The invention relates to a system for combined multi-dimensional electromagnetic- and seismic field characterization for use in geophysical surveying, i.e. full electromagnetic and seismic field characterization in different positions and orientations relative to the source, according to the preamble of claim 1.

The invention also relates to a method for simultaneous electromagnetic- and seismic geophysical surveying, i.e. full electromagnetic and seismic field characterization over all selected orientations and different source-receiver offsets, according to the preamble of claim 12.

Especially the invention relates to a system and method for the imaging of subsurface structures and electrical resistivities.

Background

Hydrocarbons in the subsurface show high resistivity to electromagnetic waves and can be indicated by transmitting an electromagnetic signal into the subsurface while recording the returning signal a range of distances from the source.

Several electromagnetic methods for mapping sub-seafloor resistivity have been developed. Marine controlled source electromagnetic (CSEM) surveying is one technique for mapping hydrocarbons, and is found described in e.g. the articles:

"Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola", in journal: Leading Edge (Volume 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 (volume 20, year 2002, pages 144-152), by T. Eidesmo; S. Ellingsrud; L. M. McGregor; S. Constable; M. C. Sinha; S. Johansen; F. N. Kong; H.

Westerdahl.

In a typical CSEM set-up, single-node receivers are placed on the seabed. Further, an electrical dipole-antenna transmits electromagnetic energy into the seabed, on a constant or variable frequency in different positions relative to the receivers. The receivers are retrieved and the recorded data are processed and interpreted.

A method and apparatus for offshore electromagnetic sounding utilizing wavelength effects to determine optimum source and detector positions is disclosed in US patent publication 4,617,518 A (L.J. Srnka (1986)) which comprise using a dipole source towed from a vessel, together with an array of electric dipole detectors. The potential differences between pairs of electrodes are measured. In addition to the dipole detectors being towed collinearly to the current source, a gradient detector array is towed laterally separated from- or beneath the current source.

US patent publication 6,236,211 Bl (Wynn (2001)) presents an induced polarization method for identifying minerals on the ocean floor, by towing a streamer cable, equipped with transmit and receiver electrodes, such that the free-end is close to or trenches the seabed.

From statutory invention registration USH1490H (Thompson et al. (1995)) it is proposed a geophysical prospecting method consisting of a streamer cable equipped with electromagnetic field sensors and potentially also hydrophones. Optionally, for noise-suppression purposes, a second streamer cable is located above the lower cable. A second vessel generating compressional energy may be used, and at appropriate porous subsurface formations the acoustic energy is converted to electromagnetic energy. The upwardly propagating energy is then measured by electromagnetic field sensors in the near bottom cable.

From US patent publication 20070075708 ( . Reddig; P. Heelan (2007)) it is known an electromagnetic survey system with multiple sources.

Patent publication WO2008008127 (P.J. Summerfield; L. S. Gale; B. J. Fielding (2008)) describes a method to maintain towed dipole source orientation.

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

Patent publication WO2007104949 (A. Ziolkowski (2007) disclose optimization of MTEM parameters.

Patent publication WO2008066389 (P. Barsukov; E. B. Fainberg; B. S. Singer (2008)) describes a method and apparatus for mapping hydrocarbon reservoirs in shallow waters and also an apparatus for use when practising the method.

From patent publication US2010/0045295 (R. Mittet; O. M. Aakervik; F. A. Maao; S. Ellingsrud (2008)) it is known a system and method for combined electromagnetic and seismic surveying comprising towing an EM field transmitter and seismic source and towing at least one streamer behind the vessel, said streamer or streamers having EM field receivers for measuring an electric field and seismic receivers for measuring a seismic response.

In patent publication US 2010/0172205 a combined electromagnetic and seismic surveying system and method is also described, comprising towing at least one sensor streamer, the streamer including a plurality of spaced apart electromagnetic field receivers disposed at spaced apart locations.

Both in US 2010/0172205 and US2010/0045295 a system and method is described for measuring the electrical field along the separate receiver streamers, thus making only the in-line sensor-streamer electrical field component measurable. Neither is describing a system or method for measuring the electrical field component between streamers relative to the in-line streamer component, hence there is no possibility to measure the electrical field in e.g. 3 dimensions at different separations to the source.

