MX2008005594A - A method for hydrocarbon reservoir mapping and apparatus for use when performing the method - Google Patents

Abstract

A method is proposed for a marine electromagnetic survey based on the TM mode, for the purpose of prospecting for and detecting subsurface hydrocarbon reservoirs . The method includes an electromagnetic field source (1113) that, in a submerged, essentially vertical transmitter antenna, generates and injects electric current pulses (81,82) with a sharply defined termination. An electromagnetic field generated by these pulses (81,82) is measured by at least one receiver (1109) provided with an essentially vertical receiver antenna (1111) submerged in water, during the interval when the current in the transmitter antenna (1108) of the electromagnetic field source (1113) is switched off . The distance between the electromagnetic field source (1113) and the at least one receiver (1109) is smaller than the depth of the target object. An apparatus is also described, for implementation of the method.

Description

A METHOD FOR THE CARTOGRAPHY OF HYDROCARBONS AND AN APPARATUS FOR USE WHEN IT IS TAKEN A CABO THE METHOD FIELD OF THE INVENTION The invention relates to a method and apparatus for mapping submarine hydrocarbon reservoirs, more particularly by means of the use of a magnetic transverse mode (TM mode) of a source of electromagnetic fields to record a TM response which is measured by one or more receivers submerged in the water, by the immersed transmitter oriented essentially vertically, generating intermittent pulses of electric current with markedly defined terminations and where an electromagnetic field generated by these impulses is measured by the submerged receiver and oriented in a essentially vertical, in the interval when the current in the source of electromagnetic fields is deactivated. The distance between the antenna of the electromagnetic field source and the receiving antenna is smaller than the depth of the target object. Seismology is a commonly used technique when mapping potential areas for the search for oil. The seismic data provide information on the existence, location and shape of a hydrocarbon structure located in sediments in the soil. However, a seismic survey provides information on the structure through the recording of the velocity of elastic waves sensitive to the mechanical properties of the underground rocks, but the seismic data does not reveal much about the nature of the pore fluids present in the structure. Regarding the references, please refer to the complete bibliography that goes after the description of the invention. The marine search wells are drilled to determine if there are hydrocarbons present in the form of oil or gas, but the costs associated with this are very high and there are no guarantees of finding hydrocarbons in the drilled structures. In this situation, essential additional information about the contents of the deposit can be obtained by means of electromagnetic (EM) methods. The typical and simplest geoelectric model of an offshore sedimentary structure containing a hydrocarbon reservoir can be represented as half conductive spaces that have a typical resistivity of 1 - 2 O? T? , where a resistive, thin, encapsulated layer containing oil or gas with a thickness of 10 - 100 m has a resistivity of 20 - 100 Op? The typical depth of the resistive layer is approximately 500-5000 m. The sediments are covered by more conductive seawater that has a resistivity of 0.25-0.3 Qm, as well as non-conductive air. The greater resistivity of hydrocarbon-containing deposits is used in all electromagnetic methods for the search for hydrocarbons as the main indicator of the presence of oil and gas. Magnetotelluric (MT) sounding is a well-known method that is used extensively in land EM applications. Sometimes, the MT method is used for marine applications. The MT method uses the natural geomagnetic variations excited through the interaction between the solar wind and the main geomagnetic field. The low sensitivity of the MT method with respect to the resistive hydrocarbon layers is explained by the properties of the MT field. A magnetotelluric field is a flat wave that falls from the atmosphere and propagates vertically through the earth as TE (TE = electrical transverse) fields. It is well known that the TE field is insensitive to a resistive, horizontal, thin layer that is encapsulated in a more conductive structure. This is illustrated later. In this way, the MT method is of limited use in the marine MS search for hydrocarbons.

