Abstract:
A marine electromagnetic survey source includes a power cable configured to couple to a high voltage power supply at one axial end and to a head unit at the other axial end. The head unit includes equipment configured to output a lower voltage at higher current than the current imparted thereto by high voltage power supply. The head unit has an electrically conductive exterior coupled to one output terminal of the equipment. An electromagnetic antenna cable having an electrode thereon is coupled to the head unit and configured to receive the output of another terminal of the head unit equipment. In some implementations, electromagnetic fields are induced in formations by conducting current to the equipment. Marine geophysical surveys are conducted utilizing such induction of electromagnetic fields.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     The invention relates generally to the field of marine electromagnetic geophysical surveying apparatus and methods. More specifically, the invention relates to structures for marine electromagnetic geophysical survey sources and uses thereof. 
     Marine electromagnetic geophysical surveying is used, among other purposes, to infer spatial distribution of electrical conductivity of rock formations below the bottom of a body of water such as a lake or ocean. The spatial distribution of conductivity is used to assist determining the presence of hydrocarbon bearing rock formations in the subsurface, potentially resulting in cost savings by better targeting drilling operations. One type of such surveying is known as “controlled source” electromagnetic surveying (“CSEM”), which generally includes inducing a time varying electromagnetic field in the subsurface formations and measuring one or more parameters related to a response of the subsurface rock formations to the induced electromagnetic field. 
     Devices for inducing such electromagnetic fields are generally referred to as electromagnetic “sources” and include, among other devices, spaced apart electrodes or wire coils disposed along or at the end of a cable. The cable is typically towed by a vessel in the body of water. Time varying electric current is imparted across the electrodes or through the coils, generally from a power supply located on the vessel, to induce a time varying electromagnetic field in the water and subsequently in the subsurface formations. The electrodes may be suspended at a selected depth in the water by the use of floatation devices such as buoys, or the cable itself may be neutrally or otherwise buoyant. 
     In some circumstances, it may be desirable to operate the electromagnetic source at a substantial distance from the tow vessel. In particular, it is desirable in some circumstances to operate the electromagnetic source proximate the bottom of the body of water or at great depth in the water. There is a need for an electromagnetic source cable system that can operate at substantial distances from the source of current and/or at substantial depth in the water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example embodiment of a marine electromagnetic survey system using an example embodiment of an electromagnetic source according to the invention. 
         FIG. 1A  shows another example embodiment of a marine electromagnetic survey system. 
         FIGS. 1B and 1C  show two different possible embodiments of a head unit used in embodiments of marine electromagnetic survey systems. 
         FIG. 2  shows an example embodiment of electromagnetic source circuits according to the invention. 
         FIG. 3  shows an example embodiment of electromagnetic source having a high voltage power supply disposed on shore, while the electromagnetic source is located in the water at a selected distance from the shore. 
         FIG. 4  shows an example embodiment similar to that in  FIG. 3  wherein the electromagnetic source is disposed on the water bottom. 
         FIG. 5  shows an example embodiment of an electromagnetic source having a high voltage power supply disposed on a drilling or production platform, wherein the source is suspended in a body of water. 
         FIG. 6  an example embodiment similar to that shown in  FIG. 5  wherein the electromagnetic source is disposed on the water bottom. 
     
    
    
     DETAILED DESCRIPTION 
     An example marine electromagnetic survey system is shown schematically in  FIG. 1 . The marine electromagnetic survey system may include a sensor cable  10  having thereon at longitudinally spaced apart positions a plurality of sensors  12 . The sensor cable  10  is shown being towed by a survey vessel  18  moving on the surface of a body of water  22  such as a lake or ocean. The sensor cable  10  may alternatively be deployed on the water bottom  23  or towed by another vessel (not shown). As another alternative, or one or more additional sensor cables may be deployed behind the survey vessel  18 , behind another vessel (not shown), or on the water bottom  23 . The invention may also be used with sensor nodes (not shown), e.g., static nodes disposed on the water bottom  23 . The type(s) and configuration(s) of sensors  12  are not intended to limit the scope of the present invention. The sensors  12  may be used to measure the electromagnetic response of formations below the water bottom  23  to electromagnetic field(s) imparted by an electromagnetic source, as discussed below. The sensors  12  may measure one or more various electric field properties, such as voltage, magnetic field amplitude, and/or magnetic field gradient. 