As seen in literature and presented patent applications, CSEM systems operate with either cable (streamer) or node-based receiver systems. Whereas cable-based systems have a limitation in e.g. measuring the cross-line and vertical component, the node-based systems face reduced operational efficiency, limited receiver-electrode separation in all directions (to the vicinity of the node) and lack of online data quality control. With the node receiver systems the acquisition on the seabed is spatially 'point-wise' on each receiver location, contrary to the towed cable-systems able to do continuous recording in-between positions. It follows that a cable-based system is superior for integrating seismic- and electromagnetic acquisition into one measurement system.

In marine environments and highly attenuating media, for depth penetration, the transmitted signals are of very-low-frequency character. The corresponding wavelengths will be very-long and for increasing dipole-antenna momentum the electrode separation should be increased accordingly.

The primary field from an electromagnetic source (e.g. a conducting loop), when turned on, will induce currents in the subsurface, in turn creating secondary electromagnetic fields.

For a CSEM frequency domain EM system a receiver measures the secondary field generated in the earth in the presence of the primary field.

Time domain EM systems usually record the secondary field in the absence of the primary signal from a source (i.e. source shut-off).

The implementation of the source function in CSEM systems may vary, among related to source-receiver separation and source and receiver electrode orientations.

None of the above mentioned articles or patent publications discloses a system or method for measuring the electrical field potential between electrodes arranged on separate streamers, for full electrical field characterization over different orientations at different source receiver separations.

By electrically interconnecting separate sensor-streamers and employing steerable streamer technology selectable receiver electrode separation, short to long distances, and sampling of any desired electrical field orientation at different and selectable separations to the source is obtained. Until present, none have sought to implement a hybrid measurement system including the advantages of all present CSEM systems. Integrating near- and far-field multi-dimensional electromagnetic measurements, combined with high-resolution seismic acquisition, yield improvement in sub-surface image quality and operational efficiency compared to prior art. Object

The main object of the invention is to provide a system and method which solves the above mentioned problems of prior art.

It is an object of the invention to provide a system and method for full electromagnetic- and seismic field characterizations in different positions relative to source.

An object of the invention is to provide a system and method for giving a high signal-to-noise ratio characterization, of the electromagnetic field components in all orientations and at different separations to the source, combined with high-resolution multi-component seismic acquisition.

It is an object of the invention to provide a system and method for the imaging of the earth's interior by characterization of subsurface geological structures and electrical resistivities.

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

The invention

A system according to the invention is described in claim 1. Preferable features of the system are described in claims 2-11.

A method according to the invention is described in claim 12. Preferable features of the method are described in claims 12-22. For full characterization of seismic and electrical field components a novel measurement system and method for performing measurements will be presented in the following.

For improved imaging of sub-surface geological formations the system according to the invention is arranged for giving a high signal-to-noise ratio characterization, preferably 3- dimensional, of electromagnetic field potential in water-column and in seabed, combined with high-resolution multi-component seismic acquisition.

The present invention is based on utilizing multiple streamer-cable receiver-systems, the streamer-cables electrically interconnected and provided with electrodes and sensors for performing towed and stationary electromagnetic and seismic data acquisition, potentially combined with invasive seabed measurements.

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

By employing streamers provided with 'birds' and buoyancy-effects, receiver electrode- separation and orientation is adjusted by steering the streamers in horizontal and vertical direction. Both in-line, cross-line, diagonal and vertical electromagnetic field components are extracted on and between the streamers at different source-receiver separations.

With towed- and stationary operation the invention is suited for large-area scan surveys and stationary mode combination of electrical field- and multi-component seismic- on and in seabed measurements.

A method according to the invention for data acquisition of seismic and electrical field components may be summarized in the following steps:

a) Move at least one survey vessels to a desired position over the seabed or move the survey vessel(s) with a mainly constant speed,

b) Release a specified volume of air into the water by the use of a seismic air gun to produce a steep-fronted shock wave every X second and transmit electromagnetic pulses by means of a an electrical transmit antenna,

c) Measure pressure (P-wave) and record 3-dimensional particle velocity (S-wave) by means of hydrophones and geophones (accelerometers) integrated in a multi streamer receiver system, respectively,

d) Measure magnetic flux by means of e.g. magnetic coils,

e) Measure electrical field potential between electrodes arranged and connected along and between the streamers ,

f) Perform the measurements in c), d) and e) at different positions relative to the source, and g) Store measurements locally and transfer to a control unit on the survey vessel for further processing.

Step a) includes that one survey vessel is towing a multi-streamer receiver system, an electric transmit antenna and a seismic air gun, or that one survey vessel is used for towing a multi- streamer receiver system and one survey vessel is towing the electric antenna and the seismic air gun.

