Methods and apparatus for use in detecting seismic waves in a borehole

The invention provides methods and apparatus for detecting seismic waves propagating through a subterranean formation surrounding a borehole. In a first embodiment, a sensor module uses the rotation of bogey wheels to extend and retract a sensor package for selective contact and magnetic coupling to casing lining the borehole. In a second embodiment, a sensor module is magnetically coupled to the casing wall during its travel and dragged therealong while maintaining contact therewith. In a third embodiment, a sensor module is interfaced with the borehole environment to detect seismic waves using coupling through liquid in the borehole. Two or more of the above embodiments may be combined within a single sensor array to provide a resulting seismic survey combining the optimum of the outputs of each embodiment into a single data set.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to seismic surveying of subterranean geological formations. More particularly, the present invention relates to improved seismic sensors for monitoring seismic waves at a location within a liquid-filled borehole, and methods for their use.

2. State of the Art

Seismic surveying is used, by way of example, to examine subterranean geological formations for the potential presence of reserves of hydrocarbons such as petroleum, natural gas and combinations thereof as well as the extent or volume of such reserves. Seismic waves, also termed acoustic waves, are emitted from a seismic energy source to penetrate through layers of rock and earth, and under certain conditions are reflected and refracted by variations in the composition of the subterranean formations in the path of the waves. Seismic sensors configured as motion sensors in the form of geophones or accelerometers or pressure sensors in the form of hydrophones receive the reflected and refracted waves and convert them into corresponding electrical signals, which are then analyzed for the presence and extent of formations containing oil and gas deposits.

An increasingly common technique for subterranean exploration is known as borehole seismic surveying, wherein one or more seismic sensors are placed below the earth's surface in the liquid-filled borehole of a well. The seismic energy source may be located above ground, or may also be placed in a borehole to emit seismic waves within close proximity to the area of interest. By recording the seismic waves at various depths below the surface, a profile is acquired that provides more detailed information about the surrounding area than may be acquired using surface-based seismic sensors. These higher resolution views of subterranean formations can thus be examined for the presence of hydrocarbon reserves that might otherwise remain hidden.

In order to reduce the time required for data acquisition, an array of seismic sensor modules is deployed in the borehole to take simultaneous readings at different locations along its length. The sensor modules, typically in the form of sondes containing geophones, are lowered into the borehole on an elongated structure including a conductive cable such as a wireline, tubing string or other suitable structure. The geophones are configured for measuring the seismic waves in three directions or axes, to give a reading for each of the orthogonal components of the waves. For optimum sensing by the geophones, it is necessary that there be a good interface between the sondes and the subterranean formation volume surrounding the borehole to ensure effective transmission of seismic energy. In the prior art this has often been accomplished by using extendable mechanical arms that urge the sondes into firm contact with the borehole wall, and provide an improved mechanical coupling for conducting waves to the geophones. In boreholes that are lined with metallic casings, magnetic means have also been used in an attempt to couple sondes to the borehole wall. All of the foregoing types of systems are controlled from above the surface to deploy the interface structures for the geophones, and involve complicated mechanisms for extending and retracting arms or orienting and activating magnets. Limitations on transmitting electric and hydraulic power to significant depths are another significant concern. The prior art approaches result in increased equipment cost and enhanced possibility of a malfunction causing the sondes to become stuck within the borehole and requiring an expensive retrieval, or “fishing,” operation. Further, wave components traveling perpendicular to the borehole, versus wave components traveling up and down the borehole liquid column, are subject to different influences on their propagation. Interfacing all of the sondes in the same wall-coupling manner may not improve geophone readings for all three x, y and z sensor directions.

What is needed, therefore, are robust and uncomplicated methods and apparatus that achieve an improved interface between seismic waves and sensor modules within a borehole to provide high-resolution seismic survey data, while overcoming the problems associated with the prior art.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, improved seismic sensors and methods for coupling them within a borehole are disclosed. Embodiments of the present invention are directed to sensor modules having geophones for sensing of seismic waves. The sensor modules are lowered into a borehole as part of a sensor array, and monitor signals emitted from a seismic energy source located at the earth's surface or similarly contained within a borehole. The sensor modules are interfaced with the surrounding environment in such a way that complex and unreliable coupling mechanisms are not required, while still enabling optimized geophone sensing.

