NL1038970C2 - Method and apparatus for electromagnetic exploration of the targets potentially containing hydrocarbon reservoirs. - Google Patents

Description

METHOD AND APPARATUS FOR ELECTROMAGNETIC EXPLORATION OF THE TARGETS POTENTIALLY CONTAINING HYDROCARBON RESERVOIRS

BACKGROUND OF THE INVENTION 5

The modem technique for the marine electromagnetic prospecting of the hydrocarbon deposits is based on a determining of an anomalously high electric resistivity at the sea subsurface. The methods for determining the electric resistivity of the subsurface rock formations known as the Controlled Source Electromagnetic (CSEM) methods plays a 10 significant role in a complex of geophysical tools used for the marine EM surveying. The most popular CSEM system, named «SBL» consists of a horizontal electric dipole transmitter (HED) submerged in the sea water and supplied by strong electric pulses of square form; the horizontal electric receiving dipoles are installed on the seabed at different offsets (distances) from the transmitter. Various modifications of this system 15 are described by Smka (1986), Ellingsrud et al. (2001-2005), Eidesmo et al. (2003), MacGregor et al. (2003), and in other publications referenced above. The majority of the CSEM transmitters transmit either a harmonic electric current or a sequence of the square electric pulses. After the data acquisition, the electric field is Fourier transformed to the frequency domain for the further analysis and interpretation.

20 The existing marine CSEM systems are capable to resolve the targets provided that the horizontal offset between source and receiver is greater than several depths to the reservoir. This condition assures that the EM-field propagates from the transmitter to the receiver through the bedrock. On the other hand, large offsets make the measurement vulnerable to the distorting effect of the EM-fields propagating through 25 the air. According to Constable (2006) and Constable and Weiss (2006), the contribution of the air-propagating field makes the most popular CSEM-technique inapplicable to the shallow water surveying, specifically, the traditional SBL technique is considered unreliable if the sea depth is less than 300 meters. Andreix (2009), Frenkel and Davydycheva (2009), Johnstad and Famelly (2010) and others demonstrated that 30 the depth of this sounding technique can be decreased to 50 meters and less, however, it will result in decreasing of the spatial resolution of the method.

Edwards and Chave (1986) proposed a CSEM configuration based on a step-on transient response for a horizontal in-line electric dipole-dipole system. This system is characterized by a higher resolution in comparison with the traditional frequency 35 domain SBL. However it does not provide maximal possible 1038970 2 resolution with respect to a research target.

Edwards et al. (1981, 1984, 1985) proposed a frequency domain system - the Magnetometric Off-Shore Electric Sounding (MOSES). This system uses a vertical transmitter dipole supplied by an alternating electric current as the transmitter, and the 5 magnetic sensor measuring the azimuth component of the magnetic field near the sea floor as a receiver. A significant advantage of the MOSES is its reliance on the TM-mode of the electromagnetic field which is more sensitive to the resistive targets than the TE-mode. The system has disadvantage, in particular, it operates with large offsets, which are necessary to provide a sufficient signal level and as a result, insufficient 10 sounding depth and resolution.

Ziolkovsky et al. (2010) improved MTEM method working in time domain; they proposed an approach giving possibility to remove undesirable response called “airwave” from surveying data. This response distorted useful signal from the target and prevented surveying in shallow waters. However, spatial resolution of the method is still 15 low because of large offsets needed at surveying.

From physical point of view most high resolution of EM sounding regarding resistive targets is provided by the TM mode of the field, it means that the vertical electric field component Ez is most perspective carrier of information on a resistive anomaly. Barsukov et al. (2007, 2008), Andries (2009), MacGregor et al. (2011), Velikhov et al. 20 (2009) and others denoted this fact in their patents and publications. Constable et al.

(2008) proposed a measuring system, consisting of a set of electrode pairs built-in the long lines oriented in arbitrary directions, including vertical direction. However, the patent does not contain definite proposals on how to use the vertical gradients of the vertical electric field to search hydrocarbon (HC) targets with the controlled current 25 sources.

Barsukov et al. (2008) developed CSEM system named “TEMP-OEL” working in time domain. In this system HED transmitter drive square pulses of current in horizontal line embedded in sea water; receiver (VER) installed on sea bottom measures vertical component Ez (TM-mode) of electric field during the pauses between pulses. A 30 horizontal separation between the source and receiver can be less than the depth to the target. This system has high resolution with respect to the resistive targets and can operate in shallow waters. TEMP-OEL configuration does not lose sensitivity and resolution at using in shallow waters. At the same time, its straightforward application in shallow waters seems to be problematic, because of strong dependence of the 35 measured data from vertical orientation of the receiver (Holten et al. 2009).

