Abstract:
An electromagnetic source for electromagnetic survey of a subsea formation includes a towfish configured to be towed by a surface vessel; a plurality of electrodes attached to the towfish; and an acoustic ranging system having acoustic components individually attached to each of the towfish and the plurality of electrodes, wherein the acoustic ranging system is configured to determine a geometry of the plurality of electrodes.

Description:
BACKGROUND OF INVENTION 
       [0001]    Electromagnetic (EM) prospecting techniques are commonly used in mineral and hydrocarbon exploration. Passive EM technologies, such as marine Magneto-Tellurics (MT), work by measuring the naturally occurring EM fields in the earth. Active EM techniques, on the other hand, may use a controlled EM (CSEM) energy source (e.g., an antenna) instead of recording earth&#39;s passive energy. In EM surveys, receivers measure the distortion of the source field by locally present resistors-such as those associated with the presence of hydrocarbon deposits. These measurements can then be related back to the conductivity and resistivity, and thus, the porosity and types of fluids in the rock. Therefore, these EM measurements allow the user to identify hydrocarbon deposits. 
         [0002]      FIG. 1  illustrate an example of a CSEM survey in a marine environment. As shown, a surface vessel  101  on the sea surface  117  tows a towfish  103 . The towfish  103  includes an EM source that is electrically connected to the surface vessel  101  via cables that run along the tow line  109 . The EM source emits low frequency EM fields that propagate outward through sea water and into the seafloor  111 . In a typical operation, the towfish  103  is drag by the surface vessel  101  such that the EM source will be at a relatively constant distance (typically 40-50 m) from the seafloor  111 . A plurality of receivers  102  are disposed on seafloor  111  for detecting the signals returned from the EM fields that have traversed the subsea formation. 
         [0003]    Due to the variable and lower conductivities in the sub-sea formations, the propagation of EM fields through the subsea formations will be substantially affected. On the other hand, the conductivity of the sea water is substantially constant. Therefore, any changes or variations of the detected EM signals will reflect conductivity variations in the subsea formations. Therefore, the measured EM signals may be used to derive the subsea formation electrical properties (i.e., resistivity or conductivity). 
         [0004]    For accurate mapping of the subsea formation resistivities, the positions of the towfish (EM source)  103  and the receivers  102  will need to be accurately determined. The locations of the receivers  102  and/or the towfish  103  are typically determined using an acoustic ranging system. An acoustic ranging system typically uses one or more USBL (Ultra-short baseline) device to measure the distance (range) between the objects. 
         [0005]    A typical marine USBL system consists of a transceiver (USBL) that is mounted on a pole under a ship and a transponder/responder (not shown) on the object to be ranged, e.g., the receivers  102  on the seafloor, the towfish  103 , or a remote operated vehicle (ROV). 
         [0006]    In a ranging operation, an acoustic pulse is transmitted by the transceiver and detected by the transponder, which then replies with its own acoustic pulse. This return pulse is detected by the transceiver. The time from the transmission of the initial acoustic pulse until the reply is detected by the transceiver is measured by the USBL system. In addition, the transceiver normally contains three or more transducers separated by a baseline of 10 cm or less. The “spaced” transducer array will typically not receive the reply signal simultaneously, hence the received signals would have phase differences. A “phase-differencing” method can then be used, based on the phase differences of the signals received by the different transducers in the array, to calculate the angle from the transducer array to the transponder on the object being determined. Thus, the USBL system can determine the location of a transponder based on its range (distance) and bearing measured by the transceiver (the transducer array). 
         [0007]    In deep water ranging, the towfish and the receivers are at long distances from the USBL transceiver, making it difficult to get good signals for accurate measurements. In the conventional configuration shown in  FIG. 1 , the USBL transceiver is located near the water-air interface, acoustic noises generated by this interface may interfere with or reduce the sensitivity of the measurements. To alleviate the interference or S/N problems, an inverted configuration USBL system has been proposed. See e.g., http://sonardyne.co.uk./News/baseline/baseline_magazine —l issue _1.pdf 
         [0008]    As shown in  FIG. 2 , in an inverted USBL system, the USBL transceiver is located on the remote object (e.g., the towfish  103 ), and a transponder T is located below the surface vessel  101 . By placing the transceiver (USBL) away from the air-water interface, the signal-to-noise ratio is improved with this configuration. 