In a first exemplary embodiment of the invention, a sensor module includes a sensor package, or sonde, that is magnetically coupled to the wall of a borehole having a metallic casing. During deployment, the sensor module uses a self-contained device to automatically extend and retract the sensor package. Bogey wheels on the module ride along the borehole casing and operate a mechanism that retracts the sensor package away from the casing during sensor module travel. When the sensor module is brought to a halt at a desired sensing location, the bogey wheels no longer operate the retraction mechanism, and the sensor package extends for magnetic coupling to the metallic casing. Upon renewed longitudinal motion through the borehole, the bogey wheels and associated retraction mechanism detach the sensor package from its magnetic coupling. The device thus provides a good interface with the subterranean environment surrounding the borehole, while eliminating complicated parts and lengthy connections to above surface actuation controls. In addition, because movement of the sensor module automatically retracts the sensor package, the risk of sticking the module within the borehole due to a malfunction is significantly reduced.

In a second exemplary embodiment of the invention, a sensor module is interfaced by magnetic coupling to a metallic borehole casing as described above with respect to the first exemplary embodiment, but without the requirement for any mechanical coupling devices. The sensor module comprises a sonde having a plurality of permanent magnets placed around its periphery. The magnets are attached so as to form protrusions extending from the sides of the sonde. Each magnet is oriented such that its protrusion presents a magnetic pole opposite to the pole presented by the protrusion of an adjacent magnet. This creates magnetic field lines, which pass from one protrusion to another along the periphery of the sonde. The sensor module is simply dragged along the casing wall of the borehole during deployment, with some of the protrusions in magnetic contact with the borehole casing. This approach has the added advantage of scraping away surface deposits that may exist on the casing, which will improve the magnetic coupling.

In a third exemplary embodiment of the invention, a sensor module is designed to efficiently interface with the surrounding environment without requiring direct coupling to a borehole wall or casing. Rather, the module is formed as a container or sonde having a mass-to-volume ratio that gives it an average density substantially equal to that of the borehole liquid. This equal density, and the nearly incompressible nature of a liquid, allows the sensor module to precisely match the displacement of borehole liquid due to seismic wave disturbance. This creates, in effect, a liquid coupling wherein the motion of the sensor module can be monitored to exactly track the seismic waves. The simple and lightweight construction of this embodiment is highly cost effective and reduces the need for complicated supporting architecture, facilitating its deployment on wireline. This type of sensor module is also well adapted for attachment to drill pipe or coiled tubing used to perform borehole drilling or downhole maintenance and remediation functions, and may be particularly suitable for use in seismic while drilling operations.

In yet another exemplary embodiment of the present invention, a sensor array having a number of sensor modules of the various above-described embodiments is provided for deployment within a borehole. Geophones within the sensor modules measure seismic waves emitted from a seismic energy source, and provide an output reading for each of the orthogonal components of the waves. The wave components most effectively measured by each of the sensor module embodiments are then used to generate a seismic survey, while the other components are filtered out. This optimizes the survey data by combining the advantages of each sensing technique into a single result.

Other and further features and advantages will be apparent from the following detailed description of the invention taken in conjunction with the accompanying drawings. The following examples are provided for purposes of illustration only, and are not intended to be limiting. It will be understood by one of ordinary skill in the art that numerous combinations and modifications are possible for the embodiments presented herein.

DETAILED DESCRIPTION OF THE INVENTION

Although the subsequent examples will be discussed in terms of deployment in a well used for petroleum or gas exploration and production, it should be understood that the present invention would also work well for seismic surveying applications not related to these fields. Any technology that uses sensors deployed within a borehole to monitor seismic waves may benefit from the present invention.

Referring initially toFIG. 1, three different sensor module embodiments14,52and66of the invention are illustrated in a sensor array2. Sensor array2is deployed in a liquid or slurry filled well borehole4for seismic surveying of the subterranean formation volume proximate the well. Sensor array2is lowered into the borehole4on an elongated structure depicted as wireline6, however, other suitable structure such as a tubing string may be used. The liquid or slurry may comprise, for example, water or a water and hydrocarbon based drilling fluid, or “mud.” In the case of petroleum exploration, the interior of borehole4will usually be surrounded by a metallic, typically steel, casing8which has been floated into the borehole4subsequent to the drilling thereof and then cemented in place, as known to those of ordinary skill in the art. Seismic waves10generated by seismic energy source12are passed through the subterranean formations surrounding borehole4, and sensor array2monitors these waves from within borehole4after reflection from and refraction by these formations to provide geological information.

Turning toFIG. 2, the seismic waves may be considered as being comprised of orthogonal components traveling in the x, y and z directions. The sensor modules of array2carry geophone type sensors that are configured and oriented for measuring the seismic waves in these three directions, or axes, to give a reading for each of the orthogonal components of the waves. The geophones operate by measuring displacement between a stationary first part and a second part that is allowed to move along a defined axis in response to the seismic waves transmitted thereto as vibrations. This method of operation requires a good interface between the geophones and the final transmission medium for the seismic waves acting upon the geophones in order to effectively receive the vibrations.

FIGS. 3A and 3Bschematically illustrate a first sensor module embodiment14of the present invention having a self-contained device to automatically extend and retract a sensor package. Sensor module14includes a housing16, bogey wheels18and sensor package or sonde20. At least one of the bogey wheels18is biased to swing away from housing16so as to force the bogey wheels18on both sides of housing16into firm contact with borehole casing8.FIG. 3Ashows that when the sensor module14is stationary within borehole4, sensor package20extends out for magnetic coupling to borehole casing8via one or more permanent magnets M carried by the sensor package. When sensor module14is placed in motion, for example during initial deployment or repositioning within borehole4, bogey wheels18are rotated due to their contact with borehole casing8. The rotation operates a mechanism attached to bogey wheels18that retracts sensor package20away from borehole casing8, as depicted inFIG. 3B. Sensor package20is held in the retracted position during travel of sensor module14. After movement has ceased, bogey wheels18no longer rotate to operate the retracting mechanism, and sensor package returns to the extended position for magnetic coupling to borehole casing8.

FIG. 4Ashows one implementation of the first exemplary embodiment, characterized as sensor module14′. Sensor module14′ employs a hydraulic pump and cylinder arrangement as the mechanism for retracting sensor package120carrying magnets M and one or more geophones G. Bogey wheel118ais biased by spring22to swing outwardly toward the wall of casing8in the direction of arrow24and press against casing8. Bogey wheels118band118care pressed against the side of borehole casing8opposite bogey wheel118a. A hydraulic pump P for driving pistons within a cylinder26through control manifold C is incorporated with bogey wheel118a. The hydraulic pump P, for example a gear pump, is actuated by rotation of the wheels. Other types of pumps using drives configured for translating the wheel motion into pumping force would also be suitable. In operation, hydraulic pump P pressurizes a hydraulic fluid responsive to rotation of bogey wheel118aas sensor module14′ moves longitudinally within casing8, the hydraulic fluid pressure being communicated to the center of cylinder26to move a piston therein (not shown, seeFIG. 4B) outwardly in cylinder26to tension cable32and retract sensor package120against an outward spring bias provided by linkages36.

InFIG. 4B, which schematically represents the hydraulic pump and cylinder retracting mechanism, upon movement of sensor module14′, the bogey wheel rotation actuates the hydraulic pump P, which in turn provides pressurized hydraulic fluid to cylinder26, extending piston28in the direction of arrow30. A cable32attached at one end32ato ram28is routed along guide pins or pulleys34and attaches at its other end32bto sensor package120. As piston28extends in the direction of arrow30, it pulls cable32, which in turn pulls on sensor package120and draws it into a retracted position. During sensor module14′ travel, cylinder26will be maintained in a retracted state with piston28extended. Once motion is stopped, the hydraulic pump P will no longer pressurize cylinder26, and sensor package120, which is biased outwardly by linkages36represented schematically as springs inFIG. 4B, will return to the extended position for magnetic coupling by magnets M with the casing8. Hydraulic fluid is permitted to bleed back into a reservoir in control manifold C for reuse through a check valve which opens when hydraulic pump P is no longer pressurizing the hydraulic fluid.

It is possible that the retracting mechanism could use hydraulically actuated devices other than cylinder26, such as by extending a bellows or diaphragm with hydraulic pump P to pull cable32. It is also contemplated that hydraulic pump P may be used to power a drive to wind a cable to retract the sensor package120without using a hydraulic cylinder, using a slip clutch to prevent damage to the cable when the sensor package is fully retracted. With any of the above described hydraulically actuated devices, the retracting mechanism should include a balanced hydraulic system capable of compensating for high pressures that may be encountered within the operating environment in order to allow the hydraulic fluid to bleed back into the reservoir in control manifold C. Such balanced hydraulic systems typically involve mechanisms such as bellows or diaphragms for equalizing the pressure between the hydraulic fluid reservoir and the borehole fluid, as previously known in the art.

Returning toFIG. 4A, to aid in overcoming the magnetic attraction of magnets M of sensor package120when initially detaching it from borehole casing8, sensor package120is supported in housing116with linkages36. As sensor module14′ begins to move, linkages36permit rocking or twisting of sensor package120in the direction of arrow38about an axis extending perpendicular to the plane of the drawing sheet to break the magnetic coupling with casing8. This feature reduces the force that would be required for detachment if sensor package120were simply pulled back from the face of casing8in a direction perpendicular thereto, and thus assists retraction of sensor package120.

It is also desirable that sensor module14′ include an automatic latching mechanism to maintain the sensor package120in a retracted position whenever the hydrostatic pressure within the borehole is low, i.e., near the surface. The latching mechanism could be mechanical, hydraulic, electronic or comprise any other form generally know in the art form providing such a function.

FIG. 5shows another implementation of the first exemplary embodiment, characterized as sensor module14″. Sensor module14″ uses a slip or friction clutch and cam bar or member arrangement as the mechanism for retracting a sensor package220. Bogey wheel218ais mounted on spring biased arm40to swing out in the direction of arrow42and press against casing8. Bogey wheels218band218care pressed against the opposite side of casing8due to the spring bias of arm40on the opposing side of sensor module14″. Sensor package220is mounted to a cam bar44. Cam bar44is eccentrically interfaced with bogey wheels218band218cvia respective slip or friction clutches46aand46b. When sensor module14″ is stationary within borehole4, sensor package220is in the extended position for magnetic coupling to casing8, as depicted byFIG. 5. Upon longitudinal movement of sensor module14″, the rotation of bogey wheels218band218cin either direction will force cam bar44in the direction of arrow48, and retract sensor package220. At a certain point, the force required for further displacement of cam bar44in direction48will be sufficient to cause friction clutches46aand46bto engage. Thereafter, cam bar44will be maintained at a constant retracted position during longitudinal travel of sensor module14″ through casing8. When sensor module14″ is repositioned to a desired location, the longitudinal direction of travel of sensor module14″ is reversed for a short distance, which may be a matter of inches, sufficient to release friction clutches46aand46band causing cam bar44to move back out and extend sensor package220for magnetic coupling by magnets M with casing8.

Sensor package220is supported on cam bar44by a pair of staggered pins50. These act to aid in overcoming the magnetic attraction of sensor package220when initially detaching it from casing8in much the same way as linkage assembly36of sensor module14′. As sensor module14″ begins to move, one of the pins50pulls on extended sensor package220before the other, causing one side of sensor package220to lift from magnetic coupling with casing8before the other side to assist with breaking the magnetic coupling to casing8.

Turning toFIGS. 6A and 6B, a second exemplary sensor module embodiment52of the present invention is illustrated. Sensor module52comprises a sonde54containing sensors (not shown) and having a plurality of permanent magnets56placed around its periphery. The poles of magnets56terminate in protrusions58that extend outwardly from sonde54.FIG. 6B, taken along section line B inFIG. 6A, shows permanent magnets56of U-shaped cross-section with one pole at each tip. Other magnet shapes are within the scope of the invention, as long as they present a protrusion terminated by a magnetic pole. Each magnet56is oriented such that each protrusion58has a magnetic pole oppositely charged from the pole of the protrusion adjacent to it. This creates magnetic field lines60passing from one protrusion58to another along the periphery of sonde54and through borehole casing8. During deployment or repositioning within borehole4, sensor module52is simply dragged along borehole casing8, with protrusions58in magnetic contact therewith.

