

characterised in that at least one pair of components of the field in opposite horizontal directions along the axis are combined on at least one signal channel, the electromagnetic field (9) in use produced using at least one transmitter (12) with the receiver (16, 416, 516) being positioned in spaced apart relationship from the transmitter (12), on a transmitterreceiver axis which is perpendicular to a horizontal dipole axis (A, A+, A-) of the transmitter (12), either forward or rearward with respect to the dipole centre.

14. A receiver (16, 416, 516) as claimed in claim 13, wherein the sensors comprise sensor elements (26a-26i), such as electrodes, with sensor positions respectively along the axis, and which comprise at least two pairs of sensor elements along the axis, wherein the sensor elements (26a-26i) of the one pair are configured to detect a component of the field (9) in one direction along the axis and the sensor elements (26a-26i) of the other pair are configured to detect a component of the field (9) in an opposite direction along the axis.

15. A receiver (16, 416, 516) as claimed in claim 13 or 14, which further comprises at least two pairs of sensors to detect at least two pairs of opposite components of the field along the axis, wherein sensor element spacings between sensor elements (26a-26i) of the one of the pairs is greater than between the sensor elements (26a-26i) of the other of the pairs.

16. A receiver (16, 416, 516) as claimed in any of claims 13 to 15, wherein the sensor elements (26a-26i) of each of the sensors of an opposite pair may be configured to be positioned apart by a distance, e.g. in the region of 100 to 500 m.

17. A receiver (16, 416, 516) as claimed in any of claims 13 to 16, further comprising at least one support member configured to extend radially away from a body of the receiver (16, 416, 516) for supporting one or more sensor elements (26a-26i) of sensors and/or for positioning or aligning the sensor elements (26a-26i) along the axis.

18. A receiver (16, 416, 516) as claimed in any of claims 13 to 17, wherein the sensors comprise at least one pair of opposite sensors, wherein outer sensor elements (26a-26h) are arranged about an inner, central sensor element (26i) so that one sensor of the pair is formed by the inner, central sensor element (26i) and the outer sensor element (26a-26h) along the axis for detecting the component in one direction, and another sensor is formed by the inner, central sensor element (26i) and the other outer sensor element (26a-26h) along the axis for detecting the component in the opposite direction.

19. A receiver (16, 416, 516) as claimed in any or claims 13 to 18, wherein the sensors are arranged along four axes for detecting the electromagnetic field (9) so that pairs of components of the field (9) in opposite directions radially from a central part of the receiver apparatus (16, 416, 516) on each of the four axes are combined on the signal channel, whereby the sensors detect the field in a total of eight directions.

20. A receiver (16, 416, 516) as claimed in any of claims 13 to 19, wherein the sensors comprise main sensor elements (26a-26h) for detecting main components of the field along the opposite directions which combine on the signal channel, and wherein the sensors further comprise further sensor elements (32a-32d) for detecting one or more calibration components, wherein the calibration combine with the combined main components on at least one calibrated signal channel.

21. A receiver (16, 416, 516) as claimed in claim 20, wherein the calibration sensor elements (32a-32d) are arranged apart by a distance, in use horizontally, e.g. in the range of 1 to 10 m.

22. A receiver (16, 416, 516) as claimed in any of claims 13 to 21, further comprising electrical combining circuitry for coupling the sensors and combining the components from the sensors onto the signal channel.

23. Survey apparatus for performing a marine controlled source electromagnetic survey, the apparatus comprising:

at least one receiver (16, 416, 516) in accordance with any of claims 13 to 22;

at least one electromagnetic field transmitter for propagating an electromagnetic field (9) through the subsurface; and

at least one processor (102) configured to process data obtained from the receiver (16, 416, 516) to facilitate detecting the resistive target (7), the receiver (16, 416, 516) in use being configured to obtain opposite horizontal components and being positioned in spaced apart relationship from the transmitter (12), on a transmitter-receiver axis which is perpendicular to a horizontal dipole axis (A, A+, A-) of the transmitter (12), either forward or rearward with respect to the dipole centre.

24. Computer apparatus (100) for receiving and processing the data from the signal channel in the method of any of claims 1 to 12.

25. A computer program which when executed causes the computer apparatus (100) of claim 24 to perform one or more steps to process and/or analyse the data from the signal channel in the method of any of claims 1 to 12.