         [0009]    These prior art methods are capable of determining the positions of the towfish and receivers. However, during a survey, the EM source is dragged by the surface vessel. The electrodes on the EM sources may not maintain the same configuration or orientation at all times. Therefore, there is a need for systems and methods that can be used to monitor the geometry of the EM source during the survey in order to improve the performance of CSEM prospecting. 
       SUMMARY OF INVENTION 
       [0010]    One aspect of the invention relates to electromagnetic sources for electromagnetic survey of a subsea formation. An electromagnetic source in accordance with one embodiment of the invention includes a towfish configured to be towed by a surface vessel; a plurality of electrodes attached to the towfish; and an acoustic ranging system having acoustic components individually attached to each of the towfish and the plurality of electrodes, wherein the acoustic ranging system is configured to determine a geometry of the plurality of electrodes. 
         [0011]    Another aspect of the invention relates to systems for electromagnetic survey of a subsea formation. A system in accordance with one embodiment of the invention includes a plurality of electromagnetic receivers disposed on a seafloor; and an electromagnetic source towed by a surface vessel at a selected distance above the seafloor, wherein the electromagnetic source includes a towfish configured to be towed by a surface vessel, a plurality of electrodes attached to the towfish, and a first acoustic ranging system having acoustic components individually attached to each of the towfish and the plurality of electrodes. 
         [0012]    Another aspect of the invention relates to methods for electromagnetic survey of a subsea formation. A method in accordance with one embodiment of the invention includes deploying a plurality of electromagnetic receivers on a seafloor; transmitting an electromagnetic field into the subsea formation using an electromagnetic source towed by a surface vessel at a selected distance above the seafloor, wherein the electromagnetic source comprises: a towfish configured to be towed by a surface vessel, a plurality of electrodes attached to the towfish, and an acoustic ranging system having acoustic components individually attached to each of the towfish and the plurality of electrodes; detecting electromagnetic signals returned from the subsea formation; and determining a geometry of the plurality of electrodes of the electromagnetic source. 
         [0013]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  shows a conventional CSEM survey system having a surface vessel towing an EM source at a selected distance from a sea floor, where a plurality of receivers are deployed. The surface vessel includes an acoustic ranging system (USBL) for determining the locations of the towfish and the receivers. 
           [0015]      FIG. 2  shows another CSEM survey system having a surface vessel towing an EM source at a selected distance from a sea floor, where a plurality of receivers are deployed. The towfish includes an acoustic ranging system (USBL) for determining the locations of the towfish with respect to the surface vessel. 
           [0016]      FIG. 3A  shows a conventional towfish having two electrodes in tow for generating electromagnetic (EM) fields into the subsea formation. 
           [0017]      FIG. 3B  shows a variation of a conventional towfish having four electrodes in tow for generating cross dipole EM fields into the subsea formation. 
           [0018]      FIG. 4A  shows an EM source in accordance with one embodiment of the invention. The EM source includes an acoustic ranging system for determining the geometry of the electrodes in the EM source. 
           [0019]      FIGS. 4B and 4C  show EM sources in accordance with other embodiments of the invention. The EM source includes an acoustic ranging system for determining the geometry of the electrodes in the EM source. 
           [0020]      FIG. 5  shows a system for marine EM survey in accordance with one embodiment of the invention. 
           [0021]      FIG. 6  shows a flow chart illustrating a method in accordance with one embodiment of the invention. 