Unlike the MT method, the methods based on CSEM (Electromagnetic Controlled Source Method) use both TE fields (occasionally called inductive mode) and TM (magnetic transverse) fields (occasionally called galvanic mode). The CSEM methods are the most frequently used in marine EM search, since they are more sensitive to a resistive, encapsulated, thin layer. Different forms (configurations) of the CSEM methods are used, depending on the types of transmitter and receiver. Next, the term transmitter and receiver specifies the source and detector of electromagnetic fields. Some of the existing configurations are illustrated below. The most common CSEM systems in use consist of a horizontal cable that receives an intense electrical current (transmitter), the cable is arranged on or near the seabed and the horizontal electric receivers are installed on or near the seabed at different distances from the transmitter . These systems can either be permanently installed on the seabed during a measurement period or can be towed behind a vessel. In some configurations, they are accompanied by measurements of magnetic components of the EM field. These systems consist of a transmitter that configures a powerful alternating current in a submarine cable and a set of receivers that perform measurements of electromagnetic fields in the frequency or temporal domain. The most important feature of these systems is the requirement of a large lag between the transmitter and the receivers, 5-10 times the depth of a target, ie 5-10 km. Only under these conditions can the shielding effect of seawater be suppressed and a suitable signal be measured. Furthermore, as will be illustrated below, in practice none of the existing configurations employing the above CSEM configurations can provide the resolution required to reveal and examine the target areas containing encapsulated hydrocarbons at depths greater than 3000 m, nor the resolution required in these cases where the thickness and resistivity of the hydrocarbon layer is insufficient. This limitation is the main disadvantage of all existing inventions based on a CSEM configuration.

BRIEF DESCRIPTION OF THE INVENTION The object of the invention is to remedy or reduce at least one of the disadvantages of the prior art. The objective is achieved through characteristics established in the following description and in the following claims. The invention describes a novel system consisting of a method and an apparatus for electromagnetic search for the purpose of locating a reservoir, examining its geometry and determining whether there are hydrocarbons or water in the reservoir. The method can also be used if the area and its geometry are known from seismic data or other data. The purpose of the proposed invention is to register deposits, also at depths exceeding 3000 m, to increase the resolution of the results produced by an electromagnetic method to search for targets containing hydrocarbons and to increase the efficiency of the sounding. To achieve success, it is suggested that electromagnetic fields are used only in the galvanic mode (TM mode), which has the maximum sensitivity with respect to the resistive objectives that are encapsulated in a more conductive layer. The following examples illustrate the advantage of the proposed invention. According to a first aspect of the present invention, a novel method for revealing a reservoir and its nature is provided. This method consists in exciting and measuring electromagnetic fields only in the TM mode induced in submarine strata, the processing and analysis of data with the purpose of determining the electrical properties of the section and the resistance of the layer that contains the deposit and therefore his, her nature . According to a second aspect, the invention describes an apparatus arranged to reveal a reservoir and its nature, which consists of generating and measuring electromagnetic fields only in the TM mode in the subsea strata and the subsequent processing of data for the purpose of determining the electrical properties of the section and the resistance of the layer that contains the deposit and therefore its nature. A third aspect of the invention proposes the use of a source of electromagnetic fields oriented essentially vertically, elongated, also called a transmitter, to excite electromagnetic fields only in TM mode, at least one pair of transmitting electrodes arranged one on another they are provided with an intense current from a power supply, by means of isolated cables, the transmitting electrodes allow a current to pass into the surrounding seawater. This transmitter excites electromagnetic fields only in TM mode, in horizontally uniformly stratified structures. According to a fourth aspect of the invention, the transmitter generates pulses of electromagnetic fields with markedly defined terminations and with intervals where the energy is deactivated, the transmitter pulse exhibits the shortest possible rise time from a base value to a maximum value required, a maximum stability close to the maximum value and then the shortest possible descent time back to the base value. In this way, a reference for a signal intercepted by the receiver is provided, the transmitter pulses form the basis for the processing and interpretation of signals returning from the polled structure. The receiver carries out measurements of electromagnetic field responses in the absence of the primary field. According to a fifth aspect of the invention, use is made of one or more elongated, substantially vertically oriented, submerged receivers comprising means arranged to record a field potential difference across the length of the receiver, to measure a secondary field in TM mode. Advantageously, the receiver is provided with at least one pair of receiver electrodes disposed one above the other. According to a sixth aspect of the invention, a distance R (phase shift) between the transmitter and the receiver is small enough to produce an induction zone condition. An induction zone is characterized because the condition 0 = R = (tpa (t) / μ0) 1 2 is valid. At this point, t is the period of delay from the moment the energy is deactivated at the transmitter, μ0 = 4p10"7 H / m is the magnetic permeability of the vacuum, pa is the average (apparent) resistivity of a substrate the which at time t exhibits the same response as the probed cross-section, R is the horizontal distance (offset) According to a seventh aspect of the invention, several receivers can be used for the measurements, optionally synchronous measurements, in order to increase the sounding efficiency According to an eighth aspect of the invention, the transmitter generates a special sequence of square pulses to suppress external noise, the pulse sequence is inconsistent with noise.The measured responses are then accumulated and calculated The average value According to a ninth aspect of the invention, one or more autonomous, marine, fixed background stations monitor the variations of the megnetoteluric field to fi n to reduce MT noise in CSEM measurements. According to a tenth aspect of the invention, pressure sensors are used in combination with electrodes to reduce wave noise and swelling in CSEM measurements.

