Abstract:
To perform a survey of a subterranean structure behind a subsea surface, at least one sensor module is provided in a subsea environment, where the at least one sensor module comprises at least one magnetic field sensor. Measurement data is received from the magnetic field sensor, and an electric field along a particular direction is determined based on the measurement data to perform the survey of the subterranean structure, wherein the particular direction is generally orthogonal to the subsea surface.

Description:
TECHNICAL FIELD 
       [0001]    The invention generally relates to determining an electric field based on measurement data from a magnetic field sensor for surveying a subterranean structure behind a subsea surface. 
       BACKGROUND 
       [0002]    Various electromagnetic techniques exist to perform surveys of subterranean structures for identifying structures of interest, such as structures containing hydrocarbons. One such technique is the magnetotelluric (MT) survey technique that employs time measurements of naturally occurring electric and magnetic fields for determining the electrical conductivity distribution beneath the surface. Another technique typically used in subsea environments is the controlled source electromagnetic surveying technique, in which an electromagnetic transmitter is placed or towed in sea water. Surveying units containing electric and magnetic field sensors are deployed on a seabed within an area of interest to make measurements from which a geological survey of the subterranean structure underneath a seabed can be derived. 
         [0003]    In one type of electromagnetic surveying technique, each of the surveying units includes horizontal electric field sensors, magnetic field sensors, and a vertical electric field sensor. The vertical electric field sensor is arranged in a vertical orientation relative to the generally horizontal seabed. However, this vertical electric field sensor is subjected to motion within the sea water, such as motion due to ocean currents, which provides a source of noise that may adversely affect accuracy. 
       SUMMARY 
       [0004]    In general, a sensor module is provided that has at least one magnetic field sensor to perform at least one magnetic field measurement. A vertical electric field can be determined based on the magnetic field measurement(s) such that a vertical electric field sensor does not have to be used. 
         [0005]    Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  schematically illustrates an example arrangement for performing a survey of a subterranean structure underneath a seabed (or sea floor), in accordance with an embodiment. 
           [0007]      FIGS. 2A-2B  illustrate an arrangement of magnetic field sensors for making magnetic field measurements from which a vertical electric field can be derived, in accordance with an embodiment. 
           [0008]      FIG. 3  is a chart containing several curves to illustrate simulated measured data values and calculations based on the simulated measured data values from the magnetic field sensors of  FIGS. 2A-2B , in accordance with an embodiment. 
           [0009]      FIGS. 4A-4B  depict charts containing curves illustrating differences between vertical electric field values calculated for a subterranean structure containing a hydrocarbon layer and vertical electric field values calculated for a subterranean structure that does not contain a subterranean layer, based on calculations according to an embodiment. 
           [0010]      FIG. 5  illustrates a toroidal sensor for making a magnetic field measurement from which a vertical electric field can be calculated, according to another embodiment. 
           [0011]      FIGS. 6A-6B  illustrate alternative techniques for obtaining gradients of magnetic fields, in accordance with some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0013]      FIG. 1  illustrates an example arrangement for performing controlled source electromagnetic marine surveying. As depicted in  FIG. 1 , a sea vessel  100  is capable of towing an electromagnetic transmitter  102  in sea water. The electromagnetic transmitter  102  is an electrical dipole in one example embodiment. Typically, the electromagnetic transmitter  102  is arranged a relatively short distance above the seabed (or sea floor)  104 . As examples, the relatively short distance of the transmitter  102  above the seabed  104  can be 50 meters or less. Although only one electromagnetic transmitter  102  is depicted, it is contemplated that alternative embodiments may use two or more electromagnetic transmitters  102  (described further below in connection with  FIG. 6 ). 
         [0014]    The electromagnetic transmitter  102  is coupled by a cable  106  to a signal generator  108  on the sea vessel  100 . Alternatively, the signal generator  108  can be contained within the electromagnetic transmitter  102 . The signal generator  108  controls the frequency and magnitude of the electromagnetic signal generated by the transmitter  102 . 
         [0015]    In one embodiment, a plurality of sensor modules  110  are arranged on the seabed  104 . In the example of  FIG. 1 , the plurality of sensor modules  110  are arranged in a row. In other embodiments, the sensor modules  110  can have other arrangements (such as an array of sensor modules or some random arrangement of sensor modules). 