           [0022]      FIG. 7  shows a block diagram illustrating a system for CSEM survey in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0023]    Embodiments of the invention relate to methods and systems for measuring the location, orientation, and geometry of an EM source, which may be used in CSEM (controlled source electromagnetic) prospecting. Embodiments of the invention use acoustic ranging systems to determine the geometry, orientation, and/or locations of the electrodes of an EM source. The acoustic ranging system includes various acoustic components attached to the EM source. Being able to determine these parameters with accuracy makes it possible to have more reliable subsea resistivity surveys. 
         [0024]    An acoustic ranging system comprises various acoustic components, which are transducers that can receive and transmit acoustic signals. These acoustic components may comprise similar or identical devices, but may be used for different roles (functions) in the system. In this description, different terms may be used to describe these components based on their functions. However, one of ordinary skill in the art would appreciate that these may be physically identical or similar units, even though they are used to play different roles. For example, the term “transceiver” would be used to denote the component that generates an acoustic pulse to a “transponder” and receives the reply signal from the transponder, whereas the term “transponder” is used to describe the acoustic component that receive the pulse from the transceiver and responds with a reply signal. 
         [0025]    Various EM sources are known in the art. For example,  FIG. 3A  shows a conventional CSEM source that includes dipole electrodes E 1 ,E 2  dragged behind the towfish  103 . These electrodes E 1 ,E 2  are often deployed in line with the travel direction of the towfish  103 . The electromagnetic field they generate will have a dipole in line with the towfish travel direction. Alternatively, the two electrodes E 1 ,E 2  may be deployed in other configurations to generate electric dipoles in different orientations. 
         [0026]    In addition to the traditional two-electrode system, a CSEM source may also include more than two electrodes. For example, U.S. Patent Application Publication No. 2007/0145980, by Conti et al. and assigned to the assignee of the present invention, discloses multi-component field sources for subsea exploration. One example is shown in  FIG. 3B , which shows a four-electrode CSEM source that can produce cross dipoles. In this example, the two additional electrodes are arranged in a direction that is substantially diagonal to the first pair of electrodes, E 1 ,E 2 . The electric dipole generated by E 3 ,E 4  would be in a direction that is different from that generated by E 1 ,E 2 . This EM source would allow one to survey the electrical properties of the subsea formations in two different orientations. 
         [0027]    Note that while these CSEM sources are described as having electrodes, one skilled in the art would appreciate that these EM sources may also comprise coils or antennas for generating electromagnetic fields. This description may use EM sources having electrodes to illustrate embodiments of the invention. However, this is for clarity of illustration and is not intended to limit the scope of the invention, i.e., embodiments of the invention may also be used with coil or antenna EM sources. The electrodes referred to in this invention may also refer to coils or other types of antennas that generate electromagnetic fields. 
         [0028]    When these CSEM sources are dragged in water during measurements, the geometry, orientation, and relative positions of these electrodes may change with time due to various factors, such as currents, travel speeds, etc. In order to obtain measurement data that can produce accurate estimates of the resistivities of the subsea formations, the geometry, orientation, and relative positions of these electrodes and towfish should be monitored. 
         [0029]    In accordance with embodiments of the invention, an EM source for use in CSEM survey may include one or more acoustic ranging system. The acoustic ranging systems included in the EM sources may be used to monitored the geometry, orientation, and relative positions of these electrodes during the survey. Because these acoustic ranging systems are located close to the EM source, they can determine the geometry of the various electrodes with high degree of accuracy. 
         [0030]      FIG. 4A  shows an example of a CSEM source in accordance with one embodiment of the invention. As shown, a transceiver  401  (USBL or a similar transceiver such as SBL) is included on the towfish  103 , while each electrodes E 1 ,E 2 ,E 3 ,E 4  is equipped with a transponder T 1 ,T 2 ,T 3 ,T 4 . The USBL or SBL transceiver  401  sends pulses to the transponders T 1 ,T 2 ,T 3 ,T 4  associated with the electrodes. The transponders T 1 ,T 2 ,T 3 ,T 4  send reply signals when a pulse from the transceiver  401  is received. The reply signals received by the transceiver  401  may then be used to determine the ranges (distances) and bearings of the individual electrodes E 1 ,E 2 ,E 3 ,E 4 . Therefore, the geometry, orientation, and relative positions of these electrodes E 1 ,E 2 ,E 3 ,E 4  can be monitored and/or recorded during a survey. 
