WO2011050139A2

WO2011050139A2 - Synthetic vertical electromagnetic source

Info

Publication number: WO2011050139A2
Application number: PCT/US2010/053496
Authority: WO
Inventors: Yuanzhong Fan; Jeffrey Lawrence Johnson; Liam Colman O Suilleabhain; Johannes Maria Singer; Pleun Marinus Van Der Sman
Original assignee: Shell Oil Company; Shell Internationale Research Maatschappij B.V.
Priority date: 2009-10-21
Filing date: 2010-10-21
Publication date: 2011-04-28
Also published as: WO2011050139A3

Abstract

A method for surveying a subsurface using electromagnetic signals, comprises: using at least one dipole to transmit into the subsurface a series of signals that includes pairs of opposing electromagnetic signals, using at least one receiver to receive from the subsurface a set of signals resulting from passage of the transmitted signals through the subsurface, locating within the set of received signals at least one pair of received signals having opposite field directions, and processing said pair of opposing received signals so as to obtain information about the subsurface. The transmitting dipole can be an alternating dipole towed above the seafloor and may be towed along non-parallel lines or along orthogonal grid lines. A plurality of pairs of received signals having a common center point may be processed and phase shifting and/or amplitude modulation can be used to steer said information. A listening period can be included between transmitted signals.

Description

SYNTHETIC VERTICAL ELECTROMAGNETIC SOURCE

RELATED CASES

Not applicable.

FIELD OF THE INVENTION

[0001] The invention relates to the use of paired electromagnetic measurements to synthesize a vertical field for investigating subsurface formations. The paired

measurements need not be generated concurrently.

BACKGROUND OF THE INVENTION

[0002] Controlled Source Electromagnetic (CSEM) techniques have been used in academic as well as exploration environments ever since Schlumberger made his first experiments in the early 20th century. For a long time, land CSEM techniques have been mostly applied to either deep scientific targets, such as deep crustal studies or magma chambers, or to shallow mining applications such as ore exploration or shallow hydrocarbon-induced alteration zones. Recently, CSEM techniques experienced a renaissance for deep hydrocarbon exploration, i.e. detection of the resistivity structure of the subsurface for the purpose of hydrocarbon exploration. CSEM techniques have been transferred into marine settings since the late 1960's / early 1970' s.

[0003] Hydrocarbon targets on land as well as offshore are usually examined using a CSEM configuration called "galvanic inline", i.e. galvanic (grounded contact) dipole source (deliberately denoted as x-direction, thus emitting an E_x field). The source sends an electromagnetic signal towards one or several galvanic, grounded dipole E_x receivers. This configuration is sensitive to thin resistors such as hydrocarbon accumulations in a conductive environment such as sedimentary rock. Galvanic inline configurations are the method of choice not only because they are somewhat sensitive to typical hydrocarbon targets, but also because they are relatively easy to operate. Horizontal source / receiver dipoles are easy to implement onshore as well as offshore.

[0004] Even more useful would be an approach using vertical (E_z) component electrical fields. Because the signal produced by such a field would be very sensitive to horizontal resistive structures, some attempts to create such fields have been made. While preferred from a signal point of view, however, generation of a true vertical field is not simple. Various approaches, including true vertical sources as well as horizontal focusing source arrays have been proposed by others. Focusing source arrays, while generating the desired signal, are very equipment- and cost-intensive to realize, while at the same time being not particularly effective. A true vertical dipole is difficult to achieve both offshore, as vertical dipoles in seawater suffer dramatically from misalignment and movement noise, and onshore, as a true vertical dipole would have to penetrate the subsurface. The focusing system is costly and difficult to handle and to operate.

[0005] Thus, it would be advantageous to provide a system that could generate vertical field data while avoiding the difficulties associated with existing systems.

SUMMARY OF THE INVENTION

[0006] To avoid the disadvantages of real vertical sources and real focusing arrays, the present invention replaces vertical dipoles and vertical focusing arrays by a synthetic processing step that allows the generation of selected vertical focusing arrays by recombining standard inline onshore as well as offshore source and receiver systems. This approach is complemented by a synthetic steering concept, completing the technique into a synthetic aperture equivalent for ultra-low frequency electromagnetic signals.

[0007] The present invention relates to data processing and can therefore be applied to onshore or offshore data that have been collected without using special acquisition techniques. More specifically, the present technique uses a synthetic recombination and focusing approach to utilize conventional inline acquisition concepts for the formation of a new, synthetic vertical field.

[0008] In accordance with preferred embodiments of the invention there is provided a method for surveying a subsurface using electromagnetic signals. The method preferably comprises the steps of: a) using at least one dipole to transmit into the subsurface a series of signals that includes pairs of opposing electromagnetic signals; b) using at least one receiver to receive from the subsurface a set of signals resulting from passage of the transmitted signals through the subsurface; c) identifying within the set of received signals at least one pair of received signals having opposite field directions; and d) processing said pair of opposing received signals so as to obtain information about the subsurface.

