Abstract:
Systems and methods for marine surveying of strata beneath a seafloor are disclosed, including, in certain aspects, systems employing one or more cables with a plurality of opto-electrical detector electrodes and an electrical wire with a fixed reference potential imposed thereon along the cable length. In a multi-cable system, the same fixed reference potential is applied to all wires in all cables. This abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure and is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims, 37 C.F.R. 1.72(b).

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       SEQUENCE LISTING, TABLE, OR COMPUTER LISTING 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present invention generally relates to the field of electromagnetic survey apparatus for subsurface exploration in the Earth. More particularly, the invention relates to structures for opto-electrical detector electrodes and arrays thereof for detection of electric fields resulting from electromagnetic fields imparted into the Earth; and, in certain particular aspects, to such systems in which a fixed reference potential is provided on a wire for all opto-electrical detector electrodes. 
         [0006]    2. Description of Related Art 
         [0007]    The structure and character of subsurface geological formations underlying a body of water are investigated, surveyed, and mapped using a variety of marine exploration techniques and systems. 
         [0008]    To identify hydrocarbon reserves in formations below the bottom of a body of water such as a lake or the ocean (called marine controlled source electromagnetic—“CSEM” —surveying) a geophysical surveying technique is used that uses electromagnetic (EM) energy. In a typical 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 disposed on the water bottom over the area of interest obtain signals related to the distribution of electrical resistivity in the subsurface strata 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 overlaying seawater and downwardly into the formations below the water bottom. 
         [0009]    The receivers most commonly used detect and record the induced electric field. The time varying EM field may be induced by passing electric current through an antenna.  FIG. 1A  shows a known marine CSEM surveying system, as illustrated in International Publication No. WO 02/14906, which includes a vessel  31  towing a cable (or streamer)  32  just above the seabed  33 . The cable  32  carries a transmitter dipole antenna  34  and several receiver dipoles  35 . The transmitter dipole antenna  34  is controlled from the vessel  31  via the cable  32 , and the responses detected by the receiver dipoles  35  are relayed back to the vessel  31  in real time via the cable  32 . The publication also shows an arrangement, as illustrated in  FIG. 1B , in which the vessel  31  tows three parallel cables  41 ,  42 ,  43 , each carrying a series of receivers  45 ,  46 ,  47 . The spacing between the receivers  45 ,  46 ,  47  is achieved by means of a spar  44 . A transmitter  48  is located on the cable  42 . The transmitter  48  has two dipole antennae arranged mutually at right angles. Each receiver also comprises two dipoles mutually at right angles. Measurements can be taken with the transmitter and receiver both inline and parallel. A characteristic difference in values indicates a highly resistive layer located beneath a highly conductive layer. 
         [0010]      FIG. 1C  shows a known system with fiber optic sensors useful in a marine exploration system (a system as disclosed in U.S. Pat. No. 6,314,050, co-owned with the present invention and fully incorporated herein for all purposes). 
         [0011]    In systems and methods as shown in  FIG. 1C , an electrical signal is converted to an optical signal in a fiber optic system. The electrical signal produced by a sensor based upon a parameter being measured is connected across a material that changes dimension responsive to an applied electrical signal. An optical fiber is coupled to the material where dimension changes of the material produce strain in the optical fiber. This strain is operable to affect light traveling through the optical fiber to produce an optical signal for a fiber optic system. Also, the material that changes dimension responsive to an applied electrical signal can be, for example, a piezoelectric ceramic cylinder, a PVDF film, or other piezo-polymer material. 
         [0012]    The system of  FIG. 1C  uses a Mach-Zehnder interferometer with one arm wrapped around a piezoelectric ceramic cylinder or “PZT” to convert an electrical signal, such as from an electrical sensor or a summed group of sensors, into differential interferometric phase. 
         [0013]    As shown, a sensor  50  creates a voltage output related to the parameter it is measuring. The output voltage is then placed across a material  52  that changes dimension (e.g., contracts and expands) responsive to the applied output voltage. An optical fiber  54  is wrapped around material  52 , and optical fiber  54  is strained by the dimension change of material  52 . The system also includes a reference optical fiber  56 . To make an interferometer, optical couplers  58  can be fusion spliced, indicated at  60 , to sensing optical fiber  54  and reference optical fiber  56 , as shown. 
