Patent Abstract:
A seismic sensor module includes sensing elements arranged in a plurality of axes to detect seismic signals in a plurality of respective directions, and a processor to receive data from the sensing elements and to determine inclinations of the axes with respect to a particular orientation. The determined inclinations are used to combine the data received from the sensing elements to derive tilt-corrected seismic data for the particular orientation.

Full Description:
TECHNICAL FIELD 
     The invention relates generally to deriving tilt-corrected seismic data in a seismic sensor module having a plurality of sensing elements arranged in multiple axes. 
     BACKGROUND 
     Seismic surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, fresh water aquifers, gas injection reservoirs, and so forth. In performing seismic surveying, seismic sources are placed at various locations above an earth surface or sea floor, with the seismic sources activated to generate seismic waves directed into the subterranean structure. Examples of seismic sources include explosives, air guns, or other sources that generate seismic waves. In a marine seismic surveying operation, the seismic sources can be towed through water. 
     The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface for receipt by seismic sensors (e.g., geophones, hydrophones, etc.). These seismic sensors produce signals that represent detected seismic waves. Signals from seismic sensors are processed to yield information about the content and characteristic of the subterranean structure. 
     For land-based seismic data acquisition, seismic sensors are implanted into the earth. Typically, seismic signals traveling in the vertical direction are of interest in characterizing elements of a subterranean structure. Since a land-based seismic data acquisition arrangement typically includes a relatively large number of seismic sensors, it is usually impractical to attempt to implant seismic sensors in a perfectly vertical orientation. 
     If a seismic sensor, such as a geophone, is tilted from the vertical orientation, then a vertical seismic signal (also referred to as a “compression wave” or “P wave”) would be recorded with attenuated amplitude. Moreover, seismic signals in horizontal orientations (also referred to as “shear waves” or “S waves”) will leak into the compression wave, where the leakage of the seismic signals into the compression wave is considered noise. Since the tilts of the seismic sensors in the land-based seismic data acquisition arrangement are unknown and can differ randomly, the noise will be incoherent from seismic sensor to seismic sensor, which makes it difficult to correct for the noise by performing filtering. 
     SUMMARY 
     In general, according to an embodiment, a seismic sensor module includes sensing elements arranged in a plurality of axes to detect seismic signals in a plurality of respective directions. The seismic sensor module also includes a processor to receive data from the sensing elements and to determine inclinations of the axes with respect to a particular orientation. The processor is to further use the determined inclinations to combine the data received from the sensing elements to derive tilt-corrected seismic data for the particular orientation. 
     Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary survey arrangement that includes seismic sensor modules according to some embodiments. 
         FIGS. 2-3  illustrate an exemplary deployment of seismic sensor modules. 
         FIG. 4  is a schematic diagram of a seismic sensor module according to an embodiment. 
         FIG. 5  is a flow diagram of a process of deriving tilt-corrected seismic data in the seismic sensor module of  FIG. 4 , according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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 are possible. 
       FIG. 1  illustrates an example survey arrangement (spread) that includes an array of seismic sensor modules  102 . In accordance with some embodiments, the seismic sensor modules are multi-axis seismic sensor modules that each includes a processor to perform tilt correction to obtain seismic data along a vertical orientation (vertical direction) and to remove or reduce noise due to leakage of seismic signals propagating along horizontal orientations into the vertical seismic signal. More generally, the processor is able to obtain seismic data along a target orientation (which can be a vertical orientation, horizontal orientation, or any other orientation), and the processor is able to remove or reduce noise due to leakage of seismic signals propagating along other orientations into the seismic signal propagating in the target orientation. 
     The seismic sensor modules  102  are connected by communications links  104  (which can be in the form of electrical cables, for example) to respective routers  106  and  108  (also referred to as “concentrators”). A “concentrator” refers to a communications module that routes data between nodes of a survey data acquisition system. 
     Alternatively, instead of performing wired communications over electrical cables, the seismic sensor modules  102  can perform wireless communications with respective concentrators. 