Coupling of sensor module52using permanent magnets56provides an interface with the surroundings that does not require external power or controls and is devoid of moving parts. The simple design frees up space and conductive elements on wireline6for data transmission, allowing more sensor modules or other equipment to be added to array2. Movement of sensor module52with protrusions58in contact with borehole casing8also has the added advantage of scraping away possible surface deposits, which will improve magnetic coupling.

Sensor module52may be connected to wireline6at a central attachment point62asuch that it is symmetrically balanced, as depicted inFIGS. 6A and 6B. Under this arrangement, sensor module52will present attachment surfaces around its perimeter that are uniformly disposed to attachment with borehole casing8. As seen inFIG. 7, in some instances it may be desirable for sensor module52to favor one side for attachment to borehole casing8. Sensor module52is hung from wireline6at an off-center attachment point62b, which will bias it to one side64. Thus, side64will have a predisposition for contact with borehole casing8. Under this arrangement, magnets56may also be limited to the contact area of side64, rather than being placed around the entire periphery of sonde54to further ensure that magnetic coupling to casing8will take place on side64.

FIG. 8schematically illustrates a third exemplary sensor module embodiment66of the present invention. Sensor module66is fabricated to interface with the surrounding environment within borehole4without requiring direct physical coupling to the borehole wall or casing8. Rather, sensor module66uses a liquid type coupling wherein seismic waves10are transmitted to the module via the borehole liquid104. Sensor module66comprises a container or sonde68for carrying one or more geophone type sensors70. Sensors70as depicted inFIG. 8are oriented to detect and measure the magnitude of seismic waves in the x and y directions, perpendicular to the longitudinal axis of borehole4and, ideally, horizontal in orientation. Sensor module66is constructed so that sonde68and any sensors70contained therein have a combined mass-to-volume ratio with an average density effectively equal to that of the borehole liquid104. In other words, sensor module66will be neutrally buoyant within the borehole liquid. Sonde68may comprise, by way of example, a low density solid structure surrounding sensors70or may enclose a hollow volume within which sensors70are mounted. Sonde68therefore presents a surface area that is accelerated at a rate equal to the displacement of borehole liquid104responsive to seismic waves10transmitted thereto by casing8Further, the nearly incompressible nature of a liquid means this displacement will transmit the seismic energy directly to sensor module66without any variation in wave propagation.

The response is directional and unmitigated for any frequency of concern. While the borehole liquid104is not capable of transmitting a shear wave, the result of solid shear disturbance (in the formation) is an orthogonal compressive wave which may, in turn, be detected. The geophones70of sensor module66are thereby effectively interfaced with the surrounding environment by “coupling” to borehole4via the borehole liquid104. This eliminates the need for any mechanical coupling devices and provides a highly economical and lightweight unit that is easily supported within the borehole environment. For example, each sensor module66may be fabricated for as little as ten percent of the cost of a clamping type sensor module, and the cost of supporting equipment may similarly be significantly reduced. Significantly more potential users exist due to the less extensive equipment requirements of this embodiment, and operational time may be significantly reduced in comparison to clamping type sensor modules as well. Further, it is notable that this embodiment of the invention is operable in an uncased borehole, since there is no need for affixation of the sensor module to casing for seismic coupling.

The fluid coupled type sensor module described above works best for translating the x and y, or horizontal orthogonal, seismic wave components to corresponding geophone sensors70contained therein, as the impedance mismatch between the solid (formation, cement, casing, etc.) and borehole liquid104is small, as is the length of seismic wave travel through the borehole liquid104. This is due to the fact that while the borehole liquid is nearly incompressible, the z, or vertical seismic wave component, along the longitudinal axis of borehole4will travel a much greater distance through the borehole104liquid unless the sensor is deployed at the bottom of the borehole4, and any amount of liquid compressibility will have a cumulative effect. One way to compensate for this problem is to incorporate sensor module66into an assembly having an exterior vertical geophone component72that is physically coupled to the side of casing8.FIGS. 9A and 9Bshow such an assembly wherein a housing74with a plurality (for example, four) bow springs76circumferentially disposed thereabout holding vertical (z-axis) geophone sensors72mounted thereon in physical contact with the side of borehole4through magnetic coupling using one or more magnets M. Sensor module66is connected to housing74so as not to hamper its ability to be displaced by the borehole fluid.FIG. 9A, for example, shows a sensor module66suspended below housing74, whileFIG. 9Bshows a unitary housing74′ that allows free movement of sensor module66suspended within its confines (only one bow spring76of a plurality shown for convenience). It is to be understood that other physical coupling means may be used for a Z-axis geophone sensor, the only requirement being that they allow sensor module66to be accelerated by the borehole fluid for liquid coupling. It is further contemplated that the bow spring type embodiments ofFIGS. 9A and 9Bmay be used to support and magnetically couple x, y and z-axis geophones to casing, and such a configuration is within the scope of the invention.

