WO2012036559A1

WO2012036559A1 - E-field sensor for marine streaming

Info

Publication number: WO2012036559A1
Application number: PCT/NO2011/000249
Authority: WO
Inventors: Yme Asgeir Kvistedal
Original assignee: Norges Geotekniske Institutt
Priority date: 2010-09-13
Filing date: 2011-09-13
Publication date: 2012-03-22
Also published as: NO20101277A1; NO332630B1

Abstract

The invention is a marine geophysical sensor cable comprising a hose skin (1) generally filled with gel (3) and comprising electrical sensors comprising pairs of first and second sea water exposed electrodes (5a1, 5b1) connected to an electrical amplifier (7a1). The first electrode (5b1) is connected to the amplifier (7a1) via a tubular electric conductor (11) arranged along the inner face of the streamer hose skin (1). The tubular electric conductor (11) is generally insulated from the sea water and the second electrode (5a1).

Description

E-Field sensor for marine streaming

Introduction Electric field sensors are used as receivers in controlled source electromagnetic (CSEM) surveys offshore. Such surveys are conducted for prospecting for subsea geological formations with distinct resistivity properties such as hydrocarbon reservoirs .

General background art

Marine electric field sensors typically comprise pairs of two electrodes (5a, 5b) along a streamer cable or streamer hose (1), please see Fig. 1. Fig. 1 is an illustration of

background art wherein two electrodes are arranged on a gel- filled streamer hose and in galvanic contact with the sea- water, connected by a slack electrical conductor generally centered in the streamer hose. The streamer hose may be filled with gel (3) . Each electrode (5a, 5b) is galvanically

connected to the sea, and positioned with a mutual separation ranging from a few metres to several hundred metres along the streamer cable. The electrodes (5a, 5b) are connected via ordinary electrically conducting wires (9a, 9b) to input terminals (71a, 71b) of one (or more) usually preamplifiers (7) . The output signal from each of these sensors, i.e. the pre-amplified, preferably digitized output signal indicating a differential voltage (Δν) , is typically acquired by data loggers on the seafloor or other types of seismic recording systems streaming the data directly to a survey vessel .

WO2010/002263A2 Johnstad describes an electromagnetic and seismic streamer cable and a method for use of such a streamer cable. The streamer cable comprises sensor cable sections of which each is provided with seismic and electromagnetic sensors along the cable, wherein the seismic sensors comprise a hydrophone and a seismic component sensor for seismic vector measurements for being conducted while the sensor cable resides at the seafloor, and wherein the electromagnetic sensors comprise E- field sensors and H- field sensors, the E- field sensors comprising pairs of first and second electrodes arranged with different positions along the cable and

connected to a voltage amplifier. The WO-application describes that the fact that the electrodes are moved through ion- containing water will induce electrical noise to the

measurements. The electrode pairs are connected via ordinary electrical wire conductors to each their amplifier, please see p. 4 of the WO publication.

WO2005/012947 Sodal describes a towed electromagnetic high power current source for controlled- source electromagnetic measurements. The transmitter source comprises two large transmitter electrodes which are provided with power via the screen of a coaxial cable with a large conductor cross - section, 250 mm², typically conducting 120 Volt x 1000 A, please see p. 2. Thus Sodal describes a power cable to

electrodes in a system which delivers a power of 120000 Watt through a coaxial cable, to transmitter electrodes.

US2009/0001987 Davidsson describes a marine electromagnetic survey cable which has a reference electrode that extends essentially along the entire length of the cable, and a number of separate sensor electrodes which are arranged along the cable and wherin each is electrically insulated from the reference electrode. A voltage measurement circuit is

connected between each measurement electrode and the reference electrode . Problems related to the background art