3

At exciting of EM field in time domain by a vertical electric dipole (VED) (Barsukov et al., 2007) the horizontal and vertical components of electric field on the sea floor are comparable in amplitude and both belong to TM mode. The circumstance gives possibility to use a VER with maximal admissible tilt less than < 0.5° at deviation of 5 VED transmitter from a vertical. In shallow waters, the resolution and sensitivity of the vertical component are still high, however, the signal’s level is inadmissible small. For example, the vertical component of the transient electric field in the near zone of the late stage on the sea bed is: -P u5l2rrV2 e,. --7-a*-? h*, 20n*Jn t5 2 10 where m is magnetic permeability of vacuum, cr is conductivity of sea bed sediments, h is the sea depth, and Pved is the electric dipole moment of the VED. On the base of assumption that the VED is ranged from the sea surface to sea bed, the Ez amplitude depends on the depth h proportionally to the depth in a cubic degree: Ez - h3. At the h -50-100 m, the vertical component at times /=0.1-2 s, where the hydrocarbon (HC) 15 anomalies usually take place, does not exceed 0.1 pV/(Am), and comparable with the errors of measurements. In addition, even the small variations of the VED’s tilt, as well as the tilt of the receiver will result in significant uncontrolled errors of the measurements and will give no possibility to apply such measuring apparatus for the EM prospecting of the HC targets in a shallow water (transient zone).

20 An application of the HED transmitter solves the signal’s level problem. In the near transient zone, the vertical component of the HED is equal to: r> ,.5/2~-3/2 E,.« T-''0,% Rh , where R is offset between the vertical receiver (VER) and the HED, Phed is a dipole’s moment. In this configuration, there are no limitations on the HED moment because the 25 length of the transmitter line is not limited by a sea depth and the dependence Ez of the depth h is linear. Apparently, an application of the HED-VER configuration in shallow waters is preferable than the VED-VER one. However, in this case the other problem, namely the tilt of the VER do arises. As it is well known, the horizontal component of the electric field excited by the HED is in several orders greater than the vertical 30 component, so the smallest deviation of the VER from the vertical line will result in significant distortions of the measured signal.

4

There are patents of McGregor, Constable, Besson et al. with a variety schemes of multielectrode measurement systems, however they do not relevant to the problem of suppressing the tilt influence on the measurements of vertical electric field component. The method presented in our invention solves this problem.

5

SUMMARY OF THE INVENTION

The invention discloses new method and apparatus for an electromagnetic exploration of the hydrocarbon reservoirs.

10 The objective of the proposed invention is to provide the maximal resolution of the electromagnetic prospecting with respect to the hydrocarbon containing targets, and to increase the efficiency of the surveying in shallow waters. The novel transmitter-receiver configuration solves both these problems. The examples given below demonstrate the advantages of the proposed invention.

15 According to the first aspect of the invention, a new method of exploration of the reservoir and its characteristics is provided. This method consist in the measurements of a galvanic mode of the electromagnetic field induced in the submarine strata, data processing and analysis with the purpose of determination of the electric properties of the section and resistance of the reservoir containing layer.

20 According to the second aspect, the invention discloses an apparatus consisting in the exciting of electromagnetic field in the near-bottom sea water, measurements of the galvanic mode of the field in the submarine strata, and determination of the electric resistance of the reservoir containing layer.

According to the third aspect of the invention, transmitter drives square pulses of an 25 electric current in the horizontal electric line and the receiver makes measurements of the electromagnetic field responses containing the information on the subsea structure. According to the forth aspect of the invention, receiver which is, in fact, a gradiometer consisting of a rigid rod with two pairs of electrodes mounted strictly coaxial in the vertical direction provides measurements of the vertical gradient of the vertical electric 30 field belonging to the TM mode of the field.

According to the fifth aspect of the invention, an offset R between the transmitter and the gradiometer can be less than the depth of the target reservoir.

According to the sixth aspect of the invention, the multiple gradiometers can be used for synchronous measurements to increase the surveying effectiveness.

5

According to the seventh aspect of the invention, the gradiometers can be placed over the sea bed or completely or partly embedded in the seabed rock.

According to the eighth aspect of the invention, the transmitter transmits special logic of the time lapses signals, and the multiple receivers installed on a seabed perform 5 synchronous measurements of the vertical gradients of the vertical component of an electric field in dissymmetrical points with the transmitter in the center of a symmetry; the results of measurements are subtracted and the resulting difference is determined; this difference form the data base used then for the inversion and the exploration of the hydrocarbon.