         [0031]    Note that while the embodiment in  FIG. 4A  has the transceiver  401  disposed on the towfish  103 , one of ordinary skill in the art would appreciate that the transceiver  401  can also be placed at some other places, for example attached to the vicinity of the towfish  103 , as shown in  FIG. 4B . In the description, the term “attached to” will be used in a general sense to describe the placement of a transceiver on a towfish, whether the transceiver is disposed inside the towfish, on the outside of the towfish, or in the vicinity of the towfish as shown in  FIG. 4B . Similarly, the transponders T 1 ,T 2 ,T 3 ,T 4  may be disposed directly on the electrodes E 1 ,E 2 ,E 3 ,E 4  or in the vicinity of the electrodes E 1 ,E 2 ,E 3 ,E 4 , i.e., the transponders may be “attached to” the electrodes. 
         [0032]    The acoustic ranging systems illustrated in  FIG. 4A  and  FIG. 4B  are examples for determining the geometry (i.e., relative distances and bearings) between different electrodes and the towfish. Knowing the geometry of the electrodes indirectly gives the relative orientations and distances between the electrodes. One skilled in the art would appreciate that other modifications or variations of the acoustic ranging systems shown in  FIG. 4A  and  FIG. 4B  are possible without departing from the scope of the invention. For example, one may equip the electrodes with transceivers such that one can also determine the relative distances and bearings among the electrodes. In another example in  FIG. 4C , the acoustic ranging system is implemented in a conventional CSEM having two-electrodes E 1  and E 2 . The transceiver  401  is disposed in the vicinity of the towfish  103 , while the transponders T 1  and T 2  are on the electrodes E 1  and E 2 . 
         [0033]    In accordance with embodiments of the invention, the acoustic ranging systems for determining the geometry of the EM source electrodes may use any suitable devices, including USBL (ultra-short baseline), SBL (short baseline), LBL (long baseline) devices, or the like. Depending on the types of the devices, different methods may be used to determine the geometry of the electrodes. 
         [0034]    For example, in a system having a USBL device disposed on or near a towfish and transponders disposed on or near the electrodes, a pulse can be sent from the USBL to the transponders, each of which then respond with a reply signal. The times and phases of the reply signals received by the transducer array in the USBL are then used to derive the ranges (distances) and bearings of the transponders relative to the USBL, as described above. While a single USBL is typically sufficient for such applications, one may also use more than one USBL device arranged in a proper configuration to increase the robustness of the system, see for example the following discussion related to SBL devices. 
         [0035]    If SBL (instead of USBL) devices are used, one may use more than one SBL devices. The plurality of SBL transceivers (typically 3 or more) may be disposed on or near a towfish in a proper configuration such that the location (distance and bearing) of a transponder can be determined (e.g., by triangulation). If four SBL transceivers are used, they may be arranged in a tetrahedral configuration, for example, such that the location of any transponder can be determined in the 3D space. Of course, even more SBL transceivers may be used to establish a robust ranging system. 
         [0036]    If the acoustic devices form a sufficiently robust measurement geometry, it may be possible to determine the shape and size of the EM electrode configurations by measuring some or all ranges (distances) between pairs of the acoustic devices. For example, with four electrodes (see e.g.,  FIG. 3B ), there are six distances among them. Determination of 5 of these 6 distances will be sufficient to fix the geometry of the electrode arrangement. 
         [0037]    The determination (or monitoring) of the electrode geometry may be performed in real time during a survey. If any of the electrodes fails to respond at some time points, the missing information may be interpolated or determined using a model. 