While sensor module66has been depicted as being deployed on wireline6as part of a sensor array, other downhole assemblies may also benefit from the use of fluid coupled type sensors. A borehole, for example, is typically drilled by using a bit that is suspended on a drill string comprising coupled sections of drill pipe extending downwardly into the borehole from the surface. Rotating the drill string at the surface using a rotary table or top drive rotates the bit for drilling when weight is applied to it through the drill string. The drill string may include a bottom hole assembly above the bit including, for example, a downhole motor with a bent housing or other steering element or assembly to enable guided, deviated or directional drilling of the borehole. Further, after an oil or gas well has been successfully drilled and completed, it is necessary over the productive lifetime of the well to perform maintenance or remediation operations within the well borehole. This maintenance or remediation often includes de-scaling operations, or reworking operations such as fracturing or acidization to increase production in older wells. It is quite advantageous to be able to insert equipment into a borehole necessary to perform such maintenance or remediation without removing the surface production equipment at the well head. Coiled tubing, which can be inserted into the borehole through the surface production equipment without removal thereof, has been employed to carry out this function. More recently, coiled tubing has also been used in conjunction with downhole motors for drilling operations as well as other types of borehole operations.

When drilling, it is desirable to know what strata will be drilled through at any time in order to provide appropriate drilling parameters during operation. Features of the strata ahead of the drill may thereby be anticipated, enabling optimized navigation of the borehole through subterranean formations which otherwise might damage the bit or expose the well to dangerous gas overpressure regions. It would, of course, be possible to extract the entire drilling assembly from the borehole and send down a wireline-carried sensor array for surveying, but the time and cost associated with such an approach are very high and safety concerns render this an undesirable alternative. In order to overcome this problem, it is known in the prior art to include seismic sensor arrangements directly within a drilling assembly to examine the area directly surrounding the drill bit concurrently with drilling. An example of this method, often referred to broadly as “measurement while drilling” (MWD) although more accurately termed “seismic while drilling,” is disclosed in U.S. Pat. No. 5,798,488 to Beresford et. al., which is incorporated herein by reference. Because rotation of the bit must typically be stopped and circulation of drilling fluid ceased in order to allow seismic measurements without interference from drilling vibrations and fluid turbulence, the fluid coupled sensors of the present invention would be well suited to such an MWD application. By eliminating the need for any mechanical coupling devices, fluid coupled sensors according to the present invention may be activated with minimal pauses in drilling and circulation and may be more easily incorporated into a drilling assembly.

FIGS. 10A and 10Bshow an embodiment of a fluid coupled type sensor module166that is configured for attachment to a drill pipe or coiled tubing77, which have much greater diameters than a wireline. While sensor module166is depicted as disposed within casing8, sensor module166has equal utility for deployment within an uncased borehole for use in conducting seismic operations while drilling. As seen in side viewFIG. 10A, sensor module166comprises an annular housing168carrying one or more geophone type sensors170.FIG. 10Bshows annular housing168surrounding drill pipe or coiled tubing77and attached thereto with highly resilient mounts172, allowing housing168to move freely in the x and y orthogonal directions. Mounts172may be formed, for instance, of low modulus rubber, springs or any other material having sufficient elasticity to allow housing168to move in the x and y directions without substantial resistance. Furthermore, whileFIG. 10Bshows four mounts172for supporting housing168, any number of mounts could be used, or the mounting structure could even be formed as a unitary ring entirely surrounding drill pipe or coiled tubing77. Such an approach may facilitate damping of seismic waves in the z direction along the axis of the borehole. As with sensor module66, sensor module166and sensors170contained therein have a combined mass-to-volume ratio with an average density effectively equal to that of the borehole liquid104so that sensor module166and sensors170are essentially neutrally buoyant. The geophones170of sensor module166are thereby interfaced with the surrounding environment by the movement of annular housing168.