10 According to the ninth aspect of the invention, the measurements of a resulting difference can be performed with only one pair of the gradiometer’s electrodes in dissymmetrical points.

According to the tenth aspect of the invention, special logic of the time lapses signals and the algorithm of measurements and data processing are configured in dependence 15 on the time or frequency operation mode and the data analysis and inversion.

According to the eleventh aspect of the invention, all measurements for the individual points and all offsets fulfilled in the time and frequency domain are inverted jointly for the determination of 3D electric resistivity model and the polarization medium’s properties.

20

DESCRIPTION OF THE DRAWINGS

The main ideas of the present invention, its advantages as well as disadvantages of the existing methods applied for marine electromagnetic prospecting of hydrocarbons will 25 become more apparent from the following description of the invention with references to the drawings appended hereto. The enclosed drawings illustrate the main ideas of the proposed invention and cannot be used to construe the scope of the invention.

Figure 1 illustrates the main idea of the method. The horizontal transmitter Tr drives an electric current in the sea water 1 (schematically shown by the dashed line). 30 Gradiometer Gr consisting of two pairs of spaced coaxial sensors (first pair 3 placed on sea bottom 2 and second pair placed in the point 4 just above the point 3) produces measurements of a difference of the electric fields between these points. At that, the horizontal component is suppressed and the vertical is remained.

Figure 2 depicts the vertical gradient of the horizontal component of the electric field 35 AEx(h) normalized on the horizontal component Ex(Hq) (in the percentage terms). The 6 curves denoted by square and round dots correspond to the 25 m and 37.5 m of sea j depth respectively. Resistivity p of the sea water is 0.28 Ohm-m and resistivity of the seabed rock is 2 Ohm-m.

Figure 3 depicts behavior of Ex(t) and Ez(t). The sea depth is equal to 50 m, resistivity 5 of the sea water is p=0.28 Ohm-m and resistivity of the seabed rock is 2 Ohm-m; duration of a pulses and a pauses between the pulses of the transmitter’s electric current is 4 seconds.

Figure 4 depicts behavior of a vertical gradient of vertical electric field: versus time -Ez(t) (a) and versus frequency - Ez(f) (magnitude - b and phase - c) domain. The fields 10 are normalized on the transmitter current and length of the transmitter and receiver line.

The parameters of the model are the same as in the Figure 3. The depth of the rectangular target (reservoir) of 3x3 km size is 1 km Transversal resistance of the a target is 2000 Ohm-m . The fields are calculated for time t=0.5 s and frequency ƒ= 0.25 Hz at 1 km offset.

15 Figure 5 illustrates an example of the system’s configuration according to the proposed method. Sea water 1 has resistivity p=0.28 Ohm-m, sea depth 2 is equal to 50 m and the resistivity of a seabed rock 3 is equal to 2 Ohm-m. Transmitter 4 is fixed or moving near the sea bottom along some profile. At offset 7 equal to 1 km, the gradiometer 5 having two coaxial sensors 6 placed at depth hi=50 m and h2=37.5 m 20 performs measuring of the vertical component of the electric field. The angle o=0.5° shows the tilt of the gradiometer.

Figure 6 depicts the transients for the instrument’s installation shown at the Figure 4.

An undistorted (a=0°) and a distorted electric field (a=0.5°) are shown at the left and an apparent resistivity for undistorted (a=0°) and a purified synthetic field (a=0.5°) are 25 shown at the right.

Figure 7 depicts operation of the horizontal gradient method: subtraction of the electric field for two symmetrical HED setups reduces the horizontal component and enhances the vertical one. Transmitter Tr sequentially takes a position Tr(-R) and Tr(+R) in two sides from the gradiometer Gr installed at the sea bottom 3 in the point 0.

30 The transmitter induces an electric current (shown schematically by dashed lines 1) in a water layer 2. After subtraction of the fields, the horizontal E^-R) and Ex(+R) components (4) are suppressed, and the vertical Ez(-R) and Ez(+R) components (5) are summarized.

7

Figure 8 shows schematically a side view of an electromagnetic surveying system with a horizontal transmitter dipole and a vertical receiver (gradiometer).

Figure 8a shows a schematic view, where the reference numeral 1 indicates a vessel floating on the sea surface 2; Ho is a depth of the sea. The winches 4 and 7 are used for 5 the towing of the transmitter 5 and receiver 8 cables. The cable 5 submerged in the sea water connects installed on the vessel converter forming the pulse electric current, with the electrode pair 6. The distance Ltt between these electrodes determines the length of the transmitter’s dipole and a dashed line in the middle of the electrodes fixes location Tr of the transmitter. From other side of the vessel, at offset R from TV, the receiver Rec 10 (gradiometer) is established strictly in-line with the Tr on sea bed using the cable-line 8. Gradiometer consists of a rod 9 with fixed on it two pairs of electrodes 10 and 11 which measure vertical component of the electric field Ez at depths hi and hi respectively.