According to a eleventh aspect of the invention, the response functions are subjected to a series of transformations and inversions with the subsequent construction of ID, 2D, 2½D and 3D images, T (x, y) and s (x, y, z) of the substrate. According to a twelfth aspect of the invention, all of the other available geological and geophysical information is used during the planning stage and the data transformation and data inversion stage of the analysis and interpretation, in order to increase the resolution and clarity of the structure of a section. According to a thirteenth aspect of the invention, the totality of the probing steps, that is, survey planning, data analysis, analysis and influence of the coastline, reliefs of the land on the seabed, the heterogeneity of the sediments and oil deposits, etc., will largely include the use of ID, 2D, 2¾D and 3D modeling.

DESCRIPTION OF THE FIGURES The main ideas of the present invention, its advantages and the disadvantages of the prior art used in the marine electromagnetic hydrocarbon search, will become apparent from the following description of the invention, which refers to the drawings Annexes, in which: Figure 1 represents the MT curves for the apparent resistivity in the sea surface, for a typical model of the strata with and without a thin, resistive objective layer; Figure 2 represents the MT phase curves on the sea surface, for a typical model of the strata with and without a thin, resistive objective layer; Figure 3 represents the MT curves for the apparent resistivity on the seabed, a typical model of the strata with and without a thin, resistive objective layer; Figure 4 represents the phase-MT curves on the seabed, for a typical model of the strata with and without a thin, objective, resistive layer; Figure 5 represents the typical CSEM designs used for marine MS search; Figure 6 represents the resolution of voltage curves for the PxEx (f) - and PxEx (t) configurations in frequency (f = 0, l Hz) and temporal domains; Figure 7 represents the resolution of curves for the apparent resistivity for the PxEx (f) - and PxEx (t) configurations in the frequency (f = 0, l Hz) and temporal domains; Figure 8 represents a diagram of current waveforms present in different places in the system according to the invention; Figure 9 represents the resolution of curves for the apparent resistivity for a system according to the present invention for electromagnetic soundings at sea; Figure 10 represents the resolution of the voltage curves for a system according to the present invention for electromagnetic soundings at sea; Figure 11 represents a schematic side view of a transmitter and receiver array in a system according to the present invention for electromagnetic soundings at sea; Figure 12 represents a schematic block diagram of a power supply unit; Figure 13 depicts a schematic block diagram of a receiver unit; and Figure 14 represents a schematic plan view of an arrangement of transmitter and receivers in a system according to the present invention for electromagnetic soundings at sea.

