RU2642492C1 - Method for marine electrical exploration - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: on the drifting ice floe there is an MTZ data recording station. Magneto telluric field measurements are made at each particular frequency, referenced to the corresponding point on the profile, the coordinates of which change due to the ice floe drift. The indicated coordinates are fixed for each measurement, the MTZ, AMTZ data and responses to sharp changes in the magneto telluric field are recorded, the obtained data set is used to restore the distribution of the geological environment electrical conductivity and make a prediction about the presence or absence of the desired objects associated with minerals in the study area.

EFFECT: creation of a method for marine geophysical exploration by magneto telluric field measurement during operations in the polar regions on drifting ice surface, obtaining reliable geophysical information on the structure beneath the seabed, ensuring depth studies to 10,000 m.

3 cl, 8 dwg

Description

The invention relates to marine electrical exploration and can be used to study the structure of the sedimentary cover and the structure of the upper part of the earth's crust with the aim of forecasting mineral deposits of the Arctic seas covered with ice.

For marine electrical exploration of hydrocarbon deposits, various methods are widely used associated with exposure to the seabed of pulses of an electromagnetic field, followed by recording changes in the electromagnetic parameters of bottom rocks and analyzing the data to detect existing anomalies and determine their nature. Exploration is carried out using various research systems of equipment and equipment. Known, in particular, is a method and device for marine electrical exploration of oil and gas fields, designed to predict hydrocarbon deposits in the transit zone of the shelf at depths of up to 10 m (RF patent No. 2375728, G01V 3/06). The method is implemented using a set of bottom stations and a vessel with a generator and an excitation field forming unit associated with a horizontal towed dipole immersed in water with supply electrodes. The electrical prospecting complex also includes a non-emitting ballast device, equipment for reading and writing information from bottom stations, recording the place and time of generating current pulses and initializing bottom stations. Bottom stations are equipped with “braids” located along or across the excitation line, each of which contains at least three measuring electrodes located at a distance of 50-500 m from each other.

A known method and system for geological studies of the bottom of the sea with the measurement of the conductivity of the bottom of the sea during magnetotelluric studies or electromagnetic studies using a controlled source (RF patent No. 23233456, G01V 3/08). This technical solution relates to the creation of a bottom magnetotelluric system and a method by which vertical magnetotelluric impedances are measured, as well as vertical electric fields generated by the action of an electromagnetic field emitter. Exploration blocks are placed at various places on the seabed within the area of interest for modeling the structure of the seabed. After a predetermined period of time, which can be from several hours to several days, the blocks are raised to the surface. The period of time during which data is collected depends on the speed of data collection and the amount of memory in the data processing unit. The accumulated data is transmitted to the processing system for analysis and data output.

However, geophysical studies using bottom stations, obviously, cannot be applied in areas covered by ice. It should be noted that the search for deposits in the polar regions of the Earth is impeded by a huge number of practical difficulties associated with low temperatures. The presence of ice exacerbates the situation. In ice-covered marine areas, with large waves or other obstacles, reconnaissance may become more difficult, expensive or even impossible. For example, in ice-covered waters, an exploration vessel must pave the way in ice and maneuver in waters filled with debris from an ice field. Even the presence of an icebreaker in front does not allow for effective research, since crushed ice can damage the hull of a geophysical vessel or lead to a break in an exciting or receiving installation. In addition, the debris of the ice field on the surface of the water makes towing the source and towed streamers more difficult and predispose to their damage. For example, any system components on the surface of the water can run into ice, drag under ice and get lost. In addition, ice can potentially build up on the surface of any cables or tow ropes discharged from the vessel, even from slips, potentially damaging the cables or tow ropes. Moreover, ice pulled under the hull and floating up behind the vessel can move these cables and cables.

Obviously, in such areas it is advisable to use methods in which the sources and receivers of the field are located on the surface of the ice.

Known, in particular, is a method of geophysical exploration of hydrocarbon deposits, according to which holes are created on the ice surface passing through sea ice to water (RF patent No. 2410728, G01V 3/12). The source and receiver electrodes are lowered into the indicated openings until the electrode is in contact with water under sea ice. A sufficient amount of conductive liquid is added to the hole to cover part of the electrode. The conductive liquid is left in the hole until it freezes. This technology fixes the electrode well, provides good contact of the electrodes with recording devices. The method provides the implementation of marine geophysical exploration, in which you can use the usual ground-based technology of work with a controlled source: the current circuits are deployed on ice, and signals propagating along the paths below the ice surface are received by other current circuits located at some distance from the sources. In this case, marine reconnaissance is carried out without the need for a tugboat, the placement of receivers at the bottom and their subsequent extraction.