         [0038]    Once the geometry of the electrodes (i.e., the relative locations of the electrodes) is determined, one may also want to determine the orientation of the EM source (or the towfish). The EM source attached to the towfish  103  is subject to various environmental impacts during a survey, including rotational disturbances such as roll, pitch, and yaw. These rotational disturbances may affect the orientation (attitude) of the towfish (hence, the orientation of the EM sources). An attitude determination system  402  ( FIG. 4A  or  4 B) may be disposed on (or in the vicinity of) the towfish to determine the attitude elements of the reference frame of the acoustic ranging system. The attitude determination system  402  determines the vertical tilt (or orientation) of the towfish with respect to an outside reference (e.g., the gravitational direction). 
         [0039]    Any orientation methods or devices known in the art may be used with embodiments of the invention. For example, an inertial platform (or an attitude system and a compass) may be used on the same vehicle that holds the SBL or USBL device. The inertial platform, for example, may be used to establish the orientation of the local coordinate frame of the SBL or USBL system. Alternatively, combinations of gyros and accelerometers may be used., see e.g., U.S. Pat. No. 4,038,876 issued to Morris. 
         [0040]    The above described processes can provide the orientation and geometry of an EM source. In accordance with embodiments of the invention, the geometry and orientation of a EM source determined by the acoustic system may be temporarily stored in a processor/memory on the towfish for later retrieval or transmitted to a processing system on the surface vessel  101  via a cable along the tow line  109 . 
         [0041]    In order to determine the EM source location in the survey frame, the EM source may be positioned (i.e., determination of its range and bearing) relative to the surface vessel. This may be accomplished using a transceiver (e.g., USBL, SBL or the like) attached to the bottom of the surface vessel and a transponder disposed on the towfish, as illustrated in  FIG. 1 . Alternatively, one may use a transceiver (e.g., USBL, SBL, or the like) on the towfish and a transponder attached to the surface vessel, as illustrated in  FIG. 2 . 
         [0042]      FIG. 5  shows an example of a CSEM survey in accordance with one embodiment of the invention (analogous to that shown in  FIG. 1 ). As shown, a transceiver  502  as part of an acoustic ranging system may be attached to the bottom of the surface vessel  101 . The transceiver  502  may comprise a USBL device, an SBL device, an LBL device, or the like. With this system, the transceiver  502  would send a pulse to the towfish  103 . A transponder  501  on the towfish  103  will respond with a reply signal upon receipt of the pulse from the transceiver  502 . The time and phase information received by the transceiver  502  are then used to determine the range and bearing of the towfish  103 . This information can then be used to fix the locally determined EM source geometry and orientation in the survey reference frame (e.g., with respect to the surface vessel  101 ). 
         [0043]    In an alternative configuration (analogous to that shown in  FIG. 2 ), the transceiver may be attached to (or disposed in the vicinity of) the towfish  103 , and the transponder may be disposed below the surface vessel  101 . That is, the  501  on the towfish  103  may comprise a transceiver, while the device  502  attached to the bottom of the surface vessel  101  may comprise a transponder. Note that the description uses “transceivers” and “transponders” as if they are different devices. This is for clarity of illustration only. One of ordinary skill in the art would appreciate that these may be same or similar types of devices (devices that are capable of transmitting and receiving acoustic signals) that are used for different functions. 
         [0044]    In the configuration shown in  FIG. 5 , the transceiver or transponder  501  on the towfish  103  may be the same as that used for the determination of the geometry of the EM source electrodes, i.e., a shared device (transceiver) is used for both determinations. 
         [0045]    Having determined the distance and bearing of the towfish  103  relative to the surface vessel  101 , the geometry and orientation of the EM source, as determined in the local reference frame with respect to the USBL on the towfish  103 , can then be correlated with the reference frame with respect to the surface vessel  101 . Furthermore, a global positioning system (GPS) on the surface vessel may be used to fix the location and orientation of the EM source with respect to the global reference (earth) frame. 