         [0016]    Each sensor module  110  includes various sensors, including magnetic field sensors for making magnetic field measurements. In accordance with some embodiments, the magnetic field sensors are arranged in a predetermined pattern such that a vertical electric field can be computed based on the magnetic field measurements. The ability to compute the vertical electric field using magnetic field measurements avoids the need for including a vertical electric field sensor in each of the sensor modules  110 . Eliminating the vertical electric field sensor allows for more compact sensor module designs, as well as removes a source of potential noise due to movement of the vertical electric field sensor due to sea water currents. 
         [0017]    In another embodiment, described further in connection with  FIG. 5  below, instead of using plural magnetic field sensors in each sensor module, one special type of magnetic field sensor can be employed. A vertical electric field can also be computed based on magnetic field measurement(s) made by this special type of magnetic field sensor. 
         [0018]    The vertical electric field is a useful parameter for surveying the subterranean structure  112  underneath the seabed  104 . In the example of  FIG. 1 , the subterranean structure  112  includes a layer  114  that has a reservoir of hydrocarbons. The hydrocarbon layer  114  is a relatively resistive layer (compared to the other parts of the subterranean structure  112 ). The presence of the resistive layer  114  in the subterranean structure  112  affects the vertical electric field that is readily noticeable. By using the surveying technique according to some embodiments, more efficient and accurate hydrocarbon exploration surveying of the subterranean structure  112  can be performed to enable the identification of the layer  114  containing hydrocarbons. In other implementations, the surveying technique can be used for other applications where surveying of subterranean structures is desirable. 
         [0019]    The example configuration of the subterranean structure  112  depicted in  FIG. 1  is an example of a one-dimensional halfspace configuration, which is the layer cake configuration where the subterranean structure  112  includes various layers that are generally horizontal and parallel to each other. However, the subterranean structure  112  can have a more complex configuration, such as an inhomogeneous halfspace configuration, where structures containing elements of interest (such as hydrocarbons) are two-dimensional in nature (e.g., rather than a generally horizontal layer of hydrocarbons, the inhomogeneous halfspace configuration may have a hydrocarbon-containing structure that has both horizontal and vertical components). 
         [0020]    Although the discussion herein focuses on computing a vertical electric field based on measurement data from magnetic field sensors, it should be noted that electric fields in other directions can be calculated based on magnetic field sensors having other orientations relative to a subsea surface. In one example, as discussed above, the subsea surface is the seabed  104 . However, in other examples, a subsea surface can have an inclined or even a vertical orientation. Measurement data from sensor modules arranged on such a non-horizontal subsea surface can be used to calculate an electric field in a direction that is generally orthogonal to the subsea surface. The term “generally orthogonal” is used in light of the fact that subsea surfaces, including the seabed  114 , are not perfectly flat, so that the electric field computed is usually not perfectly orthogonal to the subsea surface. The term “vertical electric field” is also intended to cover situations where the seabed  104  may be at a slight angle such that the electric field derived from measurement data from magnetic field sensors would not be perfectly in the vertical direction, but would be substantially or generally in the vertical direction. 
         [0021]    Each of the sensor modules  110  includes a storage device for storing measurements made by the various sensors, including magnetic field sensors, in the sensor module  110 . The stored measurement data is retrieved at a later time when the sensor modules  110  are retrieved to the sea vessel  100 . The retrieved measurement data can be uploaded to a computer  116  on the sea vessel  100 , which computer  116  has analysis software  118  capable of analyzing the measurement data for the purpose of creating a map of the subterranean structure  112 . The analysis software  118  in the computer  116  is executable on a central processing unit (CPU)  120  (or plural CPUs), which is coupled to a storage  122 . An interface  124  that is coupled to the CPU  120  is provided to allow communication between the computer  116  and an external device. For example, the external device may be a removable storage device containing measurement data measured by the sensor modules  110 . Alternatively, the interface  124  can be coupled to a communications device for enabling communications of measurement data between the computer  116  and the sensor modules  110 , where the communications can be wired communications or wireless communications. The wired or wireless communications can be performed when the sensor modules  110  have been retrieved to the sea vessel  100 . Alternatively, the wired or wireless communications can be performed while the sensor modules  110  remain on the sea floor  104 . 