Existing survey data both from geophysical exploration at sea and experiments conducted with EM- sensor cables of the

background art have shown that the signal to noise ratio of marine e- field sensors depends on how the sensors are deployed and operated in the sea. Sensors that have been built into seismic streamers and towed behind a boat typically gives measurements with noise levels that are several orders of magnitude higher than sensors of generally identical design, but arranged stationary on the seafloor. The increased noise has been experimentally determined as being magnetically induced, and related to the motion of the electrode cables with respect to the geomagnetic field. The streamer hose is subject to tensile forces both during handling on deck, launching, towing, and retrieval, and may be slightly

stretched. The signal conductor cable between the electrodes and the pre-amplifier is vulnerable in that it may break if tensioned when the streamer hose is stretched. In order to compensate for this the signal conductor cable has to be given some slack in order to avoid such breakage. As a result the signal conductor cable is allowed to move laterally in a more or less random pattern within the streamer hose. Undesired electric noise may thus be induced in the conductor-electrode- seawater loop, due to the varying magnetic flux about the conductor, if the streamer exhibits rotational motion relative to the geomagnetic field.

The conductor wires (9a, 9b) , particularly wire (9b)

illustrated in Fig. 1 will thus be susceptible to generating magnetically induced noise due to being a randomly moving conductor which is a nearly, but not perfectly central wire within the non-conducting cavity formed by the cable skin with the gel. The magnetic field in question may be the local geomagnetic field of between 25 000 nT and 70 000 nT. A precisely centrally arranged electrode conductor within a non- rigid body such as a streamer hose has been found to be infeasible to manufacture within the required precision.

Short summary of the invention

The above-mentioned problems may be reduced through the use of a streamer cable according to the invention which is a marine geophysical sensor cable comprising a hose skin (1) generally filled with gel (3) and comprising electrical sensors

comprising pairs of first and second sea water exposed

electrodes (5al, 5bl) connected to an electrical amplifier (7al) , the novel features of the invention being that the first electrode (5bl) is connected to the amplifier (7al) via a tubular electric conductor (11) arranged along an inner face of or within the hose wall of the streamer hose skin (1) , the tubular electric conductor (11) generally being insulated from the sea water and the second electrode (5al) .

In an embodiment of the invention the electrodes (5a, 5b) are ring- or sleeve-shaped.

In another aspect of the invention it is a marine geophysical sensor cable comprising a hose skin (1) generally filled with gel (3) and provided with electrical sensors,

- each sensor comprising one of several water exposed

electrodes (5a2, 5b2 , ...) arranged along the sensor cable, each electrode connected to locally arranged first input terminals (71a) of the amplifiers (7a2, 7b2 , ...),

- with second input terminals (71b) of each amplifier (7a2, 7b2 , ...) connected to a common reference electrode (15) for each of the amplifiers (7a2, 7b2, ...), wherein the novel features of the invention are - that the second input terminals (71b) of the amplifiers (7a2, 7b2) are connected to the common voltage reference electrode (15) via a tubular electric conductor (llg) arranged along the inner face of, or laminated within, the streamer hose skin (1) , the tubular electric conductor (llg) generally- being insulated from the sea water and from the electrodes (5al, 5a2, ... ) .

A common feature of both aspects of the invention is the feature of a sea water insulated tubular electric conductor (11, llg) arranged internally along the skin or within the skin (1) of the streamer cable, the tubular electric conductor for forming an electrical connection between an electrode and an amplifier.

Brief figure captions

Fig. 1: illustrates a background art EM streamer section wherein two electrodes are arranged on a gel-filled streamer hose and in galvanic contact with the sea-water, connected by a more or less slack electrical conductor wire generally non- centred in the streamer hose. The EM streamer may comprise other components such as instrument power cables, seismic sensors etc . which are not shown here .

Fig. 2: is an illustration of a simplified longitudinal section of a first embodiment of the invention wherein at least one of the two electrodes is connected to the amplifier or preamplifier via a tubular conductor integrated with the streamer hose wall, either arranged internally along the inner face not dissimilar to the metallic screen of a coaxial cable, or arranged integrally, laminated within the hose wall, in both embodiments to be insulated with respect to the sea water. The other of the two electrodes is connected with a short conductor to the amplifier.

Fig. 3: illustrates a second embodiment of the invention wherein a first electrical connection of at least two or more separate first electrodes are separately connected each to their own (pre) amplifier, the amplifier's other input are connected via the common tubular conductor to a common

reference second electrode, and insulated from the first electrodes.

Fig. 4 illustrates an alternative embodiment of what is illustrated in Fig. 2, the difference being that two separate tubular conductors according to the invention are arranged extending each from their connected electrode to an

intermediately arranged amplifier.