DETAILED DESCRIPTION OF THE INVENTION The well-known magnetotelluric (MT) sounding method is extensively used in electromagnetic soundings on land and sometimes on the high seas. The results of an MT sounding are normally presented in the form of apparent resistivity pa and impedance phase. Figures 1-4 in the associated drawings, which illustrate the resolution of the magnetotelluric method, show curves for both the apparent resistivity and the impedance phase for two basic models of the strata: 1) hi = 1 km, 0.3 Qm, h2 = lkm, p2 = 1 Qm, h3 = 40 m, p3 = 1 Qm, p4 = 1 Qm and 2) 50 Qm, p4 = 1 Qm. The first model and the second model describe the section without a resistive target layer (commonly called "reference model") and with a thin resistive layer (h3 = 40 m, p3 = 50 Qm) and an emulated hydrocarbon target, respectively . The resistivity of seawater and sediments is accepted as equal to 0.3 Qm and 1 Qm, respectively. The dotted and solid curves correspond to sections without and with layers containing hydrocarbons, respectively. Figures 1 and 2 show curves representing the apparent resistivity and the impedance phase on the sea surface, for the models described above. As can be seen, the effect of the hydrocarbon layer is small (less than 1%) so that it is barely detectable against background noise. The resolution of the MT curves can be improved when performing the MT measurements on the seabed. Figures 3 and 4 show curves representing the apparent resistivity and the impedance phase in the seabed, for the same models. In fact, the MT curves in the seabed are more sensitive to a resistive target (in the order of 3%), but their resolution is still preferably low. In addition, the primary EM field will be protected in this case by the conductive seawater, in such a way that the precision when determining the MT test curves is much lower on the seabed, compared to the sea surface. For several decades, several systems have been presented, which have been based on methods that include electromagnetic sources (CSEM) for marine applications. The most popular systems that can be used for marine surveys are shown in Figure 5 (Cheesman et al., 1987). At this point, columns Tx and Rx indicate transmitter and receiver. The first letter and the second letter, E or H, on the lines indicate the component of electric or magnetic field excited by a transmitter and the third letter and the fourth letter of the lines indicate the component of electric or magnetic field measured by a receiver . Occasionally, the configuration ??? f (Edwards et al., 1985) is also used. (At this point, z and f indicate the vertical component and the azimuth component of the horizontal magnetic field, respectively.This system is not suitable for deepwater surveys). A complete overview of the CSEM methods as well as MT can be found in Chave et al., 1991. Figures 6 and 7 show the resolution of the most popular ExEx configuration (Eidesmo et al., 2002); MacGregor et al., 2004; Johansen et al., 2005 and others) for a CSEM method in frequency and temporal domains. The transversal models used for the calculations are the same models 1 and 2 used for the MT modeling. Obviously, this CSEM method has a higher resolution compared to the MT method: 25% and 15% for the frequency and temporal domains, respectively. However, as can be seen from Figure 6, the measured signal is very small and can be smaller than fractions of microvolts, even in cases where the current in the transmitting line is nothing more and nothing less than 1000 A and the transmitting antenna is several hundred meters.

With small signals of this type, the noise generated by natural and artificial sources causes problems in the analysis and interpretation of the survey data. In the case where the transverse strength of the hydrocarbon layer is not sufficiently high, the existing CSEM methods can not produce any result, they can produce ambiguous results or they can produce erroneous results. A novel method proposed in the present invention differs from all known methods in that it exhibits a higher sensitivity and resolution with respect to a resistive thin layer, which is a direct indicator of the presence of hydrocarbon targets. Beyond this, this method, in combination with the proposed apparatus, provides a higher sounding efficiency. In the first place, only the TM mode is used, both for the excitation of the primary electromagnetic field, generated by the transmitter, and for the measurements by the receiver. This is achieved by using a source antenna or electromagnetic field transmitting antenna arranged essentially vertically, submerged, long, for example two vertically separated transmitting electrodes 1108 arranged one above the other, also referred to hereinafter as transmitter cable, which is connected to a power supply via cables, a transmitting electrode that acts as an anode and the other as a cathode and the transmitting antenna that receives square pulses for the excitation of EM fields in the strata and a receiving antenna, oriented essentially vertically long, submerged, also referred to hereinafter as the receiver cable, for example two vertically separated receiving electrodes disposed one above the other, for the receiver measurements of potential differences in a vertical component of the electric field. The field strength of the transmitter will be provided by the amplitude of the current pulse (ampere) and the spacing between the transmission electrodes. In a horizontally uniform section such as a source only the EM fields in the TM mode will be excited. TM modes that are insensitive to thin resistive layers in sections are completely absent and will not reduce an appropriate signal level. Second, the transmitter cable is supplied with pulse current as shown in Figure 8, curve 81. It should be noted that a real signal (curve 82) deviates from the ideal shape described by curve 81 due to the influence of technical limitations of the real system. The response measurements are displayed by the receiver cable in the time domain after the current in the transmitter has been deactivated. This sort of ordering will provide measurements of the EM field only, induced in the strata by the decreasing currents of the background when the transmitter current is absent, ie only an acceptable signal not masked by a primary field. Third, the distance R (phase shift) between the transmitter and the receiver is selected to be less than the depth of the sounding, that is, when the condition 0 = R = (tpa (t) / μ0) 1/2 has validity. This distance, known as the "induction zone", greatly improves the characteristics of the method, since it makes it possible to measure the transfer function with small distances where the signal is strong enough to provide an acceptable signal-to-noise ratio. For simplicity, the method and apparatus according to the invention are called "TEMP-VEL" (Transient Electromagnetic Marine Search with Vertical Electrical Lines). Figure 9 (which shows the apparent resistivity) and Figure 10 (which shows the voltage) illustrate the resolution of the TEMP-VEL method with respect to the reference model determined above and that does not contain a resistive hydrocarbon layer (curves 96). in the figures). Calculations have been carried out for several depths of the resistive hydrocarbon layer: 1, 2, 3, 4, 5 and oo km - curves 91, 92, 93, 94, 95 and 96, respectively. The offset for all curves is 500 m. The voltage in Figure 10 has been normalized in both cable lengths, to be valid for a length of 1 m and a current value of 1A. As can be seen, the position of the left branch 90 of the curves is determined by the thickness and resistivity of the seawater, as well as by the length and geometry of the supply cable. The target is still solved at a depth of 5000 m. The challenge is how to order signal measurements, since the signal can be weak in situations where the target is located at a great depth and has an insufficient resistivity. The TEMP-VEL configuration exhibits four parameters to improve the signal amplitude, transmit line length, transmitter current amplitude, receiver line length and phase shift value. In real situations, a manipulation of these parameters will provide the signal value in the range of hundreds of nanovolts to tens of microvolts. The measured response then becomes resistivity in relation to the depth through several methods which will be described later.