         [0014]    The sensor  50  in  FIG. 1C  can be a conventional detector electrode. Typically, a detector electrode used in seismic exploration applications has a voltage output on the order of some nanovolts (although this can be varied depending on the strength of the magnetic field). Applying this voltage across a standard piezoelectric ceramic cylinder (PZT) induces, for example, an approximately 5 nm/volt change of the mean diameter of a PZT having a one inch diameter and a 0.05 inch thick wall. This change can be translated into a length change in the optical fiber. 
         [0015]    With fiber optic sensors, sensor arrays have been significantly improved by the use of fiber optic telemetry and the increased dynamic range that is provided by an interferometric system. 
         [0016]    The present inventor has recognized and addressed the need for an effective and efficient system and method for providing measurements with a plurality of fiber optic detector electrodes in a multi-cable array. 
       BRIEF SUMMARY OF THE INVENTION 
       [0017]    The present invention, in certain aspects, discloses a cable for a marine exploration system, the cable including: a cable body, the cable body having a body length; a plurality of opto-electrical detector electrodes spaced-apart along the body length; an electrical wire extending along the body length; and a fixed reference potential imposable on the electrical wire. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]    A more particular description of embodiments of the invention briefly summarized above may be had by references to the embodiments which are shown in the drawings which form a part of this specification. These drawings illustrate certain embodiments and are not to be used to improperly limit the scope of the invention which may have other equally effective or equivalent embodiments. 
           [0019]      FIG. 1A  is a side schematic view of a prior art marine exploration system. 
           [0020]      FIG. 1B  is a top view of the system of  FIG. 1A . 
           [0021]      FIG. 1C  is a side schematic view of part of a prior art marine exploration system. 
           [0022]      FIG. 2  is a top schematic view of a system according to the present invention. 
           [0023]      FIG. 3  is a cross-section view of a sensor cable according to the present invention. 
           [0024]      FIG. 4A  is a side schematic view of a system according to the present invention. 
           [0025]      FIG. 4B  is a top schematic view of the system of  FIG. 4A . 
           [0026]      FIG. 4C  is a cross-section view of part of the system of  FIG. 4A . 
       
    
    
       [0027]    Certain embodiments of the invention are shown in the above-identified figures and described in detail below. Various aspects and features of embodiments of the invention are described below and some are set out in the dependent claims. Any combination of aspects and/or features described below or shown in the dependent claims can be used except where such aspects and/or features are mutually exclusive. It should be understood that the appended drawings and description herein are of certain particular embodiments and are not intended to limit the invention or the appended claims. On the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. In showing and describing the detailed embodiments, like or identical reference numerals are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
         [0028]    As used herein and throughout all the various portions (and headings) of this patent, the terms “invention”, “present invention” and variations thereof mean one or more embodiment, and are not intended to mean the claimed invention of any particular appended claim(s) or all of the appended claims. Accordingly, the subject or topic of each such reference is not automatically or necessarily part of, or required by, any particular claim(s) merely because of such reference. So long as they are not mutually exclusive or contradictory any aspect or feature or combination of aspects or features of any embodiment disclosed herein may be used in any other embodiment disclosed herein. 
       DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The present invention, in certain aspects, discloses systems and methods for measuring responses in different directions with a multi-cable flexible fiber optic receiver system with multiple detector electrodes. In certain particular aspects, the present invention provides a three-dimensional multi-cable array and an efficient and effective method for taking measurements between any two detector electrodes in the array. 
         [0030]    The present invention, in certain aspects, achieves measurement in different directions without electrical wiring being routed from one sensor cable to another. 
         [0031]    In certain aspects, the present invention discloses systems and methods in which each cable of a multi-cable array includes an electrical wire with a fixed reference potential imposed thereon which is the same for each wire and, therefore, this same potential is the reference for all detector electrodes on all cables. 
         [0032]    The present invention, in certain aspects, discloses a cable for a marine exploration system, the cable including: a cable body, the cable body having a body length; a plurality of detector electrodes (opto-electrical electrodes) spaced-apart along the body length for sensing signals related to underground strata; and an electrical wire with a fixed reference potential imposed thereon, the electrical wire extending along the entire body length of the cable. 
         [0033]    The present invention, in certain aspects, discloses a method for marine exploration including: receiving signals reflected from underground strata with a receiver system; the receiver system including at least one cable, the at least one cable having a cable body, the cable body having a body length, a plurality of detector electrodes (e.g. opto-electrical electrodes) spaced-apart along the body length, and an electrical wire with a fixed reference potential imposed thereon, the electrical wire extending along the entire body length; imposing a fixed reference potential on the electrical wire of the at least one cable; the detector electrode pairs outputting output voltage signals corresponding to the received signals related to the underground strata; and comparing the output voltage signals to the fixed reference potential. In one aspect the at least one cable is a plurality of cables spaced-apart vertically, spaced-apart horizontally, or in a 3-D array both vertically and horizontally. 