         [0022]    Alternatively, instead of providing the computer  116  (and the analysis software  118 ) on the sea vessel  100 , the computer  116  can instead be located at a remote location (e.g., at a land location). The measurement data from the sensor modules  11  can be communicated by a wireless link (e.g., satellite link) from the sea vessel  100  to the remote location. In yet another alternative, each sensor module  110  can include processing circuitry to process the measurement data and derive electric field values in accordance with some embodiments. 
         [0023]      FIG. 2A  is a schematic representation of various magnetic field intensities  202 ,  204 ,  206  and  208  in different respective orientations and locations. The magnetic field intensities  202 ,  204 ,  206  and  208  are measured by corresponding magnetic field sensors, such as sensors  252 ,  254 ,  256  and  258  that are part of a sensor module  110  depicted in  FIG. 2B . The magnetic field sensors  252 ,  254 ,  256  and  258  can be magnetic induction coil sensors, where each such sensor includes a high magnetic permeability metallic cylindrical core around which an electrical wire is wound. As depicted in  FIG. 2B , the magnetic field sensors  252 ,  254 ,  256  and  258  are attached to a housing  260  of the sensor module  110 . Other sensors may also be provided in the sensor module  262 , such as horizontal electric field sensors (not shown). 
         [0024]    The magnetic field intensities  202  and  204  extend in a first direction (represented as a y direction or axis), while the magnetic field intensities  206  and  208  extend in a second, orthogonal direction (the x direction or axis). The y-direction magnetic field intensities  202  and  204  are represented as H −   y  and H +   y , where the − symbol and + symbol are used to indicate relative position of the corresponding magnetic field with respect to a center vertical axis  210  (which is in another direction, the z direction or axis, that is orthogonal to both the x and y directions). The magnetic field intensity H −   x  is on the negative side of the x axis, whereas the magnetic field intensity H +   x  is on the positive side of the x axis. 
         [0025]    Similarly, the x-direction magnetic field intensities  206  and  208  are represented as H −   x  and H +   x  . The magnetic field intensity H −   y  is on the negative side of the y axis, whereas the magnetic field intensity H +   y  is on the positive side of the y axis. 
         [0026]    The magnetic field intensities H −   y  and H +   y  are magnetic field intensities in the y direction that are spaced apart along the x direction, while the magnetic field intensities H −   x  and H +   x  are magnetic field intensities in the x direction that are spaced apart along the y direction. From the magnetic field intensities H −   y , H +   y , H −   x  and H +   x , a vertical electric field, represented as E x , can be computed or derived without the need for using a vertically arranged electric field sensor. The vertical electric field E x  extends in the z direction. 
         [0027]    As depicted in  FIG. 2B , electrical wires  262 ,  264 ,  266 , and  268  extend from respective sensors  252 ,  254 ,  256 , and  258  to a measurement device  270 . In some implementations, the measurement device  270  measures voltages provided by current flows in the electrical wires  262 ,  264 ,  266 , and  268 , respectively. The current flows in the electrical wires  262 ,  264 ,  266 , and  268  are induced by corresponding magnetic field intensities H −   y , H +   y , H −   x  and H +   x . The measured voltages are stored in a storage device  272  in the sensor module  110  for subsequent processing, such as by the computer  116  ( FIG. 1 ). More generally, the measurement device  270  produces measurement data (e.g., measured voltages, measured currents, measured magnetic field values, etc.) that is stored in the storage device  272 , which measurement data is subsequently processed to produce a vertical electric field value according to some embodiments. 
         [0028]    To derive the vertical electric field from magnetic fields, techniques according to some embodiments make use of a fundamental physical relationship (Ampere&#39;s law) to relate spatial derivatives of magnetic fields to electric fields. Ampere&#39;s law states that the curl of a magnetic field, H, is equal to the electric current density, J: 
         [0000]        V×H=J,   (Eq. 1) 
         [0029]    Combining Eq. 1 with Ohm&#39;s law, 
         [0000]      J=σE,  (Eq. 2) 
         [0000]    which states that the electric current is equal to the product of the conductivity, σ, and electric field, E, yields Eq. 3 as provided below: 
         [0000]        V×H=σE,   (Eq. 3) 
         [0030]    Thus the curl of the magnetic field is proportional to the electric field. If the vertical component of the electric field (E z ) is considered, 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0000]    where 
         [0000]    
       