[0009] Step a) may comprises towing an alternating source above the subsurface, and/or transmitting signals along at least two non-parallel or orthogonal lines.

[0010] Step c) preferably includes identifying a plurality of pairs of received signals having opposite field directions, wherein the pairs have a common center point.

[0011] Step d) preferably includes using phase shifting and/or amplitude modulation to steer the synthetic signal that results from the processing step. A listening period is preferably included between transmitted signals. Step d) may include calculating a ratio between a received signal and a background signal that does not include a received signal. The signal transmission can be carried out using a pair of oppositely oriented dipoles and/or a single dipole transmitting signals of alternating polarity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more detailed understanding of the invention, reference is made to the accompanying wherein:

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0013] In preferred embodiments, the present invention includes the use of an array of electric dipoles to construct a focusing source system. Pairs of dipoles are oriented "against" each other so as to create a downward pointing field line pattern, as illustrated in Figure 1. As shown in Figure 1, several pairs of opposing dipoles 10a,b, 12a,b, and 14a,b, etc. located at or above a surface 18 can be used to interrogate a subsurface 20. If the resulting signals are summed, the result is a pair of signals 13a,b that corresponds to interrogation with a synthetic electromagnetic field 30 with a significant downward component 32. Thus, collecting and processing data in this way creates a synthetic focusing dipole array.

[0014] Preferred embodiments of the present techniques include two-dimensional dipole arrangements, rather than the linear configuration shown in Figure 1. For example, in Figure 2, the system includes additional dipoles 14c,d, which lie on a line orthogonal to the line defined by dipoles 14a,b. The EM field produced by these dipoles adds to the EM field from dipoles 14a,b, resulting a stronger subsurface signal. It is preferred but not necessary that the first two pairs lie on orthogonal lines. In preferred embodiments, additional pairs of dipoles, or the equivalent thereof, can be added. It is preferred that the dipole pairs lie in substantially the same plane, i.e. either on the surface or towed at substantially the same depth, and that the pairs be centered on a single point or vertical line. When the data are gathered using a towed source, the 2D configuration(s) can be obtained by towing the source along a grid. In some embodiments, when multiple source lines are used to enhance the focusing effect, the lines may define a virtual star or similar array configuration.

[0015] A towed offshore CSEM source is powered by an alternating, typically square- wave, current, creating alternating dipole moments at different positions in time. As a consequence since in some embodiments, particularly offshore, the source may be towed along a single or multiple profile and it is also possible to obtain alternating polarity sources in space.

[0016] Each point along a source towline or line of sources can become a synthetic focus center point, i.e. a point where two or more pairs of opposing synthetic dipoles can form a focusing source. There will be a number of points at the beginning and the end of each source line where there are not enough dipole pair will exist to provide sufficient fold, i.e. pieces of the tow line with "virtual single fold coverage." Those are preferably discarded.

[0017] The present invention results in: (a) a focusing of the source energy and (b) a emphasizing of the vertical electric field component, leading effectively to a vertical source, even though the input data is gathered using horizontal source dipoles. Using the present techniques, it is possible to detect significantly smaller targets and create source field components that are more suited to couple into a thin resistive target. Moreover, the vertical field component is very sensitive to 3D effects in the subsurface, e.g. boundaries of structures.

[0018] The techniques and concepts presented herein are usable in both onshore and marine settings, and are valid for all types of electromagnetic sources, including galvanic/dipole as well as inductive/coil sources. In addition, the present concepts are applicable in the frequency domain as well as in time domain acquisition, and they can be used with stationary or non- stationary receiver/source configurations, including towed systems. In particular, a conventional offshore system with a towed source and ocean bottom receivers is usable for this approach, without significant change. This means in turn, that the present processing techniques can be applied to vintage data and/or data that was originally intended for conventional processing.

[0019] The present data processing techniques are based on a linear combination of sub- problems. Instead of creating a real, i.e. physical array, it is possible to perform independent measurements with individual sources (dipoles) and then to sum the result in a way that simulates other source configurations. For example, data can be gathered using source dipole 14a and all receivers, followed by data gathered using source dipole 14b and all receivers etc.; followed finally by a linear combination of all data. The present post- acquisition array forming is possible as long as the underlying physical problem is linear and does not change with time, or at least changes with time only in a linear, reproducible way.