Description of embodiments of the invention Two embodiments of the invention are illustrated in Figs. 2 and 3. The E-field sensor with a tubular conductor (11) according to the invention may be implemented within and on an electromagnetic streamer cable including a flexible hose (1) in two different ways illustrated in the Fig. 2 and Fig. 3. The streamer cable hose may be of the ordinary type and provided with seismic sensors in addition to the EM-sensors. The streamer cable of the invention may be subdivided into cable sections with section connectors as in the background art .

A first embodiment of the invention illustrated in Fig. 2 shows the novel E-field sensor concept for a geophysical streamer. Fig. 2 is an illustration of a first embodiment of the invention wherein at least one of the two electrodes is connected to the (pre) mplifier via a tubular conductor integrated in the streamer hose wall, either along the inner face or interior in the wall, similar to the screen of a coaxial cable . The streamer may be a combined EM and seismic streamer comprising both EM sensors and seismic sensors. Fig. 2 is an illustration of a simplified longitudinal section of a first embodiment of the invention wherein at least one of the two electrodes (5al, 5bl) is connected to the amplifier (7al) via a tubular conductor (11) integrated with the streamer hose wall (1) . Generally required details required for a streamer cable section such as electrical or optical signal conductors etc. will be known to the person skilled in the art. The new feature of the invention is thus that the electrode-to- amplifier conductor is a generally tubular conductor (11) integrated with the streamer hose skin (1) , similar to the shield in a coaxial cable, as different from the background art single wire conductors loosely positioned within the gel (3) - filled interior of the streamer hose (1). The tubular electric conductor (11) generally fills the entire available diameter within said streamer hose skin (1) .

In the first configuration a tubular conductor (11) is used as an electrode-to-amplifier conductor. The tubular conductor (11) is integrated with the streamer hose wall (1) , either along the inner face of the wall or interior, embedded, laminated within the wall (lw) of the tube (1) . A first electrode (5bl) is arranged for being in contact with the sea water and connected to the tubular conductor (11) which is further connected to an input terminal (71b) of a differential amplifier or preamplifier (7al) . A second electrode (5al) , insulated from the tubular conductor (11) , is arranged at a distance AL from the first electrode (5bl) and connected to an other input terminal (71a) of the amplifier (7al) . The voltage difference AV_al-bi = V_al-V_bi is then amplified and digitized. The digitized voltage difference is then sent to a registration apparatus for storage and processing as will be known to the geophysicist skilled in the art. The voltage difference AV_ai__bi thus measured represents a local gradient AV_ai_-bi/ Ah of the E- field along the streamer cable between two electrodes (5al, 5bl) .

In Fig. 2 the preamplifier (7al) is arranged near electrode (5al) which is directly connected to one of the input

terminals (71a) of the amplifier. In the illustrated

embodiment of Fig. 2 the preamplifier (7al) is arranged near the electrode (5al) for simplicity; electrode (5al) may be connected to terminal (7al) by means of a short wire. The preamplifier (7al) may be arranged somewhere between the electrodes (5al, 5bl) , and each of the electrodes (5al, 5bl) connected using each their tubular conductor (11a, lib), please see Fig. 4. According to such an embodiment of the invention the second electrode (5al) is connected to the amplifier (7al) via a second tubular electric conductor (11a) separate from the above-mentioned first tubular conductor (11, here lib) and arranged along an inner face of or within the hose wall of the streamer hose skin (1) , wherein the tubular electric conductor (11a) is also generally insulated from the sea water and from the second electrode (5al) . Further, the amplifier (7al) may be arranged somewhere between the first and second electrodes (5bl, 5al) and connected to separate adjacent ends of the tubular electric conductors (lib, 11a) . In the second configuration, illustrated in Fig. 3, a tubular conductor (llg) is used as a common conductor to a common reference electrode (15) somewhere along the cable. Fig. 3 is an illustration of a second embodiment of the invention wherein at least the two or more electrodes are separately connected each to their own (pre) amplifier, the amplifier's other input terminal is connected via the tubular conductor to a common reference electrode. Electrodes (5a2, 5b2) insulated from said tubular conductor (llg) are connected to input terminals (71a) of differential amplifiers (7a2, 7b2 , ...) generally arranged near their corresponding electrodes (5a2, 5b2) . Voltage differences