The electrodes are mounted on the rod strictly coaxial with each other; the rod is established on (or in) the sea bottom practically vertical.

15 Figure 8b illustrates example of an installation providing additional suppressing of the noise creating by a horizontal component of the transmitter when inclining of the receiver line. All denotes shown in Figure 8a are valid for 8b as well. The transmitter sequentially takes up a position Tr at equal offsets -R and +R from the receiver (Res) following the vessel locations 1,12 and so on. The measured differences ArE(t, x) or 20 ArEz(f, Xf) are determined according to the claims 2-11.

DETAILED DESCRIPTION OF THE INVENTION

Basic Concepts of the system 25

The proposed method is based on the fact that for the horizontal dipole source submerged in shallow waters, the relative (normalized) vertical gradient of the horizontal component of the electric field measured near the sea floor is many times less than the relative vertical gradient of the vertical component.

30 For vertical electric field excited in the sea water layer underlain by horizontally uniform layered section the following relations are valid: AEx(h) = Ex(H0)-Ex(h) , .

EX(H0) Ez(H0)

Here the H0 is the depth of sea, the h < Ho is the depth of the upper sensor of the 35 gradiometer - Figure 1.

8

At the same time, the vertical gradient of the horizontal component of the Ex field is insignificant; normalized on the horizontal component it is shown in the Figure 2.

Thus, we have the following formulas for this model of the medium: 5 H° AEz(h) = Ez(H0)-Ez(h) = jEz(H0),

For k=—Ho k»4, and ^.1.1 4 EZ(H0) k 4 < _L s H0= 50 m , h = — H0 , EX(H0) 1000 0 4 0 i.e. the vertical gradient of Ex is 250 times less than the vertical gradient of Ez.

10 At shallow depths the horizontal component of the HED can considerably exceed the vertical component. Figure 3 shows the behavior of the Ex(t) and Ez(t) for the offsets: R=l km and R=2 km.

The vertical sensors (pairs of electrodes), fixed at the receiver’s rod measure the resulting electric field directed along the rod’s axis, i.e. the vector sum of Ez and Ex 15 field. If the rod’s tilt is equal to zero, the receiver measures the vertical component Ez just as it is; but if there is even small (tenth part of percent of degree) tilt of the receiver’s rod, the result will be distorted by a horizontal component of the field.

However, if the deviation of the receiver line from the vertical component is not very high, the influence of the horizontal component can be strongly reduced by measuring 20 of the difference of the field Ez on the depth Ho and on some other depth h<Ho: AEz(h) = Ez(H0)-Ez(h) with following synthesizing of the “purified vertical field” on the depth Ho by simple multiplying of the measured field difference AEz(h) on into the array factor k: E^(HJ = kAEM , k= „"'lb 25 Taking into account that the main idea of the proposed method is the gradient measurements, we named the method “GEMS” - the Gradient Electromagnetic Sounding.

As an example, consider the case of a ID model with the sea depth and resistivity of the sea water of 50 m and of 0.28 Ohm-m respectively, resistivity of the sea bed is of 2 30 Ohm-m, location of the transmitter (TR) and the receiver (gradiometer), as well as the offset are shown on the Figure 4. The measurements are produced in the time domain.

The purified synthetic field for this setup has a following form: i j 9 iiq n 2

Apparent resistivity pa(t) is calculated as 5/2 12/3 5 where Phed is the moment of HED, R is the offset, Ho is a sea depth.

If the tilt of the gradiometer is less than 0.5°, the synthesized and true TEM curve are differ less than 1% in the time range from 0.01s to 2 s; if the tilt of the gradiometer is less than 3°, the synthesized and true TEM curve are differ less than 1% in the time range t= 0.01-0.5 s and 5% in the range t=0.5 -2s and if the tilt is equal to 10° the 10 synthesized and true TEM curve are differ by 3% in the time range t= 0.01-0.5 s and 10-15% in the range t=0.5 -2 s. So, if the minimum of anomaly exceeds 15% at times t=0.5 -2 s, then the maximal tilt of the gradiometer should have to satisfy the condition a < 3°; in case of anomaly is more than 5%, the tilt a must be less than 1°.