         [0034]    Referring now to  FIG. 2 , a system  100  according to the present invention has a vessel  114  which tows a streamer array of multiple spaced-apart cables  111 ,  112 , and  113 . Each cable may have any desired number of opto-electrical detector electrodes  116  (e.g. as in  FIG. 1A ) and, in certain aspects, each cable has several thousand opto-electrical detector electrodes. It is within the scope of the present invention to employ any desired number of cables, one or more, in a vertical array, a horizontal array, or a three-dimensional array. Using a three-dimensional array, a three-dimensional representation of the underground strata can be produced. 
         [0035]    Each cable  111 ,  112 ,  113  has an electrical wire  111   a ,  112   a,    113   a,  respectively running along its entire length. An electrical source  120  on the vessel  114  imposes a known pre-selected fixed reference potential on all the electrical cables  111   a ,  112   a,    113   a  (indicated by the downward pointing arrow on each cable). The same fixed reference potential is imposed on all the cables. For example, in one aspect, the electrical wires  111   a ,  112   a,    113   a  are made of copper and are connected to a ground plate of an electrical source  120 . In certain aspects, a fixed reference potential is chosen which is close to zero. Interferometric methods are used, in one aspect, to convert the electrical potential to a phase shift in the fiber optic signal and, thus, there can be a large reference potential without deterioration of the detectability of the potential between detector electrodes of a pair. The detector electrodes  116  can have a sensed voltage in the range of a few nanovolts and the fixed reference potential can be from zero volts to several volts. 
         [0036]    The same fixed reference potential is at all points on the cables  111 ,  112 , and  113  and is present for the measurements by all the detector electrodes  116  on all cables. Thus, relative measurements are facilitated between any two detector electrodes on any of the cables and the measurements of any two detector electrodes can be compared to the one fixed reference potential (indicated by the multiple arrows labelled “Relative Measurements”,  FIG. 2 ). In use, the electrical wires  111   a ,  112   a,    113   a  are connected to the source  120  on the vessel. 
         [0037]    A relative measurement between detector electrodes on various cables measures and indicates the electrical field representative of underground strata based on signals related thereto. With the same fixed reference potential on the electrical wires  111   a ,  112   a,    113   a,  and with a sufficient number of electrodes  116 , accurate measurements in different directions (e.g. in three orthogonal axes with a system as in  FIG. 4A ) are possible, yielding a three-dimensional representation of underground strata—and this is done without alternately imposing a known potential on each cable separately. Also, any two detector electrodes in the entire array may be used since the same reference potential is used for all detector electrodes. This is accomplished without a specific wire connected between the two chosen detector electrodes. 
         [0038]    In certain aspects, with a relatively large number of detector electrodes, any anomalous variations are compensated for when sensed in real time. When, e.g., thousands of electrodes are used, such variations can be eliminated in real time. Measurements based on signals from several detector electrodes are summed and averaged to reduce the effects of noise. When signals from groups of detector electrodes are summed together, non-coherent noise will be attenuated; and, for example, noise caused by vibration or movement of a cable will be reduced. 
         [0039]      FIG. 3  shows schematically in cross-section a cable  111  with detector electrodes  116  and the electrical wire  111   a  which runs the entire length of the cable  111 . 
         [0040]      FIGS. 4A-4C  illustrate a multi-cable array  130  with cables  132  each with multiple spaced-apart opto-electrical detector electrodes  134 . Each cable  132  includes a wire  136  (like the wires  111   a ,  FIGS. 2 ,  3 ). 
         [0041]    The present invention, therefore, provides in some, but not in necessarily all, embodiments a cable for a marine exploration system, the cable including a cable body, the cable body having a body length; a plurality of opto-electrical detector electrodes spaced-apart along the body length; and an electrical wire extending along the body length, a fixed reference potential imposable on the electrical wire. 
         [0042]    The present invention, therefore, provides in some, but not in necessarily all, embodiments an array of cables for a marine exploration system, the array including a plurality of spaced-apart cables, each cable including: a cable body, the cable body having a body length; a plurality of opto-electrical detector electrodes spaced-apart along the body length for sensing signals related to underground strata; an electrical wire with a fixed reference potential imposed thereon, the electrical wire extending along the entire body length; and the cables positioned in spaced-apart relation to each other. 