     The survey vessel  18  may include thereon equipment, shown generally at  20  and referred to for convenience as a “recording system” that may include devices (none shown separately) for navigation, for energizing one or more electromagnetic sources for imparting an electromagnetic field in the formations below the water bottom  23 , and/or for recording and processing signals generated by the various sensors  12 . 
     The marine electromagnetic survey system shown in  FIG. 1  includes an electromagnetic source which may include a source cable  14  for inducing an electromagnetic field in the formations below the water bottom  23 . The source cable  14  may comprise one or more antenna cable(s)  14 A, a “power” or “tow” cable  14 B, and a head unit  15 . The “power” or “tow” cable  14 B may be coupled to the survey vessel  18  and may include insulated electrical conductors, optical fibers, and a strength member (not shown separately) to serve the purposes of conducting electrical and/or optical signals and electrical power and transmitting axial towing force from the survey vessel  18 . The aft (with respect to towing direction) end of the power cable  14 B may be coupled to a head unit  15 . The head unit  15  may be made from a strong, dense material, such as stainless steel or the like, or fiber reinforced plastic, and may have a weight in water selected so as to be negatively buoyant. In some embodiments, the head unit  15  may cause the aft end of the power cable  14 B to be submerged to a selected depth in the water depending on the amount of power cable extended from the survey vessel  18 . The shape of the head unit  15  may be hydrodynamically efficient to reduce its resistance to motion in the water. The head unit  15  may define a pressure resistant interior space (as in the example illustrated in  FIG. 2 ) wherein certain equipment (to be explained below with reference to  FIG. 2 ) may be disposed. The equipment disposed in head unit  15  may include, for example power conversion and/or switching circuits. The head unit  15  may also include control surfaces  15 A to provide upward or downward thrust under motion. For example, downward thrust under motion may resist lifting of the head unit  15  from the selected depth by the action of friction in the water as the survey vessel  18  moves the power cable  14 B. Such control surfaces  15 A may be fixed or may be rotatable, and may be autonomous or remotely controlled. Example embodiments of controls and mechanisms that may be used to operate the control surfaces  15 A are shown in U.S. Pat. No. 7,457,193 issued to Pramik and incorporated herein by reference As will be explained below, the marine electromagnetic survey system of the present invention may also be used with a fixed position electromagnetic source antenna. In such cases, connection of the head unit  15  to a power supply may be made using a power cable which performs substantially the same electrical function as the “power” or “tow” cable  14 B in  FIG. 1 , but which is not coupled to a survey vessel or other towing vessel. 
     In the present example embodiment, a forward (with respect to direction of travel of survey vessel  18 ) end of the antenna cable  14 A may be coupled to the head unit  15 . The antenna cable  14 A may be an electrically insulated, single conductor cable connected to an electrode  16  disposed at a selected position along the antenna cable  14 A. The head unit  15 , if made from electrically non-conductive material is preferably at least partially covered with an electrically conductive material jacket or skin, as shown at  15 B. Examples of such material may include, without limitation stainless steel alloy 316, MMO coated titanium, or copper. If a covering of electrically conductive material in the form of a skin or jacket is used, depending on the selected material the skin or jacket  15 B may require replacement at certain times. The design of the head unit  15  may be such that replacement of the jacket  15 B is facilitated. The head unit  15 , or jacket  15 B if used, may be electrically connected to one output terminal of a power converter ( 30  in  FIG. 2 ) disposed inside the head unit  15 , thus causing the head unit to act as one electrode of a bipole electromagnetic source antenna. The head unit  15  and the electrode  16  on the antenna cable  14 A may be energized, for example at selected times, or continuously by a high voltage power supply  28 . The voltage from the high voltage power supply  28  is converted by the power converter circuits in the head unit  15 . (see  FIG. 2 ). Energizing head unit  15  and the electrode  16  on the antenna cable  14 A may induce a time varying electromagnetic field in the formations below the water bottom  23 . The high voltage power supply  28  may provide alternating current (“AC”) or direct current (“DC”) at a voltage selected to minimize power loss along the length of the power cable  14 B. 
     The current applied across the bipole source antenna may be alternating current (“AC”) or switched direct current (“DC”) (e.g., switching current on, switching current off, reversing current polarity, or sequential switching such as a pseudorandom binary sequence). The configuration shown in  FIG. 1  induces a horizontal dipole electric field in the subsurface when the antenna (consisting of head unit  15  and electrode  16 ) is energized by the electric current. It is entirely within the scope of the present invention to induce vertical dipole electric fields in the subsurface. The type of current used to energize the electrodes is not limited to the foregoing as the invention is applicable to use with both frequency domain (continuous wave) and transient induced electromagnetic fields. 
     In some embodiments, the antenna cable  14 A may be substantially neutrally buoyant so that the antenna cable  14 A operates at substantially the same depth in the water as the head unit  15 . For example, the antenna cable  14 A may operate as close as about 100 m or as even as close as about 50 m to the seabed. In some embodiments, the antenna may operate at a water depth of about 50 m to about 3000 m or more. 
     In some embodiments, such as shown in  FIG. 1A , the antenna cable  14 A may include more than one electrode, e.g., a forward electrode  16 A and an aft electrode  16 B connected longitudinally by a linking cable  62 . In such example embodiments, the energizing circuits ( FIG. 2 ) in the head unit may make selectable electrical connection of one output terminal of the energizing circuits to either the forward electrode  16 A (with reference to the towing direction) or the aft electrode  16 B so as to enable selecting a length of the source bipole. 
     Two possible embodiments are shown in  FIGS. 1B and 1C  to illustrate the principle of using the head unit as an electrode in an electromagnetic source. In  FIG. 1B , the housing  15  may be made from electrically non-conductive material, and may contain circuits including a switch  32  such as an H-bridge (explained further below with reference to  FIG. 2 ). One output terminal of the switch  32  may be electrically connected to a conductive covering or skin  15 B disposed on part or all of the housing  15 . Another possible embodiment is shown in  FIG. 1C , in which the housing  15  is made from electrically conductive material, and is electrically connected to the switch  32  terminal as in  FIG. 1B . 
       FIGS. 1B and 1C  are shown to illustrate the difference between having the outer surface of the power conversion equipment housing  15  perform the function of the conductive element ( 15 B in  FIG. 1 ) and just having an additional conductive surface around the power conversion equipment housing  15  to act as the electrode ( 15 B in  FIG. 1 . The shape of the housing for electrical purposes is immaterial; specific shapes thereof may be configured to reduce resistance to movement in the body of water and/or to reduce turbulence caused by such movement, but any shape will performed the electrode function. Cylinders are shown in  FIGS. 1B and 1C  because this particular shape has been used in previous electrode designs. Such cylinders may include a hemispherical nose cone  15 D on the forward end. Also the power conversion equipment is likely to be contained in a pressure vessel which may be cylindrical because that is a desirable shape for resistance to hydrostatic pressure. 
     An example embodiment of equipment disposed on the vessel (e.g., in recording system  20 ) and along the source cable  14  will be explained with reference to  FIG. 2 . The recording unit  20  may include a high voltage power supply  28  as explained above. The high voltage power supply  28  may be a DC generator, or it may be an AC generator with or without a step up transformer. Alternatively the high voltage power supply  28  may be a motor-generator set or a synchronous converter. The voltage output by the high voltage power supply  28  may be selected so that a selected amount of electrical power may be transmitted along the power cable  14 B, while minimizing resistive losses along the power cable  14 B. The selected amount of power may be the amount needed to generate a suitable amplitude electromagnetic field in the formations below the water bottom ( 23  in  FIG. 1 ). In some embodiments, the selected amount of power may be in the range of about 100 kW to 1000 kW or more. In the present embodiment, the head unit  15  may include power conversion and switching circuits. For example, as illustrated in  FIG. 2 , the head unit  15  may include a power converter  30  and an H-bridge or similar switch  32 . In the present embodiment, power converter  30  may be a DC to DC power converter, or an AC to DC power converter. The power converter  30  in such example embodiment may convert the high voltage from the power cable  14 B to a low voltage, high current DC for energizing the head unit  15  (or jacket  15 B in  FIG. 1 ) and the electrode  16 . The preferred output voltage of the power converter  30  will depend upon factors such as the spacing between the head unit  15  and the electrode  16  and the electrical conductivity of the water ( 22  in  FIG. 1 ). Switch  32  may be interposed in the electrical connection between the power converter  30 , and the head unit  15  and electrode  16 . The switch  32  may be controlled by the recording unit  20  or may be remotely programmed to operate autonomously. The switch  32  may cause the current output from the power converter  30  to be applied to the head unit  15  (or jacket  15 B) and the electrode  16  in one or more switching sequences referenced to a time index, including switching on, switching off, reversing polarity and a multiple event switching sequence, for example and without limitation, a pseudorandom binary sequence (PRBS). 