Sometimes it is necessary to improve the quality of the compensation of a horizontal 15 component; it can be made using the horizontal gradient scheme. The idea of this method is that the horizontal component Ex(t) excited by the HED in time domain at small offsets is slightly depends on the resistive upper part of the seabed due to the transient process in the near zone:

EaP Mo2^ 1 x HEDU W^3/2’ 20 i.e. Ex is proportional to ~ Ver and electric field does not depend on the offset R.

The Figure 6 elucidates the functioning of the horizontal gradient method.

When moving along the profile, the HED falls into an equal offset R in the position x=-R and x=+R respect to the receiver located in the x=0 position. The horizontal components EX(R) have the same orientation in the receiver point, whereas the vertical 25 components Ez(-R) and Ez(+R) have opposite signs.

In the horizontally uniform media Ez(-R)=-Ez(+R) and Ex(-R)= Ex(+R). Subtraction of the total field E=EZ+EX in the points -R and +R result E(-R)- E(+R) = 2EZ(R) i.e.

excludes (in horizontally uniform media) or essentially decreases the influence of the horizontal component on the vertical field when the tilted receiver.

30 This scheme is applicable for the usual vertical receiver of the electric field consisted of one pair of electrodes and making measurements of Ez field in the time and frequency domain.

10

Description of the system

The main concepts of the method stated in the previous section are realized in the described apparatus. As indicated above, the system may use slightly tilted transmitting 5 and receiving lines. The allocation and functions of the elements of some of the possible setups of the acquisition system are schematically presented on the Figure 7.

In particular, a part of the acquisition system that transmits pulses of an electric current into shallow water is shown on Figures 7a and 7b in the form of two electrodes 6 submerged near the sea bottom. The electric moment of the transmitter is established by 10 a current intensity and the distance Ljr between the transmitter’s electrodes 6.

Depend on power of an electromagnetic noise, required accuracy of the measurements, resolution and other factors, the vessel 1 is moving slowly along the profile or works in a start-stop operation mode. The cable-line 5 connects the electrodes with the transmitter installed on the vessel board. The cable-line with the winch fiame provides 15 towing of the electrodes.

The receiver measuring the vertical electric field consists of a non-magnetic nonconducting rod 9 which is established practically vertically, collinearly with the transmitter line (inline setup). Two pairs of non-polarized electrodes 10 and 11 are mounted on the rod strictly coaxial in such a way as to be in the same axis as the rod 9; 20 at that the tilt a of both pairs is the same as the rod. A distance between electrodes in each pair and between the pairs is fixed according to the formulas given in the basic conception section of the real invention. Both pairs of the electrodes are connected in an opposite direction to provide the measurements of the difference: AEz(h) = Ez{h,)-Ez(h2) 25 The offset R should be large enough to provide the suppressing of the effect of an induced polarization, the small angle between transmitter and receiver electrodes and the strong enough signal sufficient for acceptable signal/noise ratio. In practice, the offset can be of the order of several hundred of meters (600-1000 meters or more).

In moving, all setups can be fulfilled by a single vessel. Such a mode can be used for 30 the fast surveying. The other form of surveying is described on the Figure 7b. A series of gradiometers are installed in advance on the sea bottom strictly along a profile crossing an area which is known as potentially containing the hydrocarbon targets. A vessel is slowly towing the horizontal electric dipole along this profile and transmits pulses of electric current; gradiometers make measurements of the differences AEz(h).

11

During the measurements or after that these differences are grouped in dissymmetrical pairs with respect to transmitter location and depend on mode of operation (time or frequency domain) record ArEO, Xj) or AaEzff, xj data. These data are then processed, analyzed, inverted and interpreted.

5 In this mode some simplification of setup and measurements is possible, namely using of single pair of electrodes instead of two ones vertically remote (claim 9, 19). However, in this case suppressing of the tilts is not just effective as in the case of application of gradiometers.

The preferable setup is shown in Figure 7b. The vessel works in a start-stop operation 10 mode and series of gradiometers installed on the sea bottom. The transmitter transmits pulses of electric current and the gradiometers accumulate ArEzO, xj and ArEz([, xt) data which then are processed and interpreted.

REFERENCES CITED 15

U.S. PATENT DOCUMENTS

4,644,892 10/1985 Kaufman etal.

4,617,518 10/1986 Smka 20 5,563,513 10/1996 Tasci 6,320,386 B1 11/2001 Balashov et al.

0052685 A1 03/2003 Ellingsrud et al.

0048105 A1 03/2003 Ellingsrud et al.

6,628,119 B1 10/2003 Eidesmoetal.

25 G01V003/12 08/2004 Ellingsrud et al.

G01V001/00 01/2005 Ellingsrud et al.