The TEMP-VEL method described in the previous section is done through the TEMP-VEL device. Figure 11 shows a schematic cross-section through seawater 1102. Reference numbers 1101 and 1103 indicate a sea surface and a seabed. A vessel 1104 is provided with a source of electromagnetic fields 1113, also referred to as a transmitter. One or more receivers 1109 are disposed at defined distance (s) from vessel 1104. During a measurement period, the vessel 1104 and the receiver / receivers 1109 are stationary during the time it takes to collect the data at the quality that provides the required signal / noise ratio. After verifying that the quality of the data is suitable for further processing, vessel 1104 changes its position with all sets of receivers 1109. This is the primary polling method. Occasionally, when a sounding is carried out following profiles and there is no need to accumulate data (if the depth of the hydrocarbon layer is sufficiently small), this method can be changed to a constant slow movement of the vessel 1104 with the transmitter 1113 and the receivers towed behind 1109. The vessel 1104 is provided with an antenna 1105 for communication as well as a power supply unit, also called generator 121 (see Figure 12). The strong current is generated by the power supply unit 121 and passed through the cables 1107 and the transmitting electrodes 1108a, 1108b which are disposed at different depths in the sea 1102 and form a transmitting antenna 1108. The moment of power Pz of a transmitter 1113 is equal to LTr x I, where LTr is the vertical distance between the transmitting electrodes 1108a, 1108b and I is the amperage. The larger Pz will be better, since this moment is of great importance for the registered signal value. The same condition is valid for receivers 1109. The vertical component of the electromagnetic fields induced in the strata by a current in the transmitter 1113 is measured by one or more essentially vertical receiving antennas 1111, each of which is constituted by minus one pair of lilla receiver electrodes, 1111b connected to receiver 1109 by wires 1110 and where the vertical distance between lilla receiver electrodes, 1111b is equal to LRc. The value of a received signal Vz is equal to LRc x Ez, where Ez is equal to the electrical component of the signal received in the z direction. The voltage of the measured signal is proportional to L4 if both the transmitting line and the receiving line have the same length L equal to the depth of the sea.

