NO336422B1 - System and method for simultaneous electromagnetic and seismic geophysical mapping - Google Patents

Description

System and method for simultaneous electromagnetic and seismic geophysical mapping

The invention relates to a system for simultaneous electromagnetic and seismic geophysical mapping, i.e. full electromagnetic and seismic field characterization, in accordance with the introduction to patent claim 1.

The invention also applies to a method for simultaneous electromagnetic and geophysical field characterization, in accordance with the introduction to patent claim 10.

In particular, the invention relates to a system and a method for imaging underground structures and electrical resistivities, as well as detection and monitoring of hydrocarbon reservoirs.

Background

Hydrocarbons in the subsurface are significantly more resistive to electromagnetic waves than layers that do not contain hydrocarbons, and can therefore be detected by sending an electromagnetic signal into the subsurface while simultaneously measuring the returned signal from different distances to the source.

Several electromagnetic methods for mapping the subsoil's resistivities have been developed. Marine CSEM ("Controlled Source ElectroMagnetic") mapping is a technique for mapping hydrocarbons, and is described, for example, in the following articles: - "Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola", in journal: Leading Edge (issue 21, year 2002, pages 972-982), by S. Ellingsrud; T. Eidesmo;

L.M. McGregor; S. Constable; M. C. Sinha; and

- "A new method for remote and direct identification of hydrocarbon filled layers in deep-water areas", in journal: First Break (issue 20, year 2002, pages 144-152), byT. Eidesmo; S. Ellingsrud; L.M. McGregor; S. Constable; M. C. Sinha; S. Johansen; F. N. Kong; H. Westerdahl.

In a typical CSEM setup, single-node receivers are placed on the seabed. Furthermore, an electric dipole antenna sends electromagnetic energy into the seabed, with a constant or varying frequency and from different positions in relation to the receivers. Finally, the recipients are picked up and registered data is processed and interpreted.

A method and system for offshore electromagnetic sounding that utilizes wavelength effects to determine optimal source and detector positions is also described in US 4,617,518 A (L.J. Srnka (1986)) which involves the use of a dipole source towed by a vessel, along with an array of electric dipole detectors. The voltage difference between pairs of electrodes is measured. In addition to the dipole detectors towed collinearly to the current source, an array of gradient detectors is towed laterally separated, from or below the current source.

US 6,236,211 B1 (Wynn (2001)) presents an induced polarization method for identifying minerals on the seabed, by towing a streamer cable, equipped with transmitting and receiving electrodes, so that the four ends are close to or burrow into the seabed.

In USH1490H (Thompson et al. (1995)) a geophysical prospecting method consisting of a streamer cable equipped with electromagnetic field sensors and possibly also hydrophones is proposed. Optionally, for noise reduction purposes, a second streamer cable is located above the lower cable. A second vessel that generates pressure energy can also be used, and with suitable porous formations in the underground, the acoustic energy is converted into electromagnetic energy. The energy that propagates upwards is then measured by electromagnetic field sensors in cables near the seabed.

From US 20070075708 (R. Reddig; P. Heelan (2007)) an electromagnetic mapping system with multiple sources is known.

WO2008008127 (P.J. Summerfield; L.S. Gale; B.J. Fielding (2008)) describes a method for maintaining rope dipole-source orientation.

From US 7,203,599 Bl (K.M. Strack; L.A. Thomsen; H. Rueter (2007)) a method for collecting transient electromagnetic data is known.

WO2008066389 (P. Barsukov; E. B. Fainberg; B. S. Singer (2008)) describes a method for mapping hydrocarbon reservoirs in shallow water and also a system for use in carrying out the method.

Combined electromagnetic and seismic mapping is known from GB2441786 (R. Mittet; O. M. Aakervik; F. A. Maao; S. Ellingsrud (2008)).

In WO2007104949 (A. Ziolkowski (2007) optimization of MTEM parameters ("Multi-Transient EM") is presented.

US 2010/0172205 Al describes a system and a method for nearly simultaneous execution of electromagnetic and seismic geophysical mapping.

As can be seen from the literature and presented patent publications, CSEM systems operate either with node or cable-based receiver systems. While cable-based systems have a limitation in measuring lateral and vertical components, the node system has reduced operational efficiency, limited receiver electrode separation in all directions (to the vicinity of the node) and lack of real-time data quality control. With the node systems, the data collection is point-by-point on the seabed at each receiver location, in contrast to the cable systems, which can do continuous collection also between positions. It also follows that cable-based systems are preferred for integrating seismic and electromagnetic data collection in one measurement system.

