Patent Publication Number: US-2019187314-A1

Title: Surveying Techniques using Multiple Different Types of Sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Nos. 62/599,984, filed on Dec. 18, 2017 and 62/767,379, filed on Nov. 14, 2018, each of which is hereby incorporated entirely as if fully set forth herein. 
    
    
     BACKGROUND 
     Geophysical surveys are often used for oil and gas exploration in geophysical formations, which may be located below marine environments. Various types of signal sources and geophysical sensors may be used in different types of geophysical surveys. Seismic geophysical surveys, for example, are based on the use of seismic waves. Electromagnetic geophysical surveys, as another example, are based on the use of electromagnetic waves. In some surveys, a survey vessel may tow one or more sources (e.g., air guns, marine vibrators, electromagnetic sources, etc.) and one or more streamers along which a number of sensors (e.g., hydrophones and/or geophones and/or electromagnetic sensors) are located. 
     During the course of a geophysical survey, the various sensors collect data indicative of geological structures, which may be analyzed to determine the possible locations of hydrocarbon deposits. In 4D surveying techniques, surveys may be performed at a given location at different times, e.g., to determine changes to hydrocarbon deposits. Many seismic surveys have been performed using impulsive sources (e.g., airguns). Recently, other types of seismic sources have been proposed and implemented, such as vibratory sources, for example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary marine geophysical survey system, according to some embodiments. 
         FIG. 2  illustrates a marine geophysical survey using both airgun and vibratory sources, according to some embodiments. 
         FIG. 3  illustrates an exemplary comparison of baseline and monitor surveys for a 4D survey, according to some embodiments. 
         FIG. 4  is a block diagram illustrating an exemplary transition in data acquisition for a 4D survey from a first type of source to a second type of source, including a transition survey that includes both types of sources, according to some embodiments. 
         FIG. 5  is a block diagram illustrating an overview of data associated with different types of sources, according to some embodiments. 
         FIGS. 6A and 6B  are diagrams illustrating exemplary calibration of survey passes using a comparison of an autocorrelation peak with an airgun impulse, according to some embodiments. 
         FIGS. 7A and 7B  are diagrams illustrating exemplary calibration of survey passes with different types of sources using image processing, according to some embodiments. 
         FIG. 8  is a flow diagram illustrating an exemplary method for a transitional marine geophysical survey, according to some embodiments. 
         FIG. 9  is a flow diagram illustrating an exemplary method for adjusting sensor measurements based on a transitional seismic survey, according to some embodiments. 
         FIG. 10  is a block diagram illustrating an exemplary computing device, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure initially describes, with reference to  FIG. 1 , an overview of a geophysical survey system.  FIG. 2  shows an example of a transition geophysical survey using two example types of seismic sources. Techniques for calibrating survey measurements acquired using different types of seismic sources are discussed with respect to  FIGS. 3-7 . Exemplary methods and an example computing system are described with reference to  FIGS. 8-10 . 
     In some embodiments, two types of sources operate simultaneously during a transition survey (e.g., by firing an airgun while a vibratory source is activated). As used herein, the term “transition survey” refers to a survey that facilitates a change from using one type of survey source to another, e.g., a change from airgun sources to vibratory sources in a 4D survey. In some embodiments, this survey technique may facilitate processing to calibrate seismic surveys performed using different types of sources. The calibration may compensate for differences between types of sources, e.g., by normalizing sensor measurements among the types of sources. This calibration may advantageously improve accuracy of images of geophysical formations based on geophysical surveys and may avoid discontinuity in 4D surveys due to a transition between types of sources, for example. 
     Overview of a Seismic Survey 
     Referring to  FIG. 1 , an illustration of a marine geophysical survey system  100  is shown (not necessarily to scale), according to some embodiments. In the illustrated embodiment, system  100  includes survey vessel  10 , airgun source  32  (represented using a rectangular shape), vibratory source  34  (represented using an oval shape), source cables  30 , paravanes  14 , and streamers  20  (streamers  20  are shown truncated at the bottom of  FIG. 1 .). In some embodiments, survey vessel  10  may be configured to move along a surface of a body of water  11  such as a lake or ocean. In the illustrated embodiment, survey vessel  10  tows streamers  20 , airgun source  32 , vibratory source  34 , and paravanes  14 , which may be used to provide a desired amount of spread among streamers  20 . In other embodiments, streamers  20  may be towed by a separate vessel (not shown), rather than survey vessel  10  that tows airgun source  32  and vibratory source  34 . In some embodiments, airgun source  32  and/or vibratory source  34  may be steered, using any of a variety of known devices, to maintain a determined distance between these elements during a survey. In some embodiments, survey vessel  10  may tow only vibratory sources  34  while a separate vessel simultaneously tows only airgun sources  32 . In some embodiments, streamers  20  may include sensors  22  (e.g., hydrophones, geophones, electromagnetic sensors, etc.). In other embodiments, streamers  20  may further include streamer steering devices  24  (also referred to as “birds”) which may provide selected lateral and/or vertical forces to streamers  20  as they are towed through the water, typically based on wings or hydrofoils that provide hydrodynamic lift. In some embodiments, streamers  20  may further include tail buoys (not shown) at their respective back ends. 