AV_a2-r = V_a2-V_r ,

AV_b2-r = V_b2-V_r ,

. . .

are amplified and digitized. The digitized voltage differences are then sent to a registration apparatus for storage and processing as will be known to the geophysicist skilled in the art. The voltage differences AV_a2__r thus measured represent local voltages (AV_a2, AV_b2, . . .) along the streamer cable of local electrodes (5a2, 5b2, ...) relative to the reference electrode (15). Local gradients AV_a2-b2/ AL of the E-field along the streamer cable may be calculated by subtraction. Physically, the tubular conductor (11, llg) resembles the screen of a coaxial cable. In an embodiment of the invention the tubular conductor (11, llg) may be laminated within the hose wall. The tubular conductor (11, llg) is generally electrically insulated at least with respect to the sea water, and preferably also with respect to the interior of the streamer cable. The streamer cable is usually gel-filled and non-conductive .

The skin integrated tubular conductor (11, llg) of the

invention will always have a fixed distance to the surrounding seawater due to the generally stable thickness of the hose wall (lw) . The distance is constant in all directions and independent of any rotational motion of the streamer hose (1) in the sea. The conductor (11, llg) will thus not be susceptible to generating magnetic field induced noise in the same way as a randomly moving generally non-central conductor wire of the background art within the non-conducting cavity formed by the cable skin with the gel. A skin integrated tubular conductor electrode cable will have similar

electromagnetic properties to that of a single lead positioned perfectly at the centre of a circular streamer.

Claims

1. A marine geophysical streamer cable comprising a flexible hose with a skin (1) , said hose generally filled with gel (3) , said hose comprising electrical sensors comprising pairs of a first and a second sea water exposed electrodes (5al, 5bl) connected to an electrical amplifier (7al) ,

characterized in that

said first electrode (5bl) is connected to said amplifier (7al) via a tubular electric conductor (11) arranged along an inner face of or within the hose wall of said streamer hose skin (1) , said tubular electric conductor (11) generally insulated from the sea water and said second electrode (5al) .

2. The marine geophysical sensor cable of claim 1, said amplifier (7al) arranged near said second electrode (5al) and connected via a conductor significantly shorter than said tubular electric conductor (11) .

3. The marine geophysical sensor cable of claim 1,

said second electrode (5al) connected to said amplifier (7al) via a second tubular electric conductor (11a) separate from said first tubular conductor (lib) and arranged along an inner face of or within the hose wall of said streamer hose skin (1) , said tubular electric conductor (11a) also generally insulated from the sea water and said second electrode (5al) .

4. The marine geophysical sensor cable of claim 3, said amplifier (7al) arranged between said first and second

electrodes (5bl, 5al) and connected to separate adjacent ends of said tubular electric conductors (lib, 11a) .

5. The marine geophysical streamer cable of any of the preceding claims, said tubular electric conductor (11) generally filling the entire available diameter within said streamer hose skin (1) .

6. A marine geophysical streamer cable comprising a flexible hose with a skin (1) , said hose generally filled with gel (3) and provided with electrical sensors,

- each sensor comprising one of several water exposed

electrodes (5a2, 5b2 , ...) arranged along said cable, each electrode (5a2, 5b2, ...) connected to locally arranged first input terminals (71a) of said amplifiers (7a2, 7b2, ...),

- second input terminals (71b) of each amplifier (7a2, 7b2 , ...) connected to a common reference electrode (15) for each of said amplifiers (7a2, 7b2 , ...),

characterized in that

- said second input terminals (71b) of said amplifiers (7a2, 7b2) is connected to said common voltage reference electrode (15) via a tubular electric conductor (llg) arranged along the inner face of or laminated within the streamer hose skin (1) , said tubular electric conductor (llg) generally insulated from the sea water and from said electrodes (5al, 5a2, ...)·

7. The marine geophysical streamer cable of claim 6, said tubular electric conductor (11) generally filling the entire available diameter within said streamer hose skin (1) .