     The concentrators  108  are connected by communications links  110 . Seismic data acquired by the seismic sensor modules  102  are communicated through the concentrators  106 ,  108  to a central recording station  112  (e.g., a recording truck). The recording station  112  includes a storage subsystem to store the received seismic data from the seismic sensor modules  102 . The recording station  112  is also responsible for management of the seismic sensor modules and concentrators, as well as the overall network. 
     One or more seismic sources  114  are provided, where the seismic sources  114  can be activated to propagate seismic signals into a subterranean structure underneath the earth on which the arrangement of seismic sensor modules  102  are deployed. Seismic waves are reflected from the subterranean structure, with the reflected seismic waves received by the survey sensor modules in the survey spread. 
       FIG. 2  illustrates three seismic sensor modules  102 A,  102 B,  102 C that have been implanted into the earth  200 . Each seismic sensor module  102 A,  102 B, or  102 C includes a respective implantation member (e.g., anchor)  202 A,  202 B, or  202 C that has a tip to allow for ease of implantation. The seismic sensor module  102 B has been implanted into the earth  200  to have a substantially vertical orientation (vertical direction) such that the seismic sensor module  102 B is not tilted with respect to the vertical orientation (Z axis of the sensor module  102 B is parallel to the vertical orientation). Also shown are X and Y axes, which are the horizontal axes that are orthogonal to each other and orthogonal to the Z axis. 
     The seismic sensor module  102 C has been implanted to have a slight tilt such that the Z axis is at an angle β with respect to the vertical orientation. The seismic sensor module  102 A has a much larger tilt with respect to the vertical orientation; in fact, the seismic sensor module  102 A has been improperly implanted to lay on its side such that its Z axis is greater than  900  offset with respect to the vertical orientation. 
     As further depicted in  FIG. 2 , each of the seismic sensor modules  102 A,  102 B, and  102 C includes a respective processor  210 A,  210 B, and  210 C. Each processor  210 A,  210 B, or  210 C is able to perform tilt correction according to some embodiments to correct for tilt of the respective seismic sensor module from the vertical orientation. After tilt correction, the Z, X and Y axes are properly oriented, as shown in  FIG. 3 . More specifically, in  FIG. 3 , the Z axis of each of the seismic sensor modules  210 A,  210 B, and  210 C is generally parallel to the vertical orientation. As a result, the seismic data along the Z axis is tilt-corrected with respect to the vertical orientation. 
       FIG. 4  illustrates a seismic sensor module  102  according to an embodiment. The seismic sensor module  102  has a housing  302  defining an inner chamber  303  in which various components can be provided. The components include seismic sensing elements  304 ,  306 , and  308  along the Z, X, and Y axes, respectively. In one embodiment, the seismic sensing elements  304 ,  306 , and  308  can be accelerometers. 
     The seismic sensing elements  304 ,  306 , and  308  are electrically connected to a processor  210  in the seismic sensor module  102 . The “processor” can refer to a single processing component or to multiple processing components to perform predefined processing tasks. The processing component(s) can include application-specific integrated circuit (ASIC) component(s) or digital signal processor(s), as examples. The processing component(s) can be programmed by firmware or software to perform such tasks. The “processor” can also include filtering circuitry, analog-to-digital converting circuitry, and so forth (which can be part of or external to the processing circuitry). 
     The processor  210  is connected to a storage device  212 , in which tilt-corrected seismic data  214  computed by the processor  210  can be stored. The seismic sensor module  102  also includes a telemetry module  216 , which is able to send tilt-corrected seismic data over the communications link  104  (which can be a wired or wireless link). In accordance with some embodiments, instead of sending tilt-corrected seismic data in all three axes, just the tilt-corrected seismic data along a single axis (e.g., Z axis) is sent. As a result, communications link bandwidth is conserved, since the amount of seismic data that has to be sent is reduced. In one implementation, the telemetry module  216  sends the Z-axis tilt-corrected seismic data in one single telemetry channel, instead of multiple telemetry channels to communicate seismic data for all three axes. The phrase “telemetry channel” refers to a portion of the communications link bandwidth, which can be a time slice, a particular one of multiple frequencies, and so forth. 
     Referring further to  FIG. 5 , the seismic sensing elements  304 ,  306 , and  308  (e.g., accelerometers) record (at  502 ) seismic signals (particle motion signals) in the three respective Z, X, and Y axes. Also, each seismic sensing element  304 ,  306 , and  308  records the component of the gravity field along the respective Z, X, or Y axis. The gravity field component recorded by each seismic sensing element is the DC component. In an alternative implementation, the seismic sensing elements  304 ,  306 , and  308  can be implemented with a three-component ( 3 C) moving coil geophone. 
     The processor  210  determines (at  504 ) the inclinations of the seismic sensing elements  304 ,  306 , and  308 . The inclination of each respective seismic sensing element is determined by extracting the DC component (expressed in terms of g or gravity) of the recorded signal from the seismic sensing element. The DC component can be extracted by taking an average of the recorded signal over time, or by filtering out the high-frequency components of the recorded signal (using a low-pass filter). The arccosine of the DC component provides the inclination of each axis (Z, X, or Y) with respect to the vertical orientation. Alternatively, if the seismic sensing elements  304 ,  306 , and  308  are implemented with a  3 C moving coil geophone, then inclinometers can be used to measure the Inclinations of the elements. 
     If the seismic sensing elements  304 ,  306 , and  308  are arranged to be exactly orthogonal to each other, then the inclinations of the seismic sensing elements  304 ,  306 , and  308  with respect to the vertical orientation will be the same value. However, due to manufacturing tolerances, the seismic sensing elements  304 ,  306 , and  308  may not be exactly orthogonal to each other, so that the inclinations can be slightly different. 
     Once the inclinations of the seismic sensing elements  304 ,  306 , and  308  are known, the processor  210  rotates (at  506 ) the seismic data recorded by the seismic sensing elements  304 ,  306 , and  308  to the vertical orientation and to the two orthogonal horizontal orientations, respectively. Rotating the seismic data involves extrapolating the recorded (tilted) seismic data to the respective vertical or horizontal orientation, as well as removing any noise caused by leakage into a seismic signal along a first orientation (e.g., vertical orientation) of seismic signals in other orientations (e.g., horizontal orientations). 
     Next, the vertical tilt-corrected seismic data only is sent (at  508 ) by the seismic sensor module  102 . By sending just the vertical tilt-corrected seismic data and not the horizontal seismic data, communications link bandwidth is conserved. In alternative embodiments, instead of sending just the vertical seismic data, horizontal tilt-corrected seismic data can be sent instead. In fact, the seismic sensor module  102  can be selectively programmed or instructed by the recording station  112  (such as in response to a command by a human operator) to send tilt-corrected seismic data along a particular orientation. Also, the operator can select that non-tilt-corrected seismic data along one or more orientations is sent, which may be useful for test, trouble-shooting, or quality control purposes. As yet another alternative, different signal orientations can be sent from different sensor modules, at different spatial spacing. For example, vertical direction can be selected for all sensor modules, and horizontal direction(s) can be selected for only a subset of these sensor modules. 
     In a different implementation, techniques according to some embodiments can be applied in a seismic data acquisition arrangement that uses just shear-wave seismic sources (e.g., shear-wave acoustic vibrators). As a result, a seismic sensor module will record in just the X and Y horizontal orientations. If the seismic sensor module further includes a compass or magnetometer, then the X and Y seismic signals can be rotated to account for inclinations with respect to any target azimuth (e.g., source-receiver direction or perpendicular to the source-receiver direction, to obtain radial or transverse energy from the shear wave generated by the shear-wave seismic source). After rotation, just the seismic data along one direction has to be sent. 
     In the same survey, compression-wave seismic sources can also be activated, with the seismic sensor module recording the seismic signal along the vertical orientation. In this case, only the vertical seismic data would be transmitted by the seismic sensor module for recording in the recording station  112  ( FIG. 1 ). 
     In addition to the tasks depicted in  FIG. 5 , alternative implementations can also perform seismic sensor module calibration between tasks  502  and  504 . Also, filtering can be applied between tasks  502  and  504 , and/or between  506  and  508 , to filter out noise such as ground roll noise, which is the portion of a seismic source signal produced by a seismic source that travels along the ground rather than travels into the subterranean structure. 
     While the invention has been disclosed 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 such modifications and variations as fall within the true spirit and scope of the invention.

Technology Classification (CPC): 6