     In some embodiments, survey vessel  10  may include equipment, shown generally at  12  and for convenience collectively referred to as a “recording system.” In some embodiments, recording system  12  may include devices such as a data recording unit (not shown separately) for making a record of signals generated by various geophysical sensors. Recording system  12  may also include navigation equipment (not shown separately), which may be configured to control, determine, and record the geodetic positions of: survey vessel  10 , airgun source  32 , vibratory source  34 , streamers  20 , sensors  22 , etc. In the illustrated embodiment, streamers  20  are coupled to survey vessel  10  via cables  18 . 
     In some embodiments, each of airgun source  32  and vibratory source  34  may include sub-arrays of multiple individual signal sources. For example, in some embodiments, an airgun source  32  may include a plurality of airguns and a vibratory source  34  may include a plurality of marine vibrators. In various embodiments, a geophysical survey system may include any appropriate number of towed airgun sources  32 , vibratory sources  34 , and streamers  20 . 
     Collectively, the survey data that is recorded by recording system  12  may be referred to as “marine survey input data”, according to some embodiments. In embodiments where the survey being performed is a seismic survey, the recorded data may be more specifically referred to as “marine survey seismic data,” although the marine survey input data may encompass survey data generated by other techniques. In various embodiments, the marine survey input data may not necessarily include every observation captured by sensors  22  (e.g., the raw sensor data may be filtered before it is recorded). Also, in some embodiments, the marine survey input data may include data that is not necessarily indicative of subsurface geology but may, nevertheless, be relevant to the circumstances in which the survey was conducted (e.g., environmental data such as water temperature, water current direction and/or speed, salinity, etc.). In some embodiments, the geodetic position (or “position”) of the various elements of system  100  may be determined using various devices, including navigation equipment such as relative acoustic ranging units and/or global navigation satellite systems (e.g., a global positioning system (GPS)). 
     Various items of information relating to geophysical surveying (e.g., raw data collected by sensors and/or marine survey input data generally, or products derived therefrom by the use of post-collection processing), may be recorded in a tangible computer-readable medium in the course of manufacturing a geophysical data product according to methods described herein. Some non-limiting examples of computer-readable media may include tape reels, hard drives, CDs, DVDs, flash memory, print-outs, etc., although any tangible computer-readable medium may be employed to create the geophysical data product. In some embodiments, raw analog data from streamers may be stored in the computer readable medium. In other instances, as noted above, the data may first be digitized and/or conditioned prior to being stored in the computer readable medium. In yet other instances, the data may be fully processed into a two- or three-dimensional map of the various geophysical structures, or another suitable representation, before being stored in the computer readable medium. The geophysical data product may be manufactured during the course of a survey (e.g., by equipment on a vessel) and then, in some instances, transferred to another location for geophysical analysis, although analysis of the geophysical data product may occur contemporaneously with survey data collection. In other instances, the geophysical data product may be manufactured subsequent to survey completion, e.g., during the course of analysis of the survey. 
     Example Seismic Data Acquisition System 
       FIG. 2  shows a side-elevation view of an example seismic data acquisition system  200  comprising a survey vessel  202  towing sources  204 A- 204 B and one streamer  208  beneath a free surface  212  of a body of water, according to some embodiments. The body of water may be, for example, an ocean, a sea, a lake, or a river, or any portion thereof. In this example, the streamer  208  is attached at one end to the survey vessel  202  via a streamer-data-transmission cable. In the illustrated embodiment, streamer  208  forms a planar horizontal data acquisition surface with respect to the free surface  212 . In practice, however, the data acquisition surface may be smoothly varying due to active sea currents and weather conditions. In other words, in some embodiments, although the streamer  208  is illustrated in  FIG. 2  as straight and substantially parallel to the free surface  212 , in practice, the towed streamer  208  may undulate as a result of dynamic conditions of the body of water in which the streamer  208  is submerged. A data acquisition surface is not limited to having a planar horizontal orientation with respect to the free surface  212 . The streamer  208  may also be towed at depths such that the data acquisition surface is intentionally angled or curved with respect to the free surface  212 . A data acquisition surface is not limited to one streamer as shown in the illustrated embodiment. In practice, the number of streamers used to form a data acquisition surface can range from as few as one streamer to as many as  20  or more streamers, according to some embodiments. 
