Abstract:
A method for acquiring electromagnetic data in at least two dimensions includes towing a first streamer cable behind a vessel in a body of water, the first streamer cable including a reference line extending substantially along the entire length thereof, a plurality of spaced apart measuring electrodes electrically insulated from the reference line and a voltage measuring circuit functionally coupled between each measuring electrode and the reference line. At least a second streamer cable is towed at corresponding distance from the vessel. The second streamer cable is configured substantially as the first streamer cable. The second streamer cable is displaced from the first streamer cable in one of a horizontal plane and a vertical plane. At selected times an electromagnetic field is imparted into the water. Voltage difference is determined between each measuring electrode and the reference line, and a difference between voltages measured at least one electrode on each of the first and second streamer cables is determined.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to the field of marine electromagnetic surveying. More specifically, the invention relates to a method and system for detecting electromagnetic signals in a marine environment in both in-line and cross-line directions. 
         [0005]    2. Background Art 
         [0006]    Marine controlled source electromagnetic (CSEM) surveying is a geophysical surveying technique that uses electromagnetic (EM) energy to identify possible hydrocarbon bearing rock formations below the bottom of a body of water such as a lake or the ocean. In a typical marine CSEM survey, an EM source and a number of EM receivers are located at or near the bottom of a body of water. The EM source is typically towed over an area of interest in the Earth&#39;s subsurface, and the receivers are disposed on the water bottom over the area of interest to obtain signals related to the distribution of electrical resistivity in the subsurface area of interest. Such surveying is performed for a range of EM source and EM receiver positions. The EM source emits either or both a time varying electric field and a time varying magnetic field, which propagate outwardly into the overlying seawater and downwardly into the formations below the water bottom. The receivers most commonly used detect and record the induced electric field at or near the water bottom. The time varying EM field may be induced by passing electric current through an antenna. The electric current may be continuous wave and have one or more discrete frequencies. Such current passing through an antenna is used for what is referred to as “frequency domain CSEM” surveying. It is also known in the art to apply direct current to an antenna, and produce transient EM fields by switching the current. Such switching may include, for example, switching on, switching off, inverting polarity and inverting polarity after a switch on or switch off event. Such switching may be equally time spaced or may be in a time series known as a “pseudo random binary sequence.” Such switched current is used to conduct what is referred to as a “transient CSEM” survey. One type of such survey is a multi-transient electromagnetic survey. 
         [0007]    The EM energy is rapidly attenuated in the conductive seawater, but in less conductive subsurface formations is attenuated less and propagates more efficiently. If the frequency of the EM energy is low enough, the EM energy can propagate deep into the subsurface formations. Energy “leaks” from resistive subsurface layers, e.g., a hydrocarbon-filled reservoir, back to the water bottom. When the source-receiver spacing (“offset”) is comparable to or greater than the depth of burial of the resistive layer (the depth below the water bottom) the energy reflected from the resistive layer will dominate over the transmitted energy. CSEM surveying uses the large resistivity contrast between highly resistive hydrocarbons and conductive aqueous saline fluids disposed in permeable subsurface formations to assist in identifying hydrocarbon reservoirs in the subsurface. 
         [0008]    U.S. Patent Application Publication No. 2009/0140741 discloses a system for acquiring EM data in three dimensions, that is, both in a direction along the direction of travel of a marine electromagnetic survey vessel, and a direction transverse to the direction of the survey vessel both in the vertical plane and in the horizontal plane. 
         [0009]    In order to make the cross-line measurements described in the &#39;741 publication, it is necessary to extend electrical conductors from the position of the electrodes used to make the cross-line measurements (typically corresponding electrodes on adjacent streamer cables) to the input of a voltage measuring circuit. The voltage measuring circuit may be on the survey vessel or at a convenient place, such as a lead in termination, at the forward end of one of the streamer cables. The long electrical conductors are subject to having voltages induced in them as a result of moving the streamer cables within the Earth&#39;s magnetic field. The amplitude of the induced voltage will depend on the velocity of the streamer cable, and the length of the electrical conductors from the respective electrodes to the voltage measuring circuit. 
         [0010]    A method is known in the art for reducing the magnitude of the induced voltage in EM streamer cables. See, for example, U.S. Pat. No. 7,671,958 issued to Ronaess et al. The method and apparatus disclosed in the &#39;958 patent is described with respect only to a single EM sensor streamer cable. There is no provision in the method and apparatus disclosed in the &#39;958 patent for the very long electrical conductors needed to reduce induction noise in systems capable of measuring cross-line EM signals, such as disclosed in the &#39;741 publication. 
         [0011]    There is a need for improved methods and apparatus for correcting measurements made by 2D and 3D towed marine survey systems for induced voltage noise. 
       SUMMARY OF THE INVENTION 
       [0012]    A method according to one aspect of the invention for acquiring electromagnetic data in at least two dimensions includes towing a first streamer cable behind a vessel in a body of water, the first streamer cable including a reference line extending substantially along the entire length thereof, a plurality of spaced apart measuring electrodes electrically insulated from the reference line and a voltage measuring circuit functionally coupled between each measuring electrode and the reference line. At least a second streamer cable is towed at corresponding distance from the vessel. The second streamer cable is configured substantially as the first streamer cable. The second streamer cable is displaced from the first streamer cable in one of a horizontal plane and a vertical plane. At selected times an electromagnetic field is imparted into the water. Voltage difference is determined between each measuring electrode and the reference line, and a difference between voltages measured at least one electrode on each of the first and second streamer cables is determined. 
         [0013]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view of an electromagnetic signal acquisition system that may be used in accordance with the present invention. 
           [0015]      FIG. 2  shows more detail of one example of a sensor module in the cable system of  FIG. 1 . 
           [0016]      FIG. 3  shows more detail of example measurement and communication circuitry of the sensor module shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a perspective view of an electromagnetic signal acquisition system that may be used in accordance with the present invention. A survey vessel  10  moves along the surface of a body of water  11  such as a lake or the ocean. The survey vessel  10  may include thereon equipment shown at  12  and referred to for convenience as a “recording system.” The recording system  12  may include devices (none shown separately in  FIG. 1 ) for navigation of the vessel, for imparting electric current to an electromagnetic transmitter (explained below) and for detecting and recoding signals generated by each of a plurality of electromagnetic receivers (explained below) on a plurality of streamer cables, which may be towed by the survey vessel  10  or by another vessel. 
         [0018]    The transmitter in the present example may be an armored, insulated electrical cable  14  having thereon spaced apart electrodes  16 A,  16 B. At selected times, the recording system  12  will impart electric current across the electrodes  16 A,  16 B. The electrical current may be, for example, continuous wave low frequency (e.g., 0.01 to 1 Hz) alternating current at one or more discrete frequencies for frequency domain electromagnetic surveying, or some form of switched direct current (e.g. switched on, switched off, reversed polarity or a series of switching events such as a pseudo-random binary sequence) for time domain electromagnetic surveying. An electromagnetic field induced by the current flowing across the electrodes  16 A,  16 B travels through the water, into rock formations  15  below the water bottom  13  and is detected by electromagnetic receivers in receiver modules  20  disposed on first, second and third streamer cables  18 A,  18 B,  18 C, respectively. Each streamer cable  18 A,  18 B,  18 C may include an electrode  32 A at the aft end thereof (furthest from the vessel  10 ). The electrode will be further explained with reference to  FIG. 2 . 
         [0019]    As will be explained further below with reference to  FIGS. 2 and 3 , each receiver module  20  may have circuitry proximate thereto for measuring voltage imparted between an electrode on the receiver module  20  and a reference potential line in response to the electromagnetic field imparted into the subsurface by the transmitter  14 . 
         [0020]    It should also be understood that while the present example transmitter, known as a horizontal electric dipole, uses a pair of electrodes spaced apart in the horizontal plane, other types of transmitters that may be used with the present invention include vertical electric dipoles (electrodes spaced apart in the vertical plane) or vertical or horizontal magnetic dipoles such as wire coils or loops having magnetic moment along the vertical and/or horizontal directions. 
         [0021]      FIG. 1  also shows a coordinate system  17  used in the present description and to illustrate that the second streamer cable  18 B may be displaced from the first streamer cable  18 A in the horizontal plane or Y direction, and the third streamer cable  18 C may be displaced from the first streamer cable  18 A in the vertical plane or Z direction. The receiver modules  20  on all three streamer cables  18 A,  18 B,  18 C may be positioned at corresponding longitudinal distances from the vessel  10  to simplify calculation of certain measurements. As will be explained further, the second and third streamer cables  18 B,  18 C may be used to obtain electric field measurements in the Y and Z directions, called the “cross-line” directions, by measuring voltages impressed across corresponding electrodes (i.e., longitudinally about the same distance from the survey vessel  10 ) on different streamer cables, as well as the so-called “in-line” direction across pairs of electrodes spaced apart in the X direction as explained above. 
         [0022]    One example of a receiver streamer cable  18  (representative of any one of the receiver streamer cables  18 A,  18 B,  18 C in  FIG. 1 ) and one of the receiver modules  20  is shown in more detail in  FIG. 2 . The cable  18  may include on its exterior helically wound, electrically conductive armor wires  18 D, such as may be made from stainless steel or other high strength, corrosion resistant, electrically conductive material. In one example, to be explained in more detail below, the streamer cable  18  may include one or more insulated electrical conductors and one or more optical fibers disposed inside the armor wires  18 D. Using an externally armored cable as shown in  FIG. 2  may have the advantages of high axial strength of and high resistance to abrasion. 
         [0023]    The streamer cable  18  in the present example may be divided into segments, each of which terminates with a combination mechanical/electrical/optical connector  25  (“cable connector”) coupled to the longitudinal ends of each cable segment. The cable connector  25  may be any type known in the art to make electrical and/or optical connection, and to transfer axial loading to a mating connector  27 . In the present example such mating connector  27  can be mounted in each longitudinal end of one of the receiver modules  20 . The connectors  25 ,  27  resist entry of fluid under pressure when the connectors  25 ,  27  are coupled to each other. 
         [0024]    The receiver module housing  24  is preferably pressure resistant and defines a sealed interior chamber  26  therein. The housing  24  may be made from electrically non-conductive, high strength material such as glass fiber reinforced plastic, and should have a wall thickness selected to resist crushing at the maximum hydrostatic pressure expected to be exerted on the housing  24 . The mating connectors  27  may be arranged in the longitudinal ends of the housing  24  as shown in  FIG. 2  such that axial loading along the cable  18  is transferred through the housing  24  by the coupled cable connectors  25  and mating connectors  27 . Thus, the streamer cable  18  may be assembled from a plurality of connector-terminated segments each coupled to a corresponding mating connector on a receiver module housing  24 . Alternatively, the cable  18  may include armor wires  18 D extending substantially continuously from end to end, and the receiver modules  20  may be affixed to the exterior of the armor wires  18 D. 
         [0025]    An electromagnetic receiver, which may be a measuring electrode  28 , is disposed on the outer surface of the housing  24 , and may be made, for example, from lead, gold, graphite or other corrosion resistant, electrically conductive, low electrode potential material. Electrical connection between the measuring electrode  28  and measuring circuits  34  (explained in more detail with reference to  FIG. 3 ) disposed inside the chamber  26  in the housing  24  may be made through a pressure sealed, electrical feed through bulkhead  30  disposed through the wall of the housing  24  and exposed at one end to the interior of the chamber  26 . One such feed through bulkhead is sold under model designation BMS by Kemlon Products, 1424 N. Main Street, Pearland, Tex. 77581. 
         [0026]    The measuring circuits  34  may be powered by a battery  36  disposed inside the chamber  26  in the housing  24 . Battery power may be preferable to supplying power from the recording system ( 12  in  FIG. 1 ) over insulated electrical conductors in the streamer cable  18  so as to reduce the possibility of any electromagnetic fields resulting from current flowing along the cable  18  from interfering with the electromagnetic survey measurements made in the various receiver modules  20 . 
         [0027]    The streamer cable  18  may include one or more optical fibers  38  for conducting command signals, such as from the recording system ( 12  in  FIG. 1 ) to the circuits  34  in the various receiver modules  20 , and for conducting signal telemetry from the receiver modules  20  to the recording system ( 12  in  FIG. 1 ) or to a separate data storage device (not shown). An insulated electrical conductor  32  forming part of the cable  18  may pass through the chamber  26  in the housing  24  such that electrical continuity in such conductor  32  is maintained along substantially the entire length of the cable  18 . 