[0020] The input data for use in the present methods can be created by any suitable means. This includes a modulated source signal moving in time and space. With a moving signal source, an alternating input in time also creates alternating sources in space, i.e. a dipole changes its polarity (i.e. its direction) while being towed along a line profile. More generally this works in onshore as well as offshore settings, but while in the offshore case an electromagnetic source is typically simply towed in the water, allowing an almost infinite number of positions in space with little effort, onshore sources need to be repositioned in a slightly more complicated manner, such as replanting of grounded dipoles or moving of source loops etc. The airborne EM case is similar to the offshore case in so far as the source moves easily and continuously. Airborne galvanic sources are of limited use, however.

[0021] To carry out the invention, "pairs" of individual dipole measurements that have opposite polarity and are positioned at equal offsets from a desired center point are used to create synthetic opposing dipole pairs, resulting in a relatively large synthetic vertical electromagnetic source. The dipoles may be thought of as "dipole atoms," as they are literally the smallest source elements that can form larger virtual/synthetic array systems. In its most simple form, the focusable synthetic source can be created out of conventional horizontal field data by identifying within a set of data at least one pair of opposed signals. In preferred embodiments, a set of data can be mined so as give multiple such focused pairs, or sets of pairs, each pair of set of pairs providing a different synthetic focus center point and thereby providing information about a different portion of the subsurface. The concept is identically applicable for magnetic (loop/coil) and electric (dipoles or similar galvanically contacting elements) sources.

[0022] Because of the large signal that is present at all times when a continuous source is used, it may be preferable to use a partial duty cycle for the source signal, e.g. to use a ternary output with a source 0 (switched off, +V, 0,-V) instead of a binary dual voltage (+V,-V) output. By way of example only, a 50% duty cycle having alternating signaling periods 42 and a listening period 44 between signals is shown in Figure 3. It is preferred to include a listening period of at least 0.5 seconds, so that the signal will be split not only at the source, but after a finite time propagation through the subsurface, so the effective splitting can also reach far offset receivers. The effective timing (e.g. the duty percent, period length, etc.) will depend on the individual problem and subsurface, and can be adapted either via a forward modeling exercise or experimentally in the field.

[0023] Such a time-domain acquisition technique is particularly useful in short offset configurations. Using a time domain approach, it is possible to detect the time-delayed secondary field response from the target while the primary source field has decayed. The time domain approach is particularly useful for onshore settings as well as any other short offset (short source-receiver distance) configurations. Alternatively or in addition, the resulting data can be processed so as to give an output that is the ratio between total and background field.

[0024] Even data from a low frequency or slow-moving tow source can be used, as overlapping dipoles still can be summed, so as long as they point in the desired direction. Thus, all (+ -) and all (- +) dipoles can be summed, respectively, independent of an overlap in space, so long as there is a break in time (ternary source signal, source switch off/listening period) separating the active individual dipoles for a sufficiently long period in time to allow die out. The time period will depend on subsurface and geometry considerations.

[0025] Once an array of synthetic dipoles has been established, phase and/or amplitude modulation can be used to enhance the signal and create a synthetic steerable source, as illustrated schematically in Figures 4-6. Figure 4 illustrates the application of a graduated phase shift to a set of dipoles; Figure 5 illustrates the application of a graduated amplitude shift to a set of dipoles; and Figure 6 illustrates the application of both phase and amplitude shifts to a set of dipoles. Phase/time shifts and/or amplitude scaling enhance the focusing effect and allow a steering of the signal. This creates a synthetic aperture system with focusing and steering attributes. Additionally, the signal characteristic (predominantly vertical/horizontal) can be synthetically changed. These steering effects can be used with standard source elements that are acquired in a conventional operation, as the reconstruction into synthetic or virtual source systems is a post-processing step.

[0026] The concepts described above with respect to source dipoles can be applied to and used for receiver dipoles (loops; in general "source atoms"). Receiver lines or arrays can form synthetic aperture systems, which can be steered and focused in substantially the same manner as sources.

[0027] Furthermore, a purely passive system could act as a dual synthetic array: the system would use the passive electromagnetic background as a source, and the receivers would act as a steerable synthetic aperture virtual source as well as a real receiver array. Both galvanic as well as inductive systems can be created onshore (on land) as well as offshore (in a marine or transitional setting). An implementation using a single or dual boat CSEM streamer ("towed array") can be particularly easily realized. [0028] By way of example only, the synthetic source / array systems described herein can be used to survey for:

• Smaller targets than are currently accessible with established CSEM techniques. The focusing approach allows a concentration of CSEM energy toward the target, while at the same time suppressing energy outside, resulting in a significantly improved contrast between the resistor/no resistor cases.

• Deeper targets than are currently accessible with established CSEM techniques. The focusing/steering approach allows a shift of the "boundary" of detectability towards 4-5 km below seafloor, which is greater than is typically currently possible.

« 3D subsurface structures, in particular reservoir boundaries.

[0029] The synthetic source focusing/steering approach has a significant operational advantage: By acquiring a mesh or network of standard source positions (for analogy they may be thought of as "source atoms," i.e. small standard source dipole units in the x and y directions, acquired on a grid similar to atoms sitting on a grid in a solid), it is possible to do a recombination of those standard source atoms into virtually any imaginable simple or complex source. In some embodiments, the present technique entails a synthetic aperture forming with special aperture focusing and aperture steering. The focusing approach combined with a steering of the synthetic array allows large gains in signal strength.

[0030] Two possible implementation of the concepts as discussed above are illustrated in Figures 7 and 8. In Figure 7, the optional process is applied in the field, whereas in Figure 8 the optional process is applied during processing to both the acquired and the modeled data and as such is 'transparent' and would fit into any workflow. As illustrated, the present process is linear and, as such, can be moved around in the workflow as desired. However, the generic acquisition approach followed by the optional processes discussed earlier is much more flexible. At the same time, not all processes can be carried out in the field, or cannot be carried out efficiently.

Example

[0031] We performed a modeling study of a synthetic vertical electromagnetic source (SVES), using opposing dipole data gathered from individual single dipole experiments. In the simulation, a resistive target was located in the subsurface at 1 km depth, with a lateral extension -2 km to 2 km. We modeled a SVES comprising pairs of 2.5 km long opposing dipoles (- 10 km to -7.5 km etc. and -7.5 km to -5 km etc.), and examined both E_x and E_z fields. The resistive target was clearly visible in the target response in the E_z field. Plots of the electric field recorded by surface receivers showed a much clearer image of the target when the output was given as a ratio between total and background field. Regardless, the models indicate that this acquisition configuration is indeed sensitive to the presence of a resistor in the subsurface.

[0032] Modeling of a source dipole configuration in which the source dipoles summed to a length of 7 km gave similar results, as did and a configuration in which the dipoles were shifted inwards to a distance of 3.5 km from the center.

[0033] Based on the modeling, we conclude that a typical target and acquisition configuration may lead to a signal difference (water wet vs. oil wet reservoir) of about 20-50% (i.e. a factor of 1.2 to 1.5 between the measured EM signal strengths for a hydrocarbon bearing and waterbearing reservoir). With the proposed synthetic "rearrangement" we end up of a signal difference of about 6000% (i.e. a factor of 60). While it is true that real noise-containing data may slightly diminish this gain, but it still gives several orders of magnitude improvement.

[0034] Conventional viewing of the same data leads to a typically 10-40% difference in observed signal between systems with and without a reservoir target (i.e. the subsurface is modeled as a water-wet, hydrocarbon-free system and as a hydrocarbon-present system), giving a signal ratio of 1.1 to 1.4 between the two cases. Using the present techniques, a signal ratio of 1.2 to 1.6 between the water and the hydrocarbon case was obtained. An interpretation using an inversion scheme would lead to a significantly easier detection of such a typical target, or it would allow the detection of a target which is currently beyond our technical means (such as a much smaller reservoir, or a reservoir at much greater depth).

[0035] Unlike systems that use actual focused arrays, the present system includes the use of relatively conventional CSEM data in a "reordered dataset" to create a synthetic version having similar properties, which allows a not only a technically much simpler data acquisition using existing hardware and logistics, but which allows beyond that a reinterpretation of existing, already acquired data sets using a synthetic vertical source pattern.

Claims

C L A I M S

1. A method for surveying a subsurface using electromagnetic signals, comprising the steps of:

a) using at least one dipole to transmit into the subsurface a series of signals that includes pairs of opposing electromagnetic signals;

b) using at least one receiver to receive from the subsurface a set of signals resulting from passage of the transmitted signals through the subsurface;

c) identifying within the set of received signals at least one pair of received signals having opposite field directions; and

d) processing said pair of opposing received signals so as to obtain information about the subsurface.

2. The method according to claim 1 wherein step a) comprises towing an alternating source above the subsurface.

3. The method according to claim 2 wherein step a) further comprises towing the alternating source along a set of orthogonal grid lines.

4. The method according to claim 1 wherein step a) includes transmitting signals along at least two non-parallel lines.

5. The method according to claim 2 wherein step c) includes identifying a plurality of pairs of received signals having opposite field directions, wherein said pairs have a common center point and step d) includes processing each of said pairs.

6. The method according to claim 1 wherein step d) includes using at least one of phase shifting and amplitude modulation to steer said information.

7. The method according to claim 1 wherein step a) includes having a listening period between transmitted signals.

8. The method according to claim 1 wherein step d) includes calculating a ratio between a received signal and a background signal that does not include a received signal.

9. The method according to claim 1 wherein step a) includes using a pair of opposi oriented dipoles.

10. The method according to claim 1 wherein step a) includes using a single dipole transmitting signals of alternating polarity.