There is also known a method of marine electrical exploration on a drifting ice using a research complex consisting of exciting and receiving installations, a source of alternating periodic current pulses and a data processing installation (RF patent No. 2069375, G01V 3/06, prototype). The method consists in the fact that the exciting installation and receiving installations are placed vertically under the ice in a layer of water. The supply of pulses is carried out by a source of alternating periodic pulses of current with a power of several tens of amperes using an EDS 72 or other installation, data processing using a digital electrical reconnaissance station of the CES type. Section profiling is carried out with a fixed distance between the points of excitation and reception of signals after exposure to an alternating pulse.

The disadvantages of the known technical solutions include the problems associated with the limited energy resource necessary when working with a controlled source of generator sets, which should be autonomously placed on an ice floe. In addition, the high conductivity of sea water, which is a powerful screen, significantly complicates the study of the seabed. All this limits the depth of geophysical research. In addition, the described methods measure only the components of the electric field, which also limits the possibilities of research.

The way out of this situation is the transition from measurements with a controlled source to measurements of magnetotelluric fields, which have significant advantages, such as:

- the compactness of the receiving unit allows you to perform work in difficult conditions;

- lack of need for an artificial source of electromagnetic field due to the use of magnetotelluric variations;

- relatively small weight of the equipment - about 15 kg for one sensing point;

- the absence of restrictions on the depth of research for any variations in the conductivity of the sedimentary cover.

The works related to the measurement of MTZ data on the ice surface are, in principle, known. Similar measurements were carried out, for example, in the study of the Baikal reef zone (Yu.F. Moroz, T.A. Moroz. Deep geoelectric section of the Baikal rift // Vestnik KRAUNTS. Earth Sciences. 2012, No. 2, issue No. 20). However, these data relate to measurements on still ice, when its thickness reached 1-1.5 m. In addition, the fresh water of Lake Baikal (specific electrical resistance of water 200-300 Ohm⋅m) did not pose a serious obstacle to studying the deep electrical conductivity of the lake basin. Baikal.

The objective of the invention is to expand the arsenal of technologies for marine electrical exploration with the elimination of the above disadvantages of analogues, namely increasing the depth of research and ensuring the reliability of information obtained about the geoelectric section in areas covered by ice.

The technical result of the invention is the creation of a method of marine geophysical exploration by measuring the magnetotelluric field when working in polar regions on the surface of drifting ice, which allows to obtain reliable information about the geological structure under the seabed, providing a depth of research of up to 10,000 m

At the same time, as a source of geophysical information on rock properties, data can be used from both standard MTZ measurements focused on the allocation of different-period oscillations (including AMT), as well as data due to variations of the type of magnetic storms (variations with a sharp change in the field), as well variations of substorm type and sharp bursts of the MT field caused by distant thunderstorms.

The specified technical result is achieved due to the fact that in the method of marine electrical exploration, including the registration of electromagnetic data on the surface of a drifting ice floe, the direction of drift of which corresponds to the design direction of a given observation profile passing over the study area, obtaining information about the distribution of resistivity of rocks lying under the sea bottom, building a model of a geoelectric section and making a forecast about the presence of mineral deposits (including hydrocarbon deposits), according to the invention, geophysical surveys are carried out by magnetotelluric sounding (MTZ), for which a recorded MTZ data station is located on the indicated drift ice, while measurements of the magnetotelluric field at each specific frequency are carried out, tied to the corresponding point on the profile, the coordinates of which vary ice drift counting, the indicated coordinates are recorded during each measurement, the MTZ, AMTZ data and responses to sharp changes in magnetotellas are recorded uric field, according to the obtained data set, restore the distribution of electrical conductivity of the geological environment and make a prediction about the presence or absence of the desired objects associated with minerals in the research area.

When restoring the distribution of electrical conductivity of the geological environment under the seabed along a non-linear, due to the free movement of the ice in space, measurement profile, a three-dimensional problem is used by selecting a model of the geological environment that satisfies the entire set of obtained MTZ data.

In addition, in addition to measuring the MTZ data, magnetovariational sounding data (MVS) are also measured.

The invention is illustrated by drawings.

In FIG. 1 shows the drift map of the North Pole - 37 drifting station, SP - 37.

In FIG. Figure 2 shows a model geophysical section (typical of the Kara Sea shelf) along the profile, which is a multilayer medium in which a target object is located at a depth of 2000 m, the length of which is 10 km, and the thickness is 35 m, which has a high resistivity (1 Ohm).

In FIG. Figure 3 shows graphs of the anomalous values of the apparent resistivity ρ _XY calculated by x, y - components of the measured field, showing that the target object is clearly distinguished in the context.

In FIG. 4 shows the actual location of the measurement points at a specific frequency relative to the midpoints of the profile sections.

In FIG. Figure 5 shows the geoelectric section obtained as a result of the interpretation of model data.

In FIG. Figure 6 shows an example of a sharp change in the electric field during a substorm.

In FIG. 7 shows the results of modeling magnetotelluric data for a sharp change in the field.

In FIG. Figure 8 shows the results of simulation of magnetotelluric data for direct and curved measurement profiles.

The essence of the invention lies in the fact that the measurements are carried out on a drifting ice floe, which leads to the appearance of a new modification of MTZ with distributed data, in which the measurement at a specific frequency is tied to its point on the measurement profile.

In the standard MTZ method, the impedance tensor [Z], which is determined from the relations between the horizontal components of the electric and magnetic fields, is used as the main characteristics

E _τ = [Z] H _τ , where

E _τ = E _τ ( Ex, Eu ),

H _τ = H _τ ( Hx, Well )

. В данном случае, за счет дрейфа льдины, измерения на каждой частоте происходят в каждой отдельной точке профиля, то есть составляющие поля могут быть представлены в виде E

где (ξ,η) - координаты точки профиля, на которой проводится измерение на частоте ω.At the same time, at each sensor placement point, the dependence of the field components on the frequency is measured,

. In this case, due to the ice drift, measurements at each frequency occur at each individual point in the profile, i.e., the component fields can be represented as E

where ( ξ , η ) are the coordinates of the profile point at which the measurement is carried out at the frequency ω.

This leads to a new spatio-temporal (spatio-frequency) modification of the MTZ problem, the solution of which requires the use of multivariate modeling and inversion and adjustment of the methodology for analyzing the obtained data. In the general case, the inverse problem of electromagnetic sounding using a magnetotelluric field is to determine the electrical conductivity of the Earth from the dependence of the impedance tensor on the position of the observation point and the frequency of variations of the MT field. Given the incorrectness of such a problem, its solution makes sense only in the case when, based on a priori information about the structure of the medium under study, the search area is limited and an approximate solution of the inverse problem is sought inside a compact set of plausible models. This allows you to quite effectively apply the proposed method.

(типпер), определяемый из связи вертикальной и тангенциальных компонент магнитного поля:As an addition to the MTZ method, the method of magnetovariational sounding (MVZ) can be used, which is not so much influenced by near-surface inhomogeneities and allows one to obtain sufficiently reliable information about immersed geoelectric structures. In this method, the main characteristic is the Wiese-Parkinson vector

(tipper), determined from the relationship of the vertical and tangential components of the magnetic field:

Hz = [ W ] H _τ

Here, as in the case of MTZ, there are features associated with the fact that measurements at each frequency occur at a separate point in the profile. In this case, the inverse problem is to determine the Earth's electrical conductivity by the dependence of the impedance tensor and tipper components on the position of the observation point and the frequency of field variations.

At the points where there is a sharp change in the field, a transient process is recorded, the presence of which allows us to solve a separate inverse probing problem, similar to the well-known inverse probing problem for MPP. The possibility of using such measurements in magnetotellurics is shown in the paper “On the Magneto-Telluric Transient Method (MTMPP) with a sharp change in the field,” M. M. Lavrentiev, M.G. Noppe, K.G. Reznitskaya / Geophysical methods of prospecting and exploration of ore mineral deposits in Siberia. - Novosibirsk, 1975 .-- S. 31-39.

When implementing the method, the MTZ receiving sensors and the MTZ recording station associated with the receiving sensors are placed on an ice floe, the drift direction of which approximately coincides (corresponds) with the direction of a given observation profile passing over the study area, about which it is necessary to obtain geological information. The choice of ice is carried out after consultation with the relevant services. At the same time, due to the free movement (drift) of the ice floe in space, a corresponding, different from linear, MTZ observation profile is formed.

A general idea of a possible real measurement profile can be obtained from FIG. 1, which shows the drift map of the North Pole drifting station - 37, SP - 37.

The total drift of the station over the entire period of operation amounted to 2076 kilometers. The average drift speed is 7.7 kilometers per day.

One of the problems encountered when measuring on the surface of the ice is the measurement of the electrical components of the field. It is advisable to carry out such measurements using special grounding devices for marine and river electrical exploration, for example, according to RF patent No. 113026. As a MTZ station, for example, a five-channel MTU-5A station manufactured by Phoenix (Canada) can be used. At each measurement, the MT fields fix the coordinates of the point at which the measurement was performed. The resulting set of electromagnetic data is analyzed with the release of different-period oscillations and sharp changes in the magnetotelluric field.

The set of profile data obtained during the implementation of the invention allows us to find the distribution of electrical resistivity of the rocks lying under the seabed along the passable profile, which is achieved by solving the inverse three-dimensional MTZ problem by selecting a model of the geological environment that satisfies the entire set of MTZ data on this profile.