In this way, the general conditions for the TEMP-VEL system are extremely favorable when the depth of the reservoir is large and LTr and LRC exhibit a length of 500-1000 m and the amperage I = 1-5 kA. Acoustic units are provided at electrodes 1108a, 1108b, lilla, 1111b for the exact determination of the position of electrodes 1108a, 1108b, lilla, 1111b and also pressure sensors (not shown). Obviously, it is not possible to install the transmitting electrodes 1108a, 1108b and lilla, 1111b, respectively, at all vertically one above the other. In addition, vessel 1104 moves slightly during measurements due to wind and currents. The actual positions of the transmitting electrodes 1108a, 1108b are recorded and the required corrective data are calculated and taken into account in the processing and interpretation of data. The data from the pressure sensors are used to reduce the EM noise caused by waves on the sea surface. The communication between the vessel 1104 and all the receivers 1109 takes place via the antennas 1105, 1112 and the communication units described below. Figure 12 shows a block diagram of the transmitter 1113. A powerful energy generator 121 generates an alternative current which is converted by a pulse generator 122 into series of square current pulses like that shown in Figure 8. The duration of The impulse activation and deactivation stages cover the interval of 0.01-100 seconds. In practice, the pulse series are formulated by the controller 123 in a manner that suppresses the noise. The incoherence between the pulses and the noise is determined in the waiting state when the transmitter current is deactivated. A controller of the transmitter 123 controls the power generator 121, the pulse generator 122, the process for supplying power to the transmitting electrodes 1108a, 1108b, the system calibration, the data acquisition process, the real-time control of the system complete, et cetera. Cables 1107 are terminated at transmitter electrodes 1108a, 1108b, which have the ability to efficiently transfer current pulses to seawater and remain in a stable position submerged in water 1102. The main ordering for TEMP-VEL probes in a "stationary record", vessel 1104 and receivers 1109 are stationary for the time required to provide the necessary quality of the measurement data. The communication block 124 is responsible for the communication processes between the transmitter 1113 and all the receivers 1109 via an antenna 1105 and participates in the data acquisition process throughout the polling. System calibration is performed periodically during the registration process. From time to time, the operator will determine, based on the data verification, the left branch of the apparent resistivity curve 90 (with a small delay time), then compare this with the answer calculated theoretically for a real configuration geometry of TEMP-VEL and the conductivity of sea water and compare it with the real value of seawater conductivity determined in real conditions in consideration of temperature, salinity and pressure. Figure 13 shows a block diagram of the receiver 1109 in Figure 1. The induced electric field is measured by means of the receiving antenna formed by the receiver cables 1110 terminating at the unpolarized receiver electrodes lilla, 1111b. After amplification via a low noise amplifier 132, the signal is digitized through an analog / digital converter (ADC) 133 and transferred through a receiver control unit 134, a communication block 136 and the antenna 1112 to vessel 1104 for complete processing and subsequent analysis. The receiving control unit 134 changes the order of data acquisition according to instructions of vessel 1104, which houses the primary center for probing. The signals can also be transferred to a land-based control center where these decisions can be made. The fieldwork strategy has been developed based on information about the area that is surveyed, received from geological and geophysical data. Modeling ID, 2D, 2½D or 3D of the electromagnetic situation occurs and the signals expected from the TEMP-VEL system are evaluated. Both the optimal order for the installation of the system and the order of measurement are planned based on these signals and the resolution required in the vertical and horizontal directions. One of the possible polling orderings is shown in Figure 14. The entire polling area is divided into sub-areas. The vessel 1104 carrying the transmitter 1113 is stationed in the center of each subarea. The sets of receivers 1109 are deployed around the vessel 1004 at the distance that satisfies the requirement of the induction zone. In addition, a network of autonomous magnetoteluric stations 141 is deployed in the area. These stations 141 are used to reduce any noise produced by geomagnetic variations. The duration of the measurements in each subarea is determined by many factors, which include the characteristics of the section, amperage, depth of the ocean, length of the transmitting and receiving antennas 1108, 1111, noise, and so on. The synchronous or asynchronous accumulation of data is carried out during these measurements. After checking the quality of the data, vessel 1104 and all sets of receivers 1109 are placed in a new location. After preprocessing and analysis, the collected data is either converted into voltage profiles or for apparent resistivity vs. time or depth in the category of gradient sections or are reversed in resistivity vs. depth in the category of stratified structures. In those cases where the influences on the electromagnetic field structure of the lateral homogeneities are not substantial, the investment is made in models in the ID category. In other instances, the investment and interpretation of the data is done in models in the category 2D, 2½D or 3D.

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Claims (17)