If sensor module166is deployed on drill pipe77in a drilling operation it may be desirable to employ concentric stabilizers400(FIG. 10A) intermittently along drill pipe77to prevent contact of sensor module166with the wall of the borehole. Centralization of the structure (drill pipe or coiled tubing) carrying sensor module166should always be considered if the borehole segment in which sensor module166is deployed is off-vertical by any significant amount. Of course, sensor module166may be placed along a necked-down or reduced diameter central portion of drill pipe77between the diametrically enlarged male (pin) and female (box) ends, which function to centralize the drill pipe and maintain sensor module166out of contact with the borehole.

FIG. 11shows a sectional side view of an alternative to the above fluid coupled structure, wherein a sensor module266is fixedly mounted to drill pipe or coiled tubing77. In this embodiment, geophone sensors270are not interfaced with the environment by movement of housing268, but are instead movably mounted within recesses274by resilient mounts272for direct interface with borehole liquid104. As seismic waves pass through borehole liquid104in the x and y orthogonal directions, sensors270move within recesses274to monitor their transmission. Housing268may comprise an annular housing with recesses in its surface, or may simply comprise shielding structures extending from drill pipe77to at least partially surround sensors270. It is also contemplated that housing268may be completely omitted, and sensors270′ would be movably mounted on resilient mounts272′ in recesses274′ formed directly in a specially configured drill pipe77. While these fluid coupled embodiments are depicted and described as including a annular ring type housing, it will be understood by those of ordinary skill in the art that other housing configurations will be possible, and that any number of geophone sensors may be positioned in various locations about and along drill pipe or coiled tubing77.

When sensors170,270are deployed on drill pipe or coiled tubing77in a borehole that has been drilled in a direction that is not substantially vertical, sensors170,270will offset from the x and y orthogonal axis. It is necessary to mathematically compensate for this offset, which compensations are within the ability of those of ordinary skill in the art and so will not be described in further detail herein.

It is also contemplated that an array of sensors370may be deployed on a conductive cable completely within coiled tubing77as shown inFIG. 11as yet another implementation of the present invention. In such an instance, the material of coiled tubing77would be selected to “give,” or respond to, an encounter with a seismic signal to convey the same to sensors370disposed in a surrounding fluid374within coiled tubing77for substantially neutral buoyancy and effective signal transmission.

In a further embodiment of the present invention, a number of sensor modules of the various different above-described embodiments are provided for deployment within a borehole on a single sensor array. Going back toFIG. 1, sensor array2includes sensor modules14,52and66instead of only one module type or embodiment, as would be the case in prior art arrays. The sensor signals for the separate x, y and z orthogonal seismic wave components from each sensor module are output to a processor78. The signals for wave components most effectively measured by each of the sensor module embodiments are then used to generate a seismic survey, while the other component signals are filtered out. For example, the vertical wave component signal from the liquid coupled module66might be filtered out, while one or more horizontal components of modules14and52are eliminated, depending on signal strength and correlation between the sensor outputs. This approach optimizes the integrity of survey data by combining the advantages of each sensing technique into a single, composite output. The sensor module composition of array2inFIG. 1is only for purposes of illustration and not by way of any limitation of the present invention. Any number of modules in any order on wireline6may be used. Moreover, only two sensor module embodiments might be deployed, instead of the three shown inFIG. 1. It is further noted that all of the embodiments of the present invention, due to their simplicity, may enable the use of arrays of dozens or even hundreds of sensor modules due to their light weight and simplicity of operation, as wireline transmission capacity may be used for data rather than power and control functions.

It is also contemplated that the sensor modules of the present invention may be fabricated in multiple segments, wherein the geophone sensors and associated signal amplification/transmission components are housed separately. This approach reduces the sensor module mass for the geophone containing segment and thus increases the effectiveness of the magnetic coupling force securing the geophone to the casing wall or, in the case of the liquid coupling embodiment, the sensor module displacement response. Although the present invention has been described with respect to the illustrated embodiments, various additions, deletions and modifications are contemplated without departing from its scope or essential characteristics. Furthermore, while described in the context of oil and gas exploration, the invention has utility in all types of subterranean geological exploration. The scope of the invention is, therefore, indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.