In marine environments and strongly attenuating medium, for deep penetration, the emitted signals must be of a very low frequency nature. Corresponding wavelengths will be very long and for increased dipole antenna moment the electrode separation should be increased accordingly.

An electromagnetic source (eg a conducting loop), when switched on, will induce currents in the subsurface, which in turn generate secondary electromagnetic fields. The measured field at a receiver is the total field (primary + secondary). When the primary field is turned off, only the secondary field will be present, from which discharges will be converted into conductivity to the subsoil.

For a CSEM frequency domain EM system, a receiver measures the secondary field of frequency generated by a source field. However, in all frequency-domain EM systems, the secondary field generated in the earth by the presence of the primary field is always detected, limiting the detection of the secondary signal.

Time domain EM systems are usually recorded in the absence of the primary signal, making the secondary signal more detectable.

The implementation of the source function in CSEM systems varies, related to source-receiver separation, source and receiver separation, source and receiver electrode orientation, and separation and source transfer scheme. For near-field measurements, i.e. shorter separation between source and receiver, a transient (time domain) methodology is used which measures the secondary field both in the presence and absence of the primary field. For far-field measurements, i.e. longer separation between source and receiver, a plane-wave approximation and harmonic decomposition of registered field (primary and secondary) can be used.

None of the above articles or patent publications disclose a system or method for measuring the electric field potential between electrodes on separated streamers. Application of steerable streamer technology results in selectable receiver-electrode separation, from short to long distances, and sampling of any desired electric field orientation. Until now, no one has sought to implement a "cable node" hybrid measurement system encompassing the advantages of all current CSEM systems. Integration of near- and far-field measurements with the possibility of invasive measurements, combined with high-resolution seismic acquisition results in significantly improved image quality of the subsurface and operational efficiency compared to known techniques.

Purpose

The main purpose of the invention is to provide a system and a method that solves the above-mentioned challenges with known technology.

It is an object of the invention to provide a system and a method for full electromagnetic and seismic field characterization.

It is also an object of the invention to provide a system and a method for imaging the Earth's interior.

It is an aim of the invention to provide a system and a method for characterizing geological structures and electrical resistivities in the subsoil.

One purpose of the invention is to provide a system and a method for creating characterization with a high signal-to-noise ratio, preferably 3-dimensional, of electromagnetic field potential in water bodies and seabeds, combined with high-resolution multi-component seismic acquisition of the subsurface.

An object of the invention is to provide a system and a method for the detection and monitoring of hydrocarbon reservoirs, and also for groundwater and mineral exploration.

The invention

A system in accordance with the invention is stated in patent claim 1. Advantageous features of the system are described in patent claims 2-9.

A method in accordance with the invention is stated in patent claim 10. Advantageous features of the method are described in patent claims 11-17.

For full characterization of seismic and electric field components, a new system and method for carrying out measurements will be presented below.

For improved imaging of geological formations in the subsurface, the system in accordance with the invention is designed to provide characterization with a high signal-to-noise ratio, preferably 3-dimensional, of electromagnetic field potential in water bodies and seabed, combined with high-resolution multi-component seismic acquisition.

The invention is based on the use of a multi-streamer receiver system equipped with electrodes for carrying out the rope and stationary seismic and electromagnetic data collection, possibly combined with invasive seabed measurements.

By measuring voltage between electrodes (on or between separate streamers), electric field components over that orientation can be extracted.

When using streamers equipped with steering devices ("birds") and buoyancy effects, receiver electrode separation and orientation can be adjusted by steering the streamers in horizontal and vertical directions.

Electromagnetic field components along ("in-line"), across ("cross-line"), diagonally and vertically can be extracted.

With both towed and stationary operation, the invention is particularly suitable for mapping large areas, and combined with a stationary mode, measurement of electric fields in bodies of water, as well as multi-component seismic and seabed invasive measurements.

A method according to the invention for data collection for full characterization of seismic and electric field components can be summarized in the following steps: a) move at least one survey vessel to a desired position above the seabed or move the survey vessel(s) at a substantially constant speed, b) illuminate a target by releasing a specific volume of air into the water using a seismic air gun to produce a steep-fronted pressure wave every X seconds and emit

pulses of varying frequency, with stop times, using an electric antenna,

c) measure pressure (P-wave) and record 3-dimensional seabed velocity ("seabed velocity") (S-wave) using hydrophones and respectively geophones (accelerometers) in a multi-streamer receiving system, d) measure magnetic flux and electric field potential between electrodes arranged in the streamers of the multi-streamer receiver system, and e) transfer measurements and records to a control unit on board the survey vessel for further processing.