1. CLAIMS 1. A method for an electromagnetic sounding of electrically resistive target objects containing potentially hydrocarbons, characterized in that the method comprises: - determining electrical characteristics of a stratum which is probed by means of using the TM mode of at least one source of electromagnetic fields (1113) and record the TM response; as - the intermittent current pulses of the source (81, 82) with a sharply defined termination are generated in at least the source of electromagnetic fields (1113); - the intermittent impulses of the source current (81, 82) are transferred to a transmitting antenna, essentially vertical, submerged (1108) and transmitted within the strata, - the average responses are intercepted by at least one receiver (1109) deployed in the induction zone and provided with at least one receiving antenna, essentially vertical, submerged (1111), in the time between the consecutive current pulses; - the measurements of the response of the strata in the induction zone, ie in an area where the horizontal distance between at least one transmitting antenna (1108) and at least one receiver (1109) is equal to R and R = (tpa (t) / μa) 1 2, where t is the delay time counted from the instant after the source of electromagnetic fields (1113) has been deactivated, μ0 = 4p10 ~ H / m and pa (t) is the apparent resistivity of a substrate in period t; as - at least the source of electromagnetic fields (1113) and at least one receiver (1109) are submerged in a body of water (1102). A method for an electromagnetic sounding according to claim 1, characterized in that the current pulses (81, 82) succeeded to each other in a special sequence which is inconsistent with a present signal noise and the responses measured by at least one receiver (1109) is stacked to provide a signal / noise ratio which is sufficient for target detection. 3. A method for an electromagnetic sounding according to claims 1 and 2, characterized in that an additional suppression of signal noise is achieved by processing time-coded geomagnetic data and time-coded source pulse data (81, 82). 4. A method for an electromagnetic sounding according to claims 1-3, characterized in that an additional suppression of signal noise is achieved by processing water pressure records coded in time, which are collected in the immediate vicinity of the receiving antenna (1111) from at least one receiver (1109) and compared to the source coded pulses in time (81, 82). 5. A method for an electromagnetic sounding according to claims 1-4, characterized in that a decision to continue the measurements, change the mode of operation, change the measurement sites or recover one or more of the signal generating means ( 141, 1108a, 1108b, 1109, lilla, 1111b, 1113) is taken after a full or partial evaluation and / or interpretation of the acquired data. 6. A method for an electromagnetic sounding according to claims 1-5, characterized in that at least some of the collected data is transferred to a central processor and analyzed in real time. A method for an electromagnetic sounding according to claims 1-6, characterized in that at least the source of electromagnetic fields (1113) and at least one receiver (1109) are stationary during a recording interval and then reassigned to another position in the sounding area for repeating the method according to claim 1. 8. A method for an electromagnetic sounding according to claims 1-6, characterized in that at least the source of electromagnetic fields (1113) and at least the receiver (1109) are in constant motion in the sounding area during recording. 9. A method for an electromagnetic sounding according to any of the preceding claims, characterized in that two or more receivers (1109) register the vertical component of the electromagnetic field induced by the same source of electromagnetic fields (1113), simultaneously and in different locations within the induction zone. 10. A method for forming images of ID, 2D, 2½D and 3D layers, characterized in that the method comprises the step that consists of combining the apparent resistivity with the apparent depth cross section calculated for all the recording sites based on the electric field vertical measured from the induced zone, excited by a source of vertical electromagnetic fields (1113) by means of the use of a delayed response in a homogeneous half space for the transmitting antenna (1108) of the source of vertical electromagnetic fields (1113). 11. An apparatus for electromagnetic probing of electrically resistive targets that potentially contain hydrocarbons, characterized in that it comprises: - a substantially vertical, submerged transmitting antenna (1108) that acts as a source (1113) of a TM mode of an electromagnetic field; - an energy source (121) arranged to supply electrical power and a controllable pulse generator (CSEM) (122) arranged to supply intermittent series of square pulses (81, 82) with a duration of 0.01-100 seconds, an amplitude of 0.1-10000 A and a termination markedly defined to the transmitting electrodes (1108a, 1108b) of the source of electromagnetic fields (1113); - at least one receiver (1109) deployed in the induction zone and provided with at least one receiver antenna, essentially vertical, submerged (1111), the receiver 1109 is arranged to register the vertical electromagnetic field during intervals between the intermittent pulses of current (81, 82). 12. An apparatus according to claim 11, characterized in that the essentially vertical transmitting antenna (1108) of the transmitter (1113) is arranged to register the vertical electromagnetic field during intervals between the intermittent current pulses (81, 82). An apparatus according to claim 11, characterized in that the acoustic sensors are provided in the immediate vicinity of upper and lower end portions (lilla, 1111b) of the receiving antenna (1111). 14. An apparatus according to claim 11, characterized in that the pressure sensors are provided in the immediate vicinity of upper and lower end portions (lilla, 1111b) of the receiving antenna (1111). 15. An apparatus according to claims 11-14, characterized in that at least the source of electromagnetic fields (1113) of the apparatus and at least one receiver (1109) are arranged to move under control or autonomously during or between the measurements, the measurements are made continuously or sequentially. 16. An apparatus according to claims 11-15, characterized in that the source of electromagnetic fields (1113) and / or at least one receiver (1109) are provided with means (1105, 1112) of the real-time transfer of at least a selection of the data collected from a central processor. An apparatus according to claims 11-16, characterized in that the additional sensors (141) for measuring the electric field of three components and / or the magnetic field of three components in geomagnetic variations are arranged in one or more locations on the seabed (1103).