         [0046]    In addition to the method described above with reference to  FIG. 5 , other methods may also be used to determine the location and orientation of the EM source electrodes with respect to a global reference frame. The following will describe a few such examples. 
         [0047]    In an alternative approach, the acoustic ranging system  502  attached to the vessel may be used to determine the ranges and bearings of three or more of the electrodes of the EM source. These measurements will determine the locations of these three or more electrodes in the global reference frame. Having the locations of these same three or more electrodes in the local reference frame (with respect to the towfish) as well as in the global reference frame, it becomes possible to derive a transformation that can be used to transform any point in the local reference frame into the locations in the global reference frame. 
         [0048]    In yet another approach, three or more transponders may be disposed near the surface with their locations defined in the global reference frame. The locations of these three or more transponders are then determined with respect to the local reference frame of the towfish, using the transceiver disposed on the towfish. These determinations will provide locations of the three or more transponders in both the global reference frame and the local reference frame (towfish reference frame). Again, a transformation may be derived from this information and used to convert any point in the local reference frame into locations in the global reference frame. 
         [0049]    The last described method may be combined with the first method described with reference to  FIG. 5 . In the combination approach, fewer than 3 transponders near the surface may be used. For example, if the orientation of the USBL (or SBL) transducer on the towfish is know—e.g., determined with an accelerometer aligned with the transducer axis, then it will be sufficient to measure only two transponders near the surface using the USBL device on the towfish. 
         [0050]    When transforming the local coordinates to the global reference frame, it is preferably to apply a shape and size conserving transformation (congruent transformation) such that the high internal accuracy used to describe the source geometry is retained. 
         [0051]      FIG. 6  shows a flow chart illustrating a method for determining the geometry, orientation, and position of an EM source used in a CSEM marine survey. As shown, a method  60  may start with determination of the geometry of the various electrodes of the EM source (step  61 ). This may be accomplished using any suitable acoustic ranging system described above. Once the geometry of the EM sources (electrodes) are determined, one may determine the attitude (orientation) of the EM source with respect to an outside reference frame (e.g., the gravitational direction) (step  62 ). Note that the order of steps  61  and  62  may be reversed or these two steps may be performed simultaneously. Then, the locally defined geometry and orientation of the EM source may be correlated with an external frame with reference to the surface vessel (step  63 ). Finally, the EM source geometry, orientation, and location may be further fixed with respect to the earth frame by using a GPS system (step  64 ). In fixing (positioning) the EM source to the survey reference frame or the global reference frame, the processing system may apply a shape and size conserving transformation (congruent transformation) such that the accuracy of the geometry measurement of the EM source is preserved. 
         [0052]      FIG. 7  shows a block diagram of an example positioning system in accordance with one embodiment of the invention. A processing/control system  71  communicates with the acoustic ranging system and attitude system on a towfish  72 . The communication may include control of the operation of the ranging and attitude system, as well as transmission of the results from the towfish to the processing system. The processing/control system  71  similarly controls and communicates with the acoustic ranging system onboard the surface vessel  73 . The processing/control system  71  also performs the transformation of the EM source geometry and orientation from the local reference frame to the global reference frame. The processing/control system  71  also communicates with a display  74 , which may be used for inputting the control parameters and displaying the results. The processing/control system  71  may be onboard the surface vessel. Alternatively, part or all of the processing/control system  71  may be located remotely. One of ordinary skill in the art would also appreciate that the processing/control system  71  may be on a computer system or multiple computer systems. 
         [0053]    Advantages of embodiments of the invention may include one or more of the followings. Embodiments of the invention can provide accurate geometry of an EM source such that the survey can produce more accurate results. By having an acoustic ranging system on a towfish, the geometry of the EM source can be determined with high accuracy. These determination may be performed in real time to monitor the survey conditions or to provide data for correcting survey results at a later time. The acoustic ranging system can use various devices that are known in the art, including USBL, SBL, and LBL. The methods and systems of the invention can be used with various types of EM sources, including electrode type and antenna/coil type EM sources. 
         [0054]    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.