         [0028]    Optical telemetry may be preferable to electrical telemetry for the same reason as using batteries for powering the circuits  34 , namely, to reduce the incidence of electromagnetic fields caused by electrical current moving along the cable  18 . The insulated electrical conductor  32  in the present example serves as a common potential reference line between all of the receiver modules  20 . 
         [0029]    The insulated conductor  32  may be electrically in contact with the water ( 11  in  FIG. 1 ) by using an electrode ( 32 A in  FIG. 1 ) at the aft end of the streamer cable  18 . If the distance between the aft end of the streamer cable  18  and the transmitter ( 14  in  FIG. 1 ) is sufficiently large, the voltage at the electrode ( 32 A in  FIG. 1 ) and thus along the entire electrical conductor  32  is substantially zero notwithstanding the electromagnetic field induced by the transmitter. In a method according to the invention, the same cable configuration as explained herein with reference to  FIG. 2  and further explained with reference to  FIG. 3  may be used for all three streamer cables ( 18 A,  18 B,  18 C in  FIG. 1 ), and in each case the conductor  32  will represent a substantially zero voltage reference line along the entire length of each streamer cable. 
         [0030]    One example of the circuits  34  is shown in more detail in  FIG. 3 . The circuits  34  may include a resistor R electrically coupled between the measuring electrode ( 28  in  FIG. 2 ) and the insulated conductor  32 , which as explained above serves as a common reference. The resistor R is also electrically connected across the input terminals of a preamplifier  40 . Thus, voltage drop across the resistor R resulting from voltage difference between a fixed potential reference (conductor  32 ) and the measuring electrode ( 28  in  FIG. 2 ) will be input to the preamplifier  40 . Such voltage drop will be related to magnitude of the electric field gradient existing where the measuring electrode ( 28  in  FIG. 2 ) is located at any point in time. 
         [0031]    Output of the preamplifier  40  may be passed through an analog filter  42  before being digitized in an analog to digital converter (ADC)  44 . Alternatively, the preamplifier  40  output may be directly digitized and the output of the ADC  44  can be digitally filtered. Output of the ADC  44 , whether digitally filtered or not, may be conducted to an electrical to optical signal converter (EOC)  46 . Output of the EOC  46  may be applied to the one or more optical fibers ( 38  in  FIG. 2 ) in the cable ( 18  in  FIG. 2 ) such that optical signals representative of the voltage measured by each measuring electrode ( 28  in  FIG. 2 ) with respect to the reference conductor ( 32  in  FIG. 2 ) may be communicated to the recording system ( 12  in  FIG. 1 ) or to a data storage unit. The type of optical or other signal telemetry used in any implementation is a matter of discretion for the system designer and is not intended to limit the scope of the invention. 
         [0032]    Referring back to  FIG. 1 , voltage difference measurements between receiver modules  20  may be made in the in-line (X) direction, horizontal cross-line direction (Y), and vertical cross-line (Z) direction. In-line measurement is made by subtracting the voltage measurements made at a selected one of the receiver modules  20  on any streamer cable  18 A,  18 B,  18 C from another receiver module  20  on the same streamer cable. Such subtraction can be performed by the recording system  12  because in the present example, optical signals representing the voltage between the measuring electrode ( 28  in  FIG. 2 ) on each receiver module  20  and the common reference potential are transmitted to the recording system  12 . Cross-line voltage difference measurements may be made in the horizontal plane (Y direction) by subtracting the voltage measured at a selected receiver module  20  on the first streamer cable  18 A from the voltage measured at a corresponding receiver module  20  (corresponding meaning approximately the same longitudinal distance from the vessel  10 ) on the second streamer cable  18 B. Cross-line voltage difference measurements may be made in the vertical plane similarly, only using the measurements from corresponding receiver module(s)  20  on the third streamer cable  18 C. 
         [0033]    Using a method according to the invention it is possible to make cross-line electric field measurements without the need to extend voltage measurement lines along entire streamer cables and between streamer cables, thus eliminating a possible source of induced voltage caused by moving the streamer cables in the Earth&#39;s magnetic field. 
         [0034]    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.