Step a) includes instead of towing the multi-streamer receiver system by a vessel, placing it to the seabed, or arranging a part of it to the seabed.

Step a) may further include injecting electrode-equipped rods injected into the seabed.

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

The method may further include a step for optimization of the measurements in relation to noise by e.g. considering dipole moment, electrode separation and orientations, for then further processing. The method may further include a step including moving the survey vessel(s) to a new desired position and repeating steps a)-g).

The method may further includes adjusting receiver electrode separation, by steering the streamers, yielding full-field characterization of all electrical field components in all spatial directions at different source receiver separations.

Further details and preferable features of the invention will appear from the following example description.

Example

The invention will below be described in detail with the accompanying non-limiting drawings, wherein:

Figure la shows one embodiment of a system according to the invention,

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

Figure lc shows an extension of principle of the embodiment in Figure 1,

Figure 2a -e shows the system according to the invention in a stationary monitoring mode, where Figure 2e shows the system according to the invention operated by nodal measurement principle,

Figure 3 shows details of the receiver system according to the invention,

Figure 4 and 5 show some potentially sampled electrical field orientations between electrodes on and between streamers,

Figure 6 shows electrode-equipped rods injected into the seabed for noiseless measurements and two-media comparisons,

Figure 7a shows how the electrical field is measured between a front electrode on a lower streamer and an aft electrode on an upper streamer,

Figure 7b shows how the measurement system and principle can be implemented as a cable- node hybrid integrated onto streamer cables, and

Figure 8a-b show the use of several survey vessels for the operation of the system according to the invention.

A system according to the invention is based on multiple streamer receiver-systems - for towed and stationary seismic and electromagnetic data acquisition on and between streamers.

Reference is now made to Figure la-b which shows a first embodiment of system according to the invention. The system according to the first embodiment includes a survey vessel 11 provided with means for transmitting and recording electromagnetic energy in the form of a controlled source electromagnetic system. Means for transmitting energy into water column and seabed are preferably an electric antenna, such as an electromagnetic dipole source 12, and a seismic air gun 13 being towed behind the survey vessel 11.

The electromagnetic dipole source 12 and seismic air gun 13 are connected to the survey vessel 11 by means of one or more umbilical 14 via which the electromagnetic dipole source 12 and seismic air gun 13 are controlled and powered.

Means for recording electromagnetic energy is preferably on more streamers 15, arranged in a multi-streamer receiver system 16, which is arranged to measure the electrical field between electrodes on or connected between streamers. A streamer 15 refers to a cable-based receiver- solution with real-time communication and data-transfer with the survey vessel 11. Streamers are electrically connected to each other for measurements between electrodes 19 on separate streamers.

Towed and stationary acquisition is feasible, with continuous connection 17 either to the survey vessel 11 or a radio-transmitting buoy 18, as shown in Figures 2a-d.

A multi-streamer receiver-system 16 consists of multiple streamers 15, such as four in the example. Separate streamers 15 are electrically interconnected and provided with electrodes 19, arranged on said streamers 15 electrically connected to a point 20 for voltage measurements between electrodes 19. By employing steerable streamer-technology, i.e. streamers 15 provided with streamer steering devices, such as "birds" (not shown), the electric field can be sampled over short to long distances 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, here arranged in front of the streamers 15. Additionally, for improved structural imaging, the streamers 15 are provided with multi-component seismic sensors 22, for measuring pressure and 3-dimensional particle velocity. The streamers 15 include at least two receiver electrodes 19 for each streamer 15, electrically connected to electrodes on separate streamers, for in-line measurements along the streamer 15, or for cross-line, vertical or diagonal measurements between electrodes 19 on the separate streamers 15. As regards seismic there is at least a need for one hydrophone for pressure and geophone for 3-D movement, but for increasing redundancy and enhancing seismic data quality, several are implemented.

As shown in Figure la-b the electromagnetic dipole source 12 and seismic air gun 13 may be arranged either in the front of the multi-streamer receiver systems 16 or behind the multi- streamer receiver systems 16. The choice of where they are positioned is of practical matter.

A system according to the invention will thus provide combined full seismic and electromagnetic field acquisition, and the multiple-streamer receiver systems 16 may be utilized with towed- and stationary optionality. Reference is now made to Figures 3 and 4 where the multiple-streamer receiver systems 16 are highlighted. In addition Figure 4 is showing the different parameters which are measured, i.e. magnetic flux H, electrical field potential E, pressure P and particle velocity V.