         [0043]    The present invention, therefore, provides in some, but not in necessarily all, embodiments a method for marine exploration of underground strata, the method including: receiving signals related to underground strata with a receiver system; the receiver system having at least one cable, the at least one cable comprising a cable body, the cable body having a body length, a plurality of opto-electrical detector electrodes spaced-apart along the body length for sensing signals related to underground strata, and an electrical wire, the electrical wire extending along the entire body length; imposing a fixed reference potential on the electrical wire of the at least one cable; the opto-electrical detector electrodes outputting optical signals corresponding to received signals related to the underground strata; and comparing the output optical signals to the fixed reference potential. 
         [0044]    Accordingly, the present invention includes features and advantages which are believed to enable it to advance multi-cable flexible fiber optic receiver systems for marine exploration technology. Characteristics and advantages of the present invention described above and additional features and benefits will be readily apparent to those skilled in the art upon consideration of the detailed description provided herein and in the accompanying drawings. 
         [0045]    Certain embodiments of this invention are not limited to any particular individual feature disclosed here, but include combinations of them distinguished from the prior art in their structures, functions, and/or results achieved. Features of the invention have been broadly described so that the detailed descriptions that follow may be better understood, and in order that the contributions of this invention to the arts may be better appreciated. There are, of course, additional aspects of the invention described herein and which may be included in the subject matter of the claims to this invention. Those skilled in the art who have the benefit of this invention, its teachings, and suggestions will appreciate that the conceptions of this disclosure may be used as a creative basis for designing other structures, methods and systems for carrying out and practicing the present invention within the scope of the claims herein. The claims of this invention are to be read to include any legally equivalent devices or methods which do not depart from the spirit and scope of the present invention. 
         [0046]    The present invention recognizes and addresses the problems and needs in this area and provides a solution to those problems and a satisfactory meeting of those needs in its various possible embodiments and equivalents thereof. To one of skill in this art who has the benefits of this invention&#39;s realizations, teachings, disclosures, and suggestions, various purposes and advantages will be appreciated from the description of certain embodiments, given for the purpose of disclosure, when taken in conjunction with the accompanying drawings. The detail in these descriptions is not intended to thwart this patent&#39;s object to claim this invention no matter how others may later attempt to disguise it by variations in form or additions of further improvements. 
         [0047]    The Abstract that is part hereof is to enable the U.S. Patent and Trademark Office and the public generally, and scientists, engineers, researchers, and practitioners in the art who are not familiar with patent terms or legal terms of phraseology to determine quickly from a cursory inspection or review the nature and general area of the disclosure of this invention. The Abstract is neither intended to define the invention, which is done by the claims, nor is it intended to be limiting of the scope of the invention or of the claims in any way. 
         [0048]    It will be understood that the various embodiments of the present invention may include one, some, or all of the disclosed, described, and/or enumerated features, aspects, improvements and/or technical advantages and/or elements in claims to this invention. 
         [0049]    Certain aspects, certain embodiments, and certain preferable features of the invention are set out herein. Any combination of aspects or features shown in any aspect or embodiment can be used except where such aspects or features are mutually exclusive. 
         [0050]    In conclusion, therefore, it is seen that the present invention and the embodiments disclosed herein and those covered by the appended claims are well adapted to carry out the objectives and obtain the ends set forth. Certain changes can be made in the subject matter without departing from the spirit and the scope of this invention. It is realized that changes are possible within the scope of this invention and it is further intended that each element or step recited in any of the following claims is to be understood as referring to the step literally and/or to all equivalent elements or steps. The following claims are intended to cover the invention as broadly as legally possible in whatever form it may be utilized. The invention claimed herein is new and novel in accordance with 35 U.S.C. §102 and satisfies the conditions for patentability in §102. The invention claimed herein is not obvious in accordance with 35 U.S.C. §103 and satisfies the conditions for patentability in §103. This specification and the claims that follow are in accordance with all of the requirements of 35 U.S.C. §112. The inventor may rely on the Doctrine of Equivalents to determine and assess the scope of the invention and of the claims that follow as they may pertain to apparatus not materially departing from, but outside of, the literal scope of the invention as set forth in the following claims. All patents and applications identified herein are incorporated fully herein for all purposes. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.