Step a) involves a survey vessel towing a multi-streamer receiver system, an electrical antenna and a seismic air gun, or a survey vessel towing a multi-streamer receiver system and another vessel towing the electrical antenna and seismic air gun.

Step a) comprises, instead of towing the multi-streamer receiver system with a vessel, deploying it to the seabed, or deploying part of it to the seabed.

Step a) can further include injecting rods equipped with electrodes into the seabed.

Step e) includes further processing of the registered data depending on source-receiver separation and source-transmission scheme.

The method can further include a step f) comprehensive optimization of the measurements in relation to noise by taking into account dipole moment, electrode separation and orientation, and then process the signals electronically.

The method may further comprise a step g) comprising moving the mapping vessel(s) to a new desired position and repeating steps a)-e) or a)-f).

The method can further include adjusting electrode separation by controlling the streamers in all spatial directions, resulting in full field characterization.

Further details and preferred features of the invention will appear from the following exemplary description.

Example

The invention will be described below in detail with reference to the non-limiting figures, where:

Figure la shows an embodiment of the system in accordance with the invention,

Figure 1b shows an alternative solution of the embodiment in Figure 1,

Figure 2a-b shows the system in accordance with the invention in stationary monitoring mode,

Figure 3 shows details of the receiver system in accordance with the invention,

Figures 4 and 5 show some potentially sampled electric field orientations between electrodes on and between streamers, Figure 6 shows rods equipped with electrodes injected into the seabed for noise-free measurements and two-medium comparisons, Figure 7 shows a possible embodiment of the electromagnetic source and the receiver system, Figure 8 shows how the electric field is measured between a front electrode on a lower streamer and an aft electrode on the upper streamer,

Figure 9 shows another embodiment in accordance with the invention, and

Figure 10a-b shows the use of several surveying vessels for the operation of the system in accordance with the invention.

A system in accordance with the invention is based on a plurality of streamer-receiver systems for the rope and and stationary seismic and electromagnetic data collection, optionally combined with invasive seabed measurements.

Reference is now made to Figure la-b which shows a first embodiment of the system in accordance with the invention. The system in accordance with the first embodiment comprises a survey vessel 11 provided with means for sending and recording electromagnetic energy in the form of a CSEM system ("controlled source ElectroMagnetic"). Means for sending energy into the body of water and the seabed are preferably an electric antenna, such as an electromagnetic dipole source 12, and a seismic air cannon 13 which is towed behind a survey vessel 11.

The electromagnetic dipole source 12 and the seismic air gun 13 are connected to the mapping vessel 11 by means of one or more cables ("umbilical") 14, via which(s) the electromagnetic dipole source 12 and the seismic air gun 13 are controlled and operated.

Means for recording electromagnetic energy are preferably one or more streamers 15, arranged in a multi-streamer receiver system 16, which are arranged to measure a scattered field in a body of water and seabed. A streamer 15 refers to a cable-based receiver solution with real-time communication and data transmission with the mapping vessel 11.

Both roped and stationary collection is possible, with continuous connection 17 either to the mapping vessel 11 or a radio transmitting buoy 18 as shown in Figures 2a-b, for online data quality control.

A multi-streamer receiver system 16 consists of a plurality of streamers 15, such as four in the example. The streamers 15 are provided with electrodes 19, for example of silver chloride material, arranged on said streamers 15, and are electrically connected to a point 20 for voltage measurements. By using controllable streamer technology, i.e. streamers 15 provided with control devices ("birds") (not shown), the electric field can be sampled over distances from meters to hundreds of meters across any desired orientation.

For increased electromagnetic redundancy, the system is preferably provided with additional means for measuring the magnetic field, such as a three-axis coil 21, arranged in front of the streamers 15.1 In addition, for improved structural imaging, the streamers 15 are provided with multi-component seismic sensors 22 for measuring pressure and 3-dimensional seabed velocity ("seabed velocity"), as well as extracting pressure and shear waves.