G01V003/08 10/2005 Wright etal.

324334000 02/2006 Constable 7592814 B2 09/2009 Andreis 30 0267608 Al 10/2009 Johnstad et al.

0001985 Al 01/2009 Besson etal.

7795873 B2 09/2010 Ziolkovsky et al.

7924014 B2 04/2011 MacGregor et al.

UK PATENT DOCUMENTS

35 12 GB 2437225 09/2010 Johnstad et al.

GB2450158A 12/2008 MacGregor et al.

OTHER PATENT DOCUMENTS 5 WO 01/57555 Al 09/2001 Ellingsrud et al.

WO 02/14906 Al 02/2002 Ellingsrudetal.

WO 03/025803 Al 03/2003 Smka et al.

WO 03/034096 Al 04/2003 Sinha et al.

10 WO 03/048812 Al 06/2003 MacGregor et al.

WO 2007/053025 Al 05/2007 Barsukov et al.

WO 2008/066389 Al 06/2008 Barsukov et al.

WO 2008/028083 A2 03/2008 Constable et al.

15 OTHER REFERENCES

Chave A. D., Constable S.C., Edwards R.N., 1991. Electric Exploration Methods for the Seafloor. Chapter 12. Ed. by Nabighian, Applied Geophysics, 2, Soc. Explor.Geophysics, Tusla, Oklahoma, P. 931-966 20 Constable S.C., 2006. Marine electromagnetic methods - A new tool for offshore exploration, The Leading Edge, 25, P. 438-444.

Constable S.C. and Weiss C. J., 2006. Mapping thin reservoirs and hydrocarbons with marine EM methods: Insights from ID modeling, Geophysics, 71, P. G43-G51.

Constable S.C. and Smka L.J., 2007. An introduction to marine controlled-source 25 electromagnetic methods for hydrocarbon exploration. Geophysics, v.72, N2, P. W A3-WA12.

Edwards R. N., Law L. K., Delaurier, J. M., 1981. On measuring the electric conductivity of the oceanic cmst by a modified magnetometric resistivity method: J. Geophys.Res., 68, P. 11609-11615.

30 Edwards R.N., Law L.K., Wolfgram P.A., Nobes D.C., Bone M.N., Trigg D.F., DeLaurier J.M., 1985. First results of the MOSES experiment: Sea sediment conductivity and thickness determination. Bute Inlet, British Columbia, by magnetometric off-shore electric sounding. Geophysics, 450, P. 153-160

Edwards R. N. and Chave A. D., 1986. On the theory of a transient electric dipole-35 dipole method for mapping the conductivity of the sea floor: Geophysics, 51, P. 984- 987.

13

Eidesmo T., Ellingsrud S., MacGregor L.M., Constable S., Sinha M.C., Johansen S.E., Kong N. and Westerdahl H., 2002. Sea Bed Logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers in deepwater areas. First Break, 5 20, March, P. 144 - 152.

Frenkel M. A. and Davydycheva S., 2009. A modeling study of low-frequency CSEM in shallow water. 71st EAGE Conference & Exhibition - Amsterdam, the Netherlands.

Greer A.A., MacGregor L.M. and Weaver R. 2004. Remote mapping of hydrocarbon 10 extent using marine Active Source EM sounding. 66th EAGE Conference & Exhibition, Paris, France, 6-10 June 2004.

Haber E., Ascher U. and Oldenburg D. W., 2002. Inversion of 3D time domain electromagnetic data using an all-at-once approach: submitted for presentation at the 72nd Ann. Internat. Mtg: Soc. of Expl. Geophys.

15 Holten T., Veiberg D., & Flekkoy E.G., 2009. Vertical Electric Time-domain

Responses from a vertical Current Source for Offshore Hydrocarbon Exploration. 71st EAGE Conference & Exhibition Amsterdam, the Netherlands.

Johansen S.E., Amundsen H.E.F., Rosten T., Ellinsgrud S., Eidesmo T., Bhuyian A.H., 2005. Subsurface hydrocarbon detected by electromagnetic sounding. The First 20 Break, 23, P.31-36.

Johnstad S.E., Farelly B.A., 2010. Shallow marine hydrocarbon prospecting. UK patent.

Kaufman A. A., and Keller G. V., 1983. Frequency and transient soundings: Amsterdam, Elsevier Science Publ. Co., P. 411-454.

Wright D. A., Ziolkowski A., and Hobbs B. A., 2001. Hydrocarbon detection with a 25 multi-channel transient electromagnetic survey, 70th Ann. Internat.Mtg,, Soc. of Expl. Geophys.