         
           
             
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         [0000]    is the partial spatial derivative of H in the x direction, 
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         [0000]    is the partial spatial derivative of H in the y direction, and k represents a unit vector (in the z direction). 
         [0031]    Eq. 4 relates the spatial derivatives of the horizontal magnetic fields to the vertical electrical field. These spatial derivatives can be approximated using finite differences which, to a second order approximation, are 
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         [0000]    where H +   y , H −   y , H +   x , and H −   x  are the magnetic field intensities illustrated in  FIG. 2A  that are capable of being measured using sensors  254 ,  252 ,  258 , and  256 , respectively. 
         [0032]    In  FIG. 2A , the H +   x  and H −   x  fields are separated (or spaced apart) by a distance in the y direction (Δy). Similarly the H +   y  and H −   y  field intensities are separated by a distance in the x direction (Δx). By measuring the changes of the horizontal magnetic field intensities in these directions (according to Eqs. 5 and 6 above), it is possible to calculate the vertical current density, J z , and, using the electrical conductivity, the vertical electrical field E z . 
         [0033]    In operation, according to the arrangement of  FIG. 1 , the sensor modules  110  are arranged in the x direction, with the sensor modules  110  spaced apart from each other by some predetermined distance (e.g., 100 meters). Each sensor module  110  records measurement data based on magnetic field intensities sensed by corresponding magnetic field sensors in the sensor module  110 . The electromagnetic transmitter  102  produces an electromagnetic signal at a predetermined frequency (e.g., between 0.1 Hz and 100 Hz) and at a predetermined magnitude. The measurements are taken along the x direction at every point (a point corresponds to a location of each sensor module, where two points are spaced apart) relative to the source, the electromagnetic transmitter  102 . The measurement data recorded by the sensor modules  110  are stored (such as in the storage devices  272  ( FIG. 2 ) in corresponding sensor modules). 
         [0034]    Once the measurement data is provided to the analysis software  118  in the computer  116  ( FIG. 1 ), the magnetic field intensities H +   y , H −   y , H +   x , and H −   x  are readily derived. From the magnetic field intensities, the vertical electric field at each point (corresponding to a respective sensor module  110 ) along the x direction can be computed by the analysis software  118  using Eqs. 4-6 above. 
         [0035]    In some embodiment, the analysis software  118  processes measurement data collected from the sensor modules  110  one at a time to derive the vertical electric field at the location of the corresponding sensor module  110 . However, in accordance with another embodiment, measurement data from multiple sensor modules can be combined and processed to produce the vertical electric field. Thus, the measurement data from the multiple sensor modules can be used to derive magnetic field intensities H +   y , H −   y , H +   x , and H −   x  associated with the multiple sensor modules  110 , with the magnetic field intensities combined (such as averaged), which combined magnetic field intensities are used to compute the vertical electric field. In some implementations, if measurement data from multiple sensor modules are to be combined, then some procedure is used to ensure that the multiple sensor modules are aligned with respect to each other (in other words, the sensors  252 ,  254  of one sensor module are parallel to the sensors  252 ,  254  of another sensor module, and the sensors  256 ,  258  of one sensor module are parallel to the sensors  256 ,  258  of another sensor module). Alternatively, if the sensor modules cannot be aligned, then the amount of misalignment between sensor modules can be determined so that the misalignment can be accounted for when combining the measurement data. 
         [0036]      FIG. 3  shows several curves corresponding to example values for magnetic field intensities H y  and H x , the spatial derivatives of these magnetic field intensity values, including 
         [0000]    
       
         
           
             
               
                 
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         [0000]    and electric fields E x  (the vertical electric field affected by the subterranean structure  112  containing the resistive layer  114 ) and E z   REF  (the vertical electric field when no resistive layer  114  is in the subterranean structure  112 ). The values of E z   REF  are plotted in  FIG. 3  to illustrate the differences between E z   REF  and E z . Note that H y  represents either H +   y  or H −   x , and H x  represents either H +   x  or H −   x . Due to the closeness of the H +   y  and H −   x  values, and the closeness of the H +   y  and H −   x  values, only one value from each pair are depicted for better clarity. 
         [0037]    The vertical axis of the chart in  FIG. 3  represents the log 10  magnitude, while the horizontal axis represents the offset (in meters) from a reference point (the electromagnetic transmitter  102 ). Note that the values represented in the charts are merely example values. 
         [0038]      FIGS. 4A and 4B  are charts for representing the percentage differences between E z  and E z   REF . The vertical axis of the charts in  FIGS. 4A and 4B  represent the percent difference expressed as 100·[(E x −E z   REF )/E z ].  FIG. 4A  represents curves from offsets 0 to 5000 meters, while  FIG. 4B  represents curves from offsets 5000 to 10,000 meters. Curve  400  represents the percentage difference due to the imaginary (or out-of-phase) component of E z , while curve  402  presents the percentage difference due to the real component of E z . As indicated in  FIGS. 4A-4B , there is a strong response in the vertical electric field E z  at offsets greater than about 3,000 meters, in the depicted example, especially in the imaginary component (curve  400 ) of E z . 
         [0039]    To provide the desired accuracy, the type of magnetic field sensor used in each sensor module  110  can be selected based on the noise levels and sensitivities of the magnetic field sensors at particular frequencies. Relatively sensitive magnetic field sensors would be able to make more accurate measurements, but may be susceptible to external noise such as minute movements in the earth&#39;s magnetic field. However, to compensate for such motion-based noise, two magnetic field sensors can be mounted on a rigid frame of the sensor module  110  in the spaced apart arrangements depicted in  FIG. 2B . 
         [0040]    The above discussion assumes use of a first type of magnetic field sensors with a cylindrical core around which electrical wires are wound. In another embodiment, a circular toroidal sensor  500  as depicted in  FIG. 5  can be used in place of the magnetic field sensors  252 ,  254 ,  256 , and  258  depicted in  FIG. 2B . The toroidal sensor  500  is based on using the line integral formulation of Ampere&#39;s law 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    which means that the line integral around a closed path is equal to the current I flowing normal to the plane of the path. If the toroidal sensor  500  is placed in a plane generally parallel to the seabed  104  ( FIG. 1 ), then the current I flowing normal to the plane of the path would be the vertical current in the z direction that is affected by a resistive layer in the subterranean structure  112  as discussed above. The circular toroidal sensor  500  is arranged in a loop of radius R. The total current I normal to the plane of the toroid is 
         [0000]      I=πR 2 J 2 ,  (Eq. 8) 
         [0041]    The toroid is wrapped on a high magnetic permeability metallic core of cross-sectional area α with a predetermined effective permeability (e.g., 200). Applying Ampere&#39;s law to the path containing the field within the core. 
         [0000]    
       