Note that the arrows in Figure 4, 5, 6 and 7 indicates some of the potentially sampled electrical field orientations between electrodes 19 on and between streamers 15.

Reference is now made to Figure 6 which shows another embodiment of the invention arranged for noiseless measurements. This is achieved by that the system includes electrode-equipped rods 25 injected into the seabed 26, where the rods 25 equipped with electrodes 19 are electrically connected to the streamer electrodes 19 and measurements transfers via the streamer 15. In this embodiment the electromagnetic dipole source 12 is arranged in the vertical direction and is positioned close to the seabed 26, possibly arranged to the seabed 26.

Reference is now made to Figure 7a which shows how the electrical field is measured between an electrode 19 arranged in the front part of a lower streamer 15 and an electrode 19 arranged in the rear part of an upper streamer 15. The signal is amplified 27 and sampled 28, stored to a memory (not shown), for example flash, and transferred 17 real-time to the vessel 11.

How the system works will now be described. A target 23 is illuminated by the electromagnetic dipole source 12 and seismic air gun 13. Simultaneously the scattered electrical field is measured between the electrodes 19, placed on the same streamer 15 or connected between separate streamers 15, together with pressure and particle velocity measured by the multi-component seismic sensors 22.

The measurements are transferred through the continuous connection 17 to the survey vessel 11. The survey 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 seismic air gun 13. The control unit may also be arranged for controlling the steerable streamers 15 via control devices arranged thereto for achieving desired electrode 19 separation.

As an example the control unit may be arranged for processing the received signals according to source switching-frequency and source-receiver separations. Accurate positioning of the source- and receiver-systems is realized by advanced inertial motion unit systems, hydro-acoustics and global positioning systems (GPS), which is not shown in any of the Figures.

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

The seismic air gun 13 shoots every X second where hydrophones in the streamer 15 record pressure (P-wave) and geophones in the streamer (accelerometers) record 3-dimensional particle velocity (S-wave). The electrical antenna, i.e. the dipole source 12, transmits signal of constant or varying frequencies, and the scattered electrical field is measured between electrodes 19 on or between streamers 15, with the possibility of invasive measurements via rods 25 equipped with electrodes 19.

An example of data acquisition with a system according to the invention for towed monitoring mode will now be described. The towed mode, where both transmit source and receiver system are towed, will usually be used for scan surveys, i.e. where large areas is to be surveyed quickly.

The vessel 11 is moving from position 1 to position 2, where the seismic air gun 13 shoots every X second and hydrophones record pressure and geophones in the streamer (accelerometers) record 3-dimensional particle velocity (S-wave) as for the stationary monitoring mode. The electrical antenna, i.e. the dipole source 12, transmits electromagnetic signal of constant or variable frequency f, at the same time as the electrical and magnetic field is measured over different orientations and at different separations to the source. As regards the processing of the measured signals, one can, for example, perform harmonic decomposition of the data, c.f. Fourier transformation. Other examples for processing of signals like this are, for example, described in the articles of Eidesmo, Ellingsrud et al. as mentioned in the background.

Possible reservoir structures and electrical resistive anomalies which are discovered during a scan survey are preferably analyzed in stationary mode.

The above examples show descriptions of the different acquisition modes, i.e. acquiring high- resolution data in stationary monitoring mode in position 1 and position 2, while utilizing towed mode when moving from position 1 and position 2.

For both stationary and towed monitoring mode the selection of electrodes 19, for electrical field measurements between, may be prefixed or selective.

As shown in the Figures the streamer configurations are adapted to desired properties for the use of the system.

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

It is thus obvious for a skilled person that the above described embodiments may be combined and modified and form new embodiments.

Modifications

With the multiple-streamer receiver and source system all kinds of operation is allowed. That is, stationary- or towed source and receivers, towed source and stationary receivers and vice-versa. The source is not necessarily a dipole-antenna with only two electrodes. Antenna beam forming may be achieved by utilizing more electrodes, and by regulating the electrical potential differences in between the electrodes.

The survey may also be performed by utilizing several vessels, as shown in Figures 8a and 8b, where one vessel ll ¹ serves/tows the dipole source 12 and one vessel ll ² serves/tows the multiple streamer systems 16.

The multiple streamer receiver systems include a number 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.