Yuan J., Edward R.N., 2001. Towed seafloor electromagnetics and assessment of gas hydrate deposits. Geophys. Res. Lett. 27, P. 2397-2400

Ziolkovsky A., Hobbs B., Wright D., 2002. First direct hydrocarbon detection and 30 reservoir monitoring using transient electromagnetics. The First Break, 20, N. 4, P. 224-225

Velikhov E.P., Zhdanov M.S., Kruglyakov M.S., Korotaev S.M., Orekhova D.S., Shchers Yu. G., 2009. Study of opportunities of the low frequency electromagnetic survey for search of hydrocarbon deposits in perspective anticline structures at an 35 example of Shtokman deposit in Baltic Sea. Geophysical Journal N4, V. 31, Naukova Dumka, Kiev, P. 3-11 10 38 9 7 0

Claims (19)

1 A method for electromagnetic exploration of electrically resistive objects, which contain potential hydrocarbon reservoirs, comprising: a system consisting of a receiver immersed in the seawater near the seabed and measuring the difference AEl = J5z (hi) - Ez ( h ^ of the vertical electric field at the depths of hj and hi, while a horizontal electrical transmitter sends rectangular pulses of electrical current characterized by a steep slope of the trailing edge, said receiver is collinear with said transmitter at a distance R which can be smaller are then the depth of the intended hydrocarbon reservoir; survey of said difference AEz is carried out over the area possibly containing the hydrocarbon reservoirs; said survey is conducted in the time domain or in the frequency domain, the data of said survey are analyzed for the construction of a 3D model of the hydrocarbon object and evaluation of its electrical resistance and. 15 An electromagnetic exploration method according to claim 1, characterized in that a ship simultaneously drags the transmitter and the receiver located at said distance R which is less than a depth to an object; said transmitter controls the rectangular pulses of electrical current with pauses; said receiver measures the difference ΔEz (t) = Ez (t, hi) - Ez (t, hi) during the 20 pauses at time t where t is the time course after the electrical power pulse disconnection. An electromagnetic exploration method according to claim 1, characterized in that many said receivers are installed on the seabed along the profile and that the ship drags the transmitter along the same profile; for all receivers installed in points x /, x ?, 25 ... Xj, ..., xn along said profile, the difference is AEz (t, x) = Ez (t, x, hi) - ^ (txhi ) measured at different distances R where said transmitter is located at a point xt + R and at a point xrR along said profile and where said difference is calculated: ArEzQ, x ^ iAEzCt, Xj + R) - AEz (t, XfR)] / 2. An electromagnetic exploration method according to claim 1, characterized in that many said receivers are fully or partially embedded in the seabed rock along the profile and that the ship drags the transmitter along said profile; for all receivers installed in the points xu x.?, ... xb ..., xy along said profile, the difference becomes AEz (t, x) = Ez (t, x, hi) -Ez (t, x, hi ) measured at different distances R where said transmitter is located at a point 1 0 3 8 9 7 o Xj + R and at a point xrR along said profile and where said difference is calculated: AfiEzfi, Xi) = [AE, r (t , Xi + R) - AE 2 (t, xr R) Y 2. 5 AREz (t, x) = [AEz (t, x, + R) - AEztf xrR)] / 2. 5. An electromagnetic exploration method according to claim 1, characterized in that a ship drags the transmitter and receiver simultaneously; said transmitter sends pulses of electrical current without pauses on a frequency ƒ and said receiver measures the amplitude and phase of the difference ΔEz (f) = Ez (f, hd-Eztf, hrf). 6. An electromagnetic exploration method according to claim 1, characterized in that many said receivers are installed on the seabed along the profile and that the ship drags said transmitter along said profile; for all receivers installed at points X], X2, -Xi, xn along said profile, the amplitude and phase of the difference ΔEz (f, x) are measured at the different distances R where said transmitter is located at a point xt + R and at a point xrR along said profile and wherein the amplitude and phase of the difference are calculated: AüEzff, xd ^ AEzff, Xi + R) - AEzff, Xi-R)] / 2. 7. An electromagnetic exploration method according to claim 1, characterized in that many said receivers are fully or partially embedded in the seabed rock along the profile and that the ship drags said transmitter along said profile; for all receivers installed at points xj, x2, ... xh ..., xn along said profile, the amplitude and phase of the difference ΔEzff, x) are measured at different distances R, said transmitter being located at a point Xj + R and at a point xrR along said profile and the amplitude and phase of the difference are calculated: ArEz ^, XiJ ^ iEzft Xi + R) -AEztf, x, -R)] / 2. An electromagnetic exploration method according to claims 1,2,3, characterized in that the transmitter sends pulses of electrical current with pauses on a frequency ƒ, and in that the measurements of the difference ΔEz (t) = Ez (t, hj) - Ez (t, I12) during the pauses, as well as the amplitude and phase 30 of the difference AEz (f) = Ez (f, hi) -Ez (f, are carried out using a frül signal. An electromagnetic exploration method according to claims 3, 4 and 6, 7, characterized in that only one pair of electrodes is used by the receiver for measuring AEz, i.e. Ez (h $ s 0). 