As an example, illustrating the implementation of the method, in FIG. Figure 2 shows a model section (typical of the Kara Sea shelf) along the profile, which is a multilayer medium in which a target object is located at a depth of 2000 m, the length of which is 10 km, the thickness is 35 m, and has a high resistivity (1 Ohm).

140 points of AMTZ are placed on the profile with equal steps. Soundings of AMTZ are carried out for 25 minutes (70 meters of ice drift, which in a first approximation will be considered concentrated at a central point). In addition, three MTZ soundings are performed per day. That is, they conduct one sounding over 1333 m of movement along the profile. On each such section of the profile, measurements are carried out at twenty different frequencies characteristic of ordinary MTZs distributed randomly.

The simulation results are shown in FIG. 3, which shows graphs of abnormal values of the apparent resistivity pkh calculated by x, y - components of the measured field, showing that the target object is clearly distinguished in section. The selected points on the graphs correspond to the midpoints of the profile sections, during which the MTZ sounding curve was measured, corresponding to a given set of frequencies. The actual location of the measurement points at a frequency of 0.0093 Hz in comparison with the position of the midpoints of the sections of the profile is shown in FIG. 4. Anomalies clearly record the position of the target on the profile.

The results of the interpretation of model data in the form of a section of resistivity are shown in FIG. 5, where an abnormal zone corresponding to the position of the target object is fixed.

At individual points of the profile, a sharp change in the magnetotelluric field can occur, as shown in FIG. 6, which makes it possible to fix and analyze the emerging transient process.

In FIG. Figure 7 shows the results of modeling magnetotelluric data for a sharp change in the field. Curves of apparent longitudinal conductivity S (t) are given for the case of a point with a sharp change in the field above the target object, point A, and at a distance of 10 km., Point B, and also a graph of abnormal values of S (t) at the central point A. From the graph (Fig. 6) shows that the presence of the target object significantly affects the behavior of the MT field. In addition, the broadband temporal sounding curves obtained by such measurements make it possible to refine the parameters of the reference section, which is very important for isolating the target object.

In FIG. Figure 8 shows the results of modeling magnetotelluric data for straight A and non-linear (curved) B profiles (Fig. 8a) that intersect the target object in plan (in the XY plane). In this case, the target object was first set to be infinitely extended along the Y axis, and then bounded with the removal of its transverse boundary from the profile for 20 km. Comparison of the results of modeling magnetotelluric data was carried out along the edge of the target object (in the direction of the X axis) and in its center (points 1 and 2, Fig. 8a). The simulation results for a limited object (as a percentage of the data obtained for an infinitely extended object) are shown for points 1 and 2 in FIG. 8b and FIG. 8c, respectively. It can be seen that the curved profile in this case is more sensitive to the geometry of the target object, therefore, this will allow more accurate restoration of the parameters of the target object during 3D inversion. The influence of the transverse boundary of the object appears closer to the edge of the object (point 1), while in the center of the object (point 2), differences from the results with an extended object are at the level of the computational error of about 1.5%.

In General, the technical solution, characterized by a set of essential features included in the claims, ensures the achievement of the claimed technical result, namely, obtaining reliable information about the geological structure under the seabed by measuring the magnetotelluric field on the surface of drifting ice with a depth of research of up to 10,000 m.

Claims (3)

1. A method of marine electrical exploration, including the registration of electromagnetic data on the surface of a drifting ice floe, the drift direction of which is consistent with the design direction of a given observation profile passing over the study area, obtaining information about the distribution of resistivity of rocks lying under the seabed, constructing a geoelectric section model and compiling prediction of the presence of mineral deposits (including hydrocarbon deposits), characterized in that the geophysical studies of This is done by means of magnetotelluric sounding, for which a MTZ data recording station is located on the indicated drift ice, while the magnetotelluric field is measured at each specific frequency, tied to the corresponding point on the profile, the coordinates of which change due to ice drift, these coordinates are fixed at each measurement, carry out registration of MTZ, AMTZ data and responses to sharp changes in the magnetotelluric field, from the resulting data set restore distribution of the electrical conductivity of the geological environment under the seabed and make a prediction about the presence or absence of objects associated with minerals in the research area. 2. The method according to p. 1, characterized in that when restoring the distribution of electrical conductivity of the geological environment under the seabed along a non-linear, due to the free movement in the space of the drifting ice, the measurement profile, a three-dimensional problem is solved by selecting a model of the geological environment that satisfies the totality of the obtained MTZ data. 3. The method according to p. 1, characterized in that in addition to the measurements of the MTZ data, magnetovariational sounding data (MVS) are measured.

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