The streamers 15 comprise at least two receiver electrodes 19 for each streamer 15, for in-line measurements along the streamer 15, or across ("cross-line"), vertical or diagonal measurements between electrodes 19 on separate streamers 15. As for seismic, it is needed only one hydrophone for pressure and one geophone for 3D motions, but for increased redundancy and better seismic data quality a plurality is preferably implemented.

As shown in Figures la-b, the electromagnetic dipole source 12 and the seismic air gun 13 can be arranged either at the front of the multi-streamer receiver systems 16 or behind the multi-streamer receiver systems 16. The choice of where they are placed is for practical reasons and has no effect as far as the measurements are concerned.

A system in accordance with the invention will in this way provide combined seismic and electromagnetic collection, and the multi-streamer receiver system 16 can be used with roped and stationary freedom of choice.

Referring now to Figures 3 and 4 where the multi-streamer receiver system 16 is highlighted. In addition, Figure 4 shows the various parameters that are measured, i.e. magnetic flux H, electric field potential E, pressure P and seabed velocity V.

It is noted that the arrows in Figures 4, 5 and 6 indicate some of the potentially sampled electric field orientations between electrodes 19 on and between streamers 15.

Reference is now made to Figure 6, which shows another embodiment of the invention designed for noise-free measurements and two-medium comparison of electromagnetic data. This is achieved by the system comprising rods 25 provided with electrodes injected into the seabed 26. In this way, the system provides the possibility of performing a combination of invasive and on-seabed electric field measurements. The rods 25 provided with electrodes 19 are electrically connected to the streamer electrodes 19 and transmit measurements via the streamer 15. In this embodiment, the electromagnetic dipole source 12 is arranged in the vertical direction and positioned close to the seabed 26, possibly arranged to the seabed 26.

Reference is now made to Figure 7 which shows a possible embodiment of the electromagnetic source and receiver system. Here, a multi-streamer receiver system 16 is arranged to the seabed 26 with a U shape and provided with rods 25 at each corner and arranged on one side of a target 23 to be explored and a multi-streamer receiver system 16 is arranged to the seabed 26 on the other side of the target 23.1 this embodiment, the streamers 15 are arranged parallel to each other.

Reference is now made to Figure 8, which shows how the electric field is measured between an electrode 19 arranged in the front part of a lower streamer 15 and an electrode 19 arranged at the aft end of an upper streamer 15. The signal is amplified 27 and sampled 28, stored in a memory ( not shown), for example flash, and is transferred 17 in real time to the vessel 11.

By using hardwired multiplexing 29, the receiver electrode configuration is optional, making it possible to switch through any electrode separation on demand.

How the system works will now be described. A target 23 is illuminated with the electromagnetic dipole source 12 and the seismic air gun 13. At the same time, the scattered electric field is measured between the electrodes 19, on the same streamer 15 or on vertically and/or horizontally separated streamers 15, together with the measurement of pressure and velocity with multi-component seismic sensors 22.

The measurements are transmitted through a continuous link 17 to the mapping vessel 11. The mapping vessel 11 is provided with a central control unit (not shown) provided with means and/or software for controlling the system, i.e. controlling the electromagnetic dipole source 12 and the seismic air gun 13 , and processing of received signals, i.e. measurements of the scattered electric and seismic field, together with pressure and velocity. The control unit can also be arranged for control of the controllable streamers 15 via control devices arranged thereto in order to achieve the desired electrode separation. The position of the streamers 15 can also be controlled by a control system designed for this, known from prior art, but is then provided with input from the control unit.

As an example, the control unit can be arranged to process the received signals with regard to source-frequency switching and source-receiver geometries, where the processing scheme is in accordance with the transmission parameters used. Accurate positioning of the source and receiver systems is realized through advanced motion sensor systems, hydroacoustics and global positioning systems (GPS), which are not shown in any of the figures.

An example of data collection with a system according to the invention for stationary monitoring will now be described. In stationary mode, the vessel is positioned in a desired position and the multi-streamer receiver systems 16 are lowered to the seabed and possibly rods equipped with electrodes are arranged into the seabed.

The seismic air cannon 13 fires every X seconds where hydrophones in the streamer 15 record pressure (P-wave) and geophones in the streamer (accelerometers) record 3-dimensional seabed velocity ("seabed velocity") (S-wave). The electric antenna, i.e. the dipole source 12, emits pulses of constant or varying frequency, with stop times, where scattered electric fields are measured between electrodes 19 on streamers 15, with the possibility of invasive measurements via rods 25 provided with electrodes 19.

An example of data collection with a system in accordance with the invention for the rope monitoring mode will now be described. In towed mode, both the sending and receiving systems are towed, which mode is usually used for investigative mapping, i.e. where large areas are to be mapped quickly. The vessel 11 moves from position 1 to position 2, where the air cannon 13 fires every X seconds and hydrophones record pressure as for the stationary monitoring mode. The electric antenna, i.e. the dipole source 12, transmits electromagnetic signals with constant or variable frequency f, while the electric and magnetic field is measured from different distances to the source, and for the far-field case, harmonic decomposition can be applied to recorded data. As regards the processing of the measured signals, one can, for example, carry out harmonic decomposition of the data, compared to Fourier transformation. Other examples of processing such signals are described, for example, in the articles by Eidesmo, Ellingsrud et al., as mentioned under background.

Possible reservoir structures and electrical resistive anomalies discovered during an exploratory survey are preferably analyzed in stationary mode.

An alternative to the investigative mapping described above is that the air cannon fires and pressure and 3D movement of the seabed are measured. The electric antenna, i.e. the dipole source 12, sends electromagnetic signals with a constant frequency f, but also signals with stop times, and that the electric and magnetic fields are measured from different distances from the source. As regards processing of the measured signals, both harmonic and transient approaches can be used, as described in the background above.

The examples described above show descriptions of different collection modes, i.e. collection of high-resolution data in stationary monitoring mode in position 1 and position 2, while using tethered mode when moving from position 1 to position 2.

With measurements comes noise. In accordance with the invention, the system includes means for optimizing the measurements using measurement principles, such as dipole moment, by adjusting electrode separation and orientation.

For both stationary and tethered monitoring modes, the choice of electrodes can be preselected or is selective.

As shown in the figures, the streamer configurations are adapted to the desired characteristics for the use of the system. For example, as shown in Figure 9, two and two streamers can be arranged in two parallel planes. Accordingly, the streamer configuration may comprise streamers in one or more planes, and each plane may comprise one or more streamers.

The streamers in accordance with the invention can be towed either in the longitudinal direction of the streamers, in the vertical direction of the streamers or a combination of these.

The above description has shown that there are many possibilities with the present invention, and many configurations are possible.

It is obvious to a person skilled in the art that the above-described embodiments can be combined and modified to form new embodiments.

Modifications

With the multi-streamer source and receiver system, all types of operations will be allowed. That is stationary or towed sources and receivers, towed source and stationary receivers and vice versa.

The source is not necessarily a dipole antenna with only two electrodes. Antenna beamforming can be achieved by using several electrodes, as well as by regulating the potential difference between them.

The mapping can be carried out using several vessels, as shown in Figures 10a and 10b where a vessel li<1>operates/tows the dipole source 12 and a vessel li<2>operates/tows the multi-streamer receiver systems 16.

The multi-streamer systems comprise at least one streamer, but may also comprise a plurality of streamers arranged in a desired configuration, such as one upper and three lower streamers, two upper and two lower streamers, but these are only a few examples.

Claims (18)