     In other embodiments, the equipment disposed in the head unit  15  may include a waveform synthesizer (not shown separately) to generate AC output at one or more selected frequencies and/or waveforms, or may directly use converted voltage AC from the high voltage power supply  28 . 
     If used in an example embodiment such as shown in  FIG. 1A , the switch  32  may also be configured to selectively connect one terminal of the power converter to either the forward electrode  16 A or the aft electrode  16 B. 
     The foregoing marine electromagnetic survey system, while explained in terms of an electromagnetic antenna towed from a vessel, alternatively may be used to many of the same possible advantages with a fixed position electromagnetic source antenna located at a substantial distance from the high voltage power supply  28 . In such embodiments, the high voltage power supply  28  may be disposed other than on a vessel, for example operated from onshore or from a rig. An example embodiment having a high voltage power supply  28  disposed on shore  21  is shown in  FIG. 3 . The power cable  14 B is extended from the high voltage power supply  28  into a body of water  22  to a selected distance from the shore  21 . The head unit  15  and antenna cable  14 A may be configured substantially as explained herein. In the present example embodiment, the antenna cable  14 A and the head unit  15  may be suspended at a selected depth in the water  22  using buoys, floats or any similar device, shown generally at  14 C. Alternatively, the head unit  15 , antenna cable  14 A and electrode  16  may be disposed on the water bottom as shown in  FIG. 4 . 
       FIG. 5  shows an example embodiment of a high voltage power supply  28  disposed on a drilling or production platform  40 . In the present example, the drilling or production platform  40  may be semisubmersible as shown, however bottom supported platforms may be used in other embodiments. The power cable  14 B may extend from the high voltage power supply  28  to a selected depth in the water  22 . The head unit  15  may be coupled to the end of the power cable  14 B. The antenna cable  14 A may be coupled to the head unit  15  as previously explained herein, wherein an electrode  16  is disposed on the antenna cable  14 A and is electrically connected to the circuits in the head unit  15  as explained with reference to  FIG. 2 . The example shown in  FIG. 5  may include having the head unit  15  and antenna cable  14 A suspended at a selected depth in the water  22 . Another example, shown in  FIG. 6 , may include extending the power cable  14 B so that the head unit  15 , antenna cable  14 A and electrode  16  may rest on the water bottom  23 . 
     For such purposes as the example embodiments shown in  FIG. 5  and  FIG. 6 , the “power” or “tow” cable indicated at  14 B in  FIG. 1  may not include strength members for transmitting axial towing force from the survey vessel  18 , but may still provide for conducting electrical or optical signals and/or power. 
     Some advantages of a combining an electrode with a head unit enclosing power converter equipment may include one or more of the following. Complexity may be reduced as there are fewer bodies to tow and handle. Any reduction in the complexity of the system being towed in deep water may provide an advantage as it may reduce risk of tangling and snagging. Source deployment, recovery and handling on the deck of the vessel may be more efficient and safer. Shielding of the control electronics may be provided because the electrode surface (the skin if used or the conductive metal housing), being at equipotential, will act as faraday cage, and so electric and/or magnetic field strengths inside the housing will be minimal. This feature may avoid potential concerns about susceptibility of the control system in the housing to the electromagnetic field induced by the source. In some embodiments, there may be a shorter total length of high current source cable (i.e., between electrodes) and therefore less power dissipated in such cables. Any reduction in such power loss may result in more efficient system operation, meaning that there may be either potential for higher source signal strength or reduced system size which in turn results in easier handling and lower cost. Finally, there may be two fewer high current “dynamic” connectors. Such connectors may be a weak point of any system, quite apart from being costly. Thus, a system according to the various aspects of the present invention may provide certain advantages over electromagnetic source systems known in the art. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.