         
           
             
               
                 
                   
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         [0042]    Using the relationship of Eq. 10, the magnetic field H derived based on measurements by the sensor  500  of  FIG. 5  can be used to calculate the vertical electric current density J z . The toroidal sensor  500  of  FIG. 5  can achieve the desired level of sensitivity to provide accurate measurements from which J z  can be computed. 
         [0043]    In the discussion above, it is assumed that there is a single electromagnetic transmitter (e.g.,  102  in  FIG. 1 ). Alternatively, multiple electromagnetic transmitters can be used. This alternative embodiment involves gradients measured by using successive measurements of H x  and H y  for different positions of the transmitter. The rigorous application of Ampere&#39;s law 
         [0000]    
       
         
           
             
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         [0000]    requires that gradients be measured across baselines that are short (in other words, distances between sensor modules  110  are short) compared to the dimensions of the model of the subterranean structure  112  and for a fixed position of the source. 
         [0044]    Since the vertical current density J z  is particularly sensitive to the presence of a resistor (resistive layer  114 ) at depth, measurements of gradients of H along the x and y directions that are proportional to J z  but not necessarily equal to it would be valuable parameters for resolving the model. Approximate gradients of H can be synthesized by differencing the fields measured by a single sensor module for two spatial positions of the source (electromagnetic transmitter), unlike the previous embodiments where differences are taken for a single source and two spatial positions of the sensor modules. 
         [0045]      FIG. 6B  depicts an x-directed first electromagnetic transmitter  610  (similar to electromagnetic transmitter  102  in  FIG. 1 ) located a distance xI from a sensor module  614 , which measures the magnetic field intensity in the y direction, H y1 , A second electromagnetic transmitter  612  is located at a second position x 2  a distance h from x 1 . The sensor module  614  in this case measures the magnetic field intensity, H y2 , in the y direction. Note that the first and second electromagnetic transmitters  610  and  612  can be two different electromagnetic transmitters that concurrently produce electromagnetic signals. Alternatively, the first and second electromagnetic transmitters  610  and  612  can be a single transmitter moved between two different positions, where the electromagnetic transmitter produces a first electromagnetic signal at a first position, and produces a second electromagnetic signal at a second position spaced apart from the first position. 
         [0046]    The difference H y2 -H y1  divided by h (gradient of H y  in the x direction) is approximately the same as the difference in field between two sensor modules  602  and  604  a distance h apart for a fixed transmitter  600  at position (x 2 +x 1 )/2, ad depicted in  FIG. 6A . This equivalence is exact over a one dimensional halfspace (layer cake arrangement of the subterranean structure where the layers are generally horizontal), but is only approximately true over an inhomogeneous halfspace (arrangement of the subterranean structure where a resistive structure may extend in three dimensions). 
         [0047]    Similarly the H x  gradient in the y direction is obtained from a transmitter (or plural transmitters) displaced by h in the y direction. This is exactly equivalent to the gradient obtained with two receivers separated by h in the y direction. 
         [0048]    A benefit of this scheme is that a particular gradient sensitivity (e.g., 1 fT/m or femto-Tesla per meter) to achieve an adequate resolution of J x  can be achieved with sensors of lower sensitivity (e.g., 100 fT resolution separated by 100 m). Consequently, existing sensors having noise levels of 200 fT at 0.3 Hz can be used to determine J z  to the desired accuracy if position accuracy or parallel transmitter tracks can be obtained. 
         [0049]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.