     In the illustrated plot, an xz-plane  214  of the Cartesian coordinate system is shown having three orthogonal, spatial coordinate axes labeled x, y and z. The coordinate system is used to specify orientations and coordinate locations within the body of water. The x-direction is parallel to the length of the streamer (or a specified portion thereof when the length of the streamer is curved) and is referred to as the “in-line” direction. The y-direction is perpendicular to the x-axis and substantially parallel to the free surface  212  and is referred to as the “cross-line” direction. The z-direction is perpendicular to the xy-plane (i.e., perpendicular to the free surface  212 ) with the positive z-direction pointing downward away from the free surface  212 . The streamer  208  is a long cable containing power and data-transmission lines that connect receivers or sensors represented by shaded rectangles  218  spaced-apart along the length of each streamer to seismic data acquisition equipment and data-storage devices located on board the survey vessel  202 . The ocean bottom node  238  is a receiver or sensor represented by a pentagon at the water bottom  222 . In some embodiments, the ocean bottom node  238  is fixed to the sea floor. 
     In the illustrated embodiment, a cross-sectional view of the survey vessel  202  towing the sources  204  above a subterranean formation  220  is shown. Curve  222 , in the illustrated embodiment, represents a top surface of the subterranean formation  220  located at the bottom of the body of water. The subterranean formation  220  is composed of a number of subterranean layers of sediment and rock in the illustrated embodiment. Curves  224 ,  226 , and  228  represent interfaces between subterranean layers of different compositions. A shaded region  230 , bounded at the top by a curve  232  and at the bottom by a curve  234 , represents a subterranean geological structure, the depth and positional coordinates of which may be determined, at least in part, by analysis of seismic data collected during a marine seismic survey. 
     Example Survey Operations Using Multiple Types of Sources Simultaneously 
     In some embodiments, as the survey vessel  202  travels over the subterranean formation  220 , the sources  204 A and  204 B are respectively fired and activated to produce an acoustic signal at spatial and/or temporal intervals, according to some embodiments. In the illustrated embodiment, sources  204  are operated simultaneously or near-simultaneously such that wavefronts from both sources propagate through the body of water at the same time. For example, airgun source  204 A may be fired while vibratory source  204 B is activated. 
     The illustrated example shows example rays that represent paths of acoustic wavefronts that travel from sources  204  into the subterranean formation  220 . In the illustrated embodiment, the ocean bottom node  238  and the receivers  218  are sensors that receive signals initiated from signal sources  204  and reflected from subsea formations. In the illustrated example, signals received by receiver  218  are shown using dashed lines and signals received by node  238  are shown using solid lines. 
     In the illustrated embodiment, rays  260  and  262  are respectively emitted from airgun source  204 A and vibratory source  204 B at approximately the same location. Therefore, in the illustrated embodiment, rays  260  and  262  are reflected from the same portion of geological structure  230  of subterranean formation  220  and recorded at a receiver  218  (e.g., among other receivers that may also receive reflected signals). In some embodiments, these measurements may allow for efficient and accurate calibration based on the recorded signals, because the signals from the two types of sources should have a known relationship, absent a need for calibration. Similarly, in the illustrated embodiment, rays  240  and  242  emitted at approximately the same location from airgun source  204 A and vibratory source  204 B, respectively, are reflected from the same location of surface  232  and recorded by node  238 . Note that for simplicity of illustration only a handful of ray paths are represented, and ray paths that extend to other subsea interfaces are not shown. 
     As used herein, “activation” of a vibratory source is intended to be construed according to its well understood meaning, which includes causing at least a portion of the source to have vibratory movement in a body of water. Typically, vibratory sources are driven using codes or functions, such as linear sweeps, random sweeps, gold codes, M-codes, other modulation sequences, etc. Note that incidental movement of a vibratory source due to environmental conditions such as ocean currents is not considered activation of the source. Further, even while activated, a vibratory source may happen to be still momentarily, e.g., at a peak or valley of a wave-style modulation when changing directions, but is still considered activated during that moment. 
     In some embodiments, an airgun is fired during the activation of a vibratory source such that both sources emit signals simultaneously. In some embodiments, as used herein, “near-simultaneous” operations/firings include those without meaningful differences in actuation time between them. As would be understood by one of ordinary skill in the art with the benefit of this disclosure, operational circumstances can cause intermittent gaps in actuations (due to equipment failure, etc.), and “near-simultaneously” should be read to include actuations with intermittent or periodic gaps, whether planned or unplanned as well as actuations without intermittent or periodic gaps, thus including “simultaneously.” 