1. System for simultaneous electromagnetic and seismic geophysical mapping, which system comprises: - at least one multi-streamer receiver system (16) which comprises at least one streamer-receiver cables (15) equipped with at least one multi-component seismic sensors (22) for measurement of pressure and 3-dimensional movements, - at least one seismic air cannon (13) adapted for towing behind a vessel (11, li<1>, li<2>), - at least one electromagnetic source (12) adapted for towing behind a vessel (11, li1, li2), - where the at least one streamer-receiver cable (15) is provided with electrodes (19) for measuring electric fields, characterized in that the system comprises at least one device for vertical, orthogonal or diagonal electric field measurements in relation to the at least one streamer (15), and with different separations from the source (12), and - a measuring device for measuring other electromagnetic field components transversely in an arbitrarily selectable orientation in relation to the at least one streamer cable, and where multi-stre the amer receiver systems (16) are adapted for towing behind a vessel (11, li<1>, li<2>) and/or for deployment on the seabed (26). 2. System in accordance with patent claim 1, characterized in that electrodes (19) separated between streamers (15) are electrically connected to a point (20) for voltage measurement between the electrodes (19), for extracting the electric field component between given electrodes (19) on the separated streamers (15). 3. System in accordance with patent claim 1, characterized in that the system comprises rods (25) provided with electrodes, which rods (25) are injected into the seabed (26) to provide a combination of invasive and on-seabed electric field measurements. 4. System in accordance with patent claim 3, characterized in that the electrodes (19) on the rods (25) are electrically connected to the streamer electrodes (19). 5. System in accordance with patent claim 1, characterized in that the system, for increased electromagnetic redundancy, includes additional devices for measuring magnetic fields, such as with a three-axis coil (21), arranged in front of the streamers (15). 6. System in accordance with patent claim 1, characterized in that the multi-streamer receiver systems (16) are arranged directly to a mapping vessel (11, li<1>, li<2>) by means of a continuous connection (17) or via a radio transmitting buoy(18). 7. System in accordance with patent claim 1, characterized in that the streamers (15) of the multi-streamer receiver systems (16) are individually controllable in water in that they are provided with control devices for control of lateral and vertical position. 8. System in accordance with one of the patent claims 1-7, characterized in that the selection of electrodes (19) for measurements is selected in advance or is selective. 9. System in accordance with one of the patent claims 1-8, characterized in that the system comprises a control unit equipped with a device for: - control of the electric antenna (12) and the seismic air cannon (13), - control of the position of the controllable streamers ( 15) for achieving the desired separation of the electrodes (19), - processing of received signals, - accurate positioning of the source and receiver systems, i.e. the electric antenna (12), the seismic air gun (13), the streamers (15), the electrodes (19) and the rods (25) provided with electrodes, - optimization of measured signals. 10. Method for simultaneous electromagnetic and seismic geophysical mapping, where the method comprises the following steps: a) moving at least one mapping vessel (11, li<1>, li<2>) to a desired position above a seabed (26) or moving the mapping vessel the /'s (11, li<1>, li<2>) at a substantially constant speed over the seabed (26), b) illuminating a target (23) by releasing a specified volume of air into the water using a seismic air gun (13) to produce a step-shaped ("steep-fronted") pressure wave every X seconds and transmit electromagnetic pulses of constant or varying frequency, with or without stop times, by means of an electrical antenna (12), c) to measure pressure and recorded 3-dimensional movements using hydrophones and geophones in at least one streamer (15) of a multi-streamer system (16), d) measuring magnetic flux and electric field potential between electrodes (19) arranged in the streamers (15) ), e) to measure electric field potential over electrodes (19) sepa routed between streamers (15), f) transferring the measurements and registrations to a control unit on the mapping vessel (11, li<1>, li<2>) for further processing; g) to perform measurements and records vertical, orthogonal or diagonal electric field measurements in relation to at least one streamer, with different separations from the source; h) performing the measurements and the recordings of other electromagnetic field components transversely in an arbitrarily selectable orientation relative to the at least one streamer; and i) to collect data in towed and/or stationary operations. 11. Method in accordance with patent claim 10, characterized by 3-dimensional electric field characterization at different distances to the source, by measurement between electrodes (19) vertically, horizontally, laterally separated over the separated streamers (15). 12. Method in accordance with patent claim 10, characterized in that step a) comprises that a mapping vessel (11, li<1>, li<2>) tows a multi-streamer receiver system (16), an electric antenna (12) and a seismic aerial gun (13), or that a survey vessel (li<1>, li<2>) is used to tow the multi-streamer receiver system (16) and a survey vessel (li<1>, li<2>) tows it the electric antenna (16) and the seismic air gun (13). 13. Method in accordance with patent claim 10, characterized in that step a) comprises arranging the multi-streamer receiver systems (16) to the seabed (26) instead of towing them with a survey vessel (11, li<1>,li<2 >). 14. Method in accordance with patent claim 10, characterized in that step a) comprises arranging the electric antenna (12) and/or the seismic air gun (13) to the seabed (26) instead of towing them with a mapping vessel (11, li) <1>, li<2>). 15. Method in accordance with patent claim 10, characterized in that step a) comprises injecting rods (25) provided with electrodes into the seabed (26). 16. Method in accordance with patent claim 10, characterized in that the method includes a step f) extensive optimization of the measurements with regard to noise. 17. Method in accordance with one of the patent claims 10-16, characterized in that the method comprises a step g) for moving the chart egg vessel(s) (11, li<1>, li<2>) to a new desired position and repetition of steps a)-e) or a)-f). 18. Method in accordance with one of the patent claims 10-17, characterized in that the method comprises adjusting the separation of the receiver electrodes (19) by controlling individual streamers (15) in the multi-streamer receiver system (16), for adjusting the orientation of measured electrical field, in all spatial directions for full field characterization.