10 AEztf) = Eztfh,) - Eztfhi). 10. An electromagnetic exploration method according to claims 1-9, characterized in that all measurements for points and all distances performed in the time domain and frequency domain are jointly inverted for a determination of a 3D model of electrical resistance and polarization properties of the medium. An apparatus for electromagnetic exploration of the electrically resistive objects containing the potential hydrocarbon reservoirs, comprising: a vertical receiver immersed in the sea water and measuring the difference AEz = Ez (hi) - Ez (hz) 10 of the vertical electric field at the depths of hi and ti2, while a horizontal electric transmitter controls the rectangular pulses of electric current characterized by a steep slope of the trailing edge; said receiver is collinear with said transmitter at a distance R which can be smaller than the depth of the intended hydrocarbon reservoir; survey of said difference AEZ is carried out over the area possibly containing the hydrocarbon reservoirs; said survey was performed in the time domain or in the frequency domain, the data from said survey are analyzed for the construction of a 3D model of the hydrocarbon object and evaluation of its electrical resistance. An apparatus according to claim 11, characterized in that a ship simultaneously drags the transmitter and the receiver located at said distance R which is less than a depth to an object; said transmitter controls the rectangular pulses of electrical current with pauses; said receiver measures the difference ΔEz (t) = Ez (t, hj) - Ez (t, h?) during the pauses at time t where t is the time course after the electrical power pulse disconnection. An apparatus according to claim 11, characterized in that many said receivers are installed on the seabed along the profile and that the ship drags said transmitter along the same profile; for all receivers installed in points x /, X2, xn along said profile, the difference ΔEz (t, x) = Ez (t, x, hi) - Enfrxfa) is measured at different distances R where said transmitter is located at a point Xj + R and at a point xrR along said profile and where said difference is calculated: ArE ^ Ci xi) = [AEz (t, Xj + R) - AEzft, Xt-R)] / 2. An apparatus according to claim 11, characterized in that many said receivers are fully or partially embedded in the seabed rock along the profile and that the ship drags the transmitter along said profile; for all receivers installed in points xj, X2, ... Xj, .... xn along said profile, the difference ΔEz (t, x) is measured at different distances R, said transmitter being located at a point Xj + R and at a point x, ~ R along the said profile and where said difference is calculated: An apparatus according to claim 11, characterized in that a ship drags the transmitter and receiver simultaneously; said transmitter sends pulses of electrical current without pauses on a frequency ƒ and said receiver measures the amplitude and phase of the difference An apparatus according to claim 11, characterized in that many said receivers are installed on the seabed along the profile and that the ship drags said transmitter along said profile; for all receivers installed at points xj, X2, ... Xj, ..., xn along said profile, the amplitude and phase of the difference ΔEztf x) are measured at different distances R where said transmitter is located at a point Xj + R and at a point xrR along said profile and where the amplitude and phase of the difference are calculated: AREztf x) = [AEttf Xi + R) - AEztf, Xi-R)] / 2. An apparatus according to claim 11, characterized in that many said receivers are fully or partially embedded in the seabed rock along the profile and that the ship drags said transmitter along said profile; for all receivers installed in points x /, X2, ... Xt, ..., xn along said profile, the amplitude and phase of the difference ΔEztf, x) are measured at different distances R with said transmitter located at a point xt + R and 25 at a point x, ~ R along said profile and where the amplitude and phase of the difference are calculated: AREztf xt) = [AEztf Xi + R) -AEztf, xrR) \ / 2. An apparatus according to claims 11, 12, 13, characterized in that the transmitter sends pulses of the electric current with pauses on a frequency ƒ and that the measurements of the difference ΔEz (t) = Ez (t, hj) - Eztf h2) during the pauses, as well as the amplitude and phase of the difference AEztf) - Eztf, hi) - Eztf, hi) are performed using full signal. 5 An apparatus according to claims 13, 14 and 16, 17, characterized in that only one pair of electrodes is used by the receiver for measurements of AEz, i.e. Ez (h2) = 0. 1 0 38 9 7 Ö