     In other implementations, the one or more of sources  204 A,  204 B, and streamer  208  may be towed by different survey vessels. In other implementations, the sources  204 A and  204 B may be a combination of several air guns and several marine vibrators (or various combinations of two or more different types of sources). It should be noted that the number of sources  204  is not limited to a single source of each type (airgun source  204 A and vibratory source  204 B). In practice, the number of sources selected to generate acoustic energy may range from as few as one source to three or more sources and the sources may be towed in groups or arrays by one or more vessels. 
     Overview of 4D Survey Techniques 
       FIG. 3  illustrates an exemplary comparison of baseline and monitoring surveys, according to some embodiments. In the context of 4D surveying, the phrase “monitor survey” refers to a subsequent survey over at least a portion of the same area covered by a previous (“baseline”) survey to detect changes in the subsurface relative to the time of the baseline survey. In the illustrated embodiment, seismic source data  310  includes baseline image  312  and monitor image  314  that are processed to produce a difference image  316  that shows differences between the baseline and monitor images. 
     In some embodiments, the seismic source data is obtained from an airgun. In some embodiments, the seismic source data is obtained from a marine vibrator. In some embodiments, the seismic source data is obtained from other types of marine signal sources (e.g. electromagnetic sources, etc.). 
     In the illustrated embodiment, baseline  312  is generated from a first survey. In the illustrated embodiment, monitor  314  is generated from a second survey obtained with the same type of signal source (e.g., in some examples the same exact source) and at the same location as baseline  312 . The second survey may be performed at a later point in time (e.g., days, months, years, etc.) than the first survey. In the illustrated embodiment, the image  316  is generated by comparing the monitor survey  314  with the baseline survey  312 . In the illustrated embodiment, image  316  may show changes in the geological structure at a particular location based on the recorded seismic source data  310 . In some embodiments, monitor survey  314  may become a baseline survey for one or more future monitor surveys. 
     In some embodiments, various preprocessing may be performed on the survey data before the baseline and monitor images are created. In some embodiments, a 4D survey performed using seismic sources over periods of time may provide information concerning changes in the geological structure of the earth e.g., the depletion over time of a natural resource such as a hydrocarbon deposit. 
     4D Survey Techniques with a Transition Survey 
       FIG. 4  is a diagram illustrating an exemplary sequence of surveys, including a transition survey for calibrating a transition from a first type of source to a second type of source, according to some embodiments. In the illustrated embodiment, the sequence includes surveys  410 A- 410 N with an airgun source, a survey  420  with both airgun and vibrator sources, and surveys  430 A- 430 N with a vibrator source. In some embodiments, surveys  410  use only airgun sources and surveys  430  use only vibratory sources. In some embodiments, the transition survey  420  may enable both backward and forward compatibility of survey measurements from different types of sources. 
     In the illustrated embodiment, survey  410 A is a baseline airgun survey with monitor surveys  410 B- 410 N also acquired with an airgun source. In the illustrated embodiment, a new transition monitor survey  420  with both airgun and vibrator sources is acquired. In some embodiments, transition survey  420  generates measurements from signals generated simultaneously or near-simultaneously by both an airgun and vibrator. In some embodiments, transition survey  420  may use several airgun sources and/or several vibrator sources. In some embodiments, transition survey  420  may use only one airgun source and several vibrator sources, or vice versa. In some embodiments, the airgun and vibrator signals of transition survey  420  are separated (e.g., by deconvolution, image comparison etc.) and used to determine calibration information. In the illustrated embodiment vibrator survey  430 A is a baseline survey for future monitor surveys (vibrator surveys  430 B- 430 N) acquired with a vibrator source only. 
     Calibration data may be used to modify various data sets to improve compatibility. For example, data from one or more surveys  410 A- 410 N (e.g., airgun only surveys) may be adjusted (e.g., using transition survey  420 ) to be compatible with data from surveys  430 A- 430 N (e.g., vibrator only surveys). In some embodiments, signals from one or both sources in the transition survey may be calibrated to be compatible with surveys  410  and/or  430 . Similarly, data from one or more surveys  430 A- 430 N may be calibrated (e.g., using transition survey  420 ) to be compatible with data from surveys  410 A- 410 N. In various embodiments, data and/or images from calibrated surveys may be compared, e.g., to determine changes in geological formations over time. 
     Overview of Use of Calibration Data 
       FIG. 5  is a diagram illustrating an overview of data associated with different types of sources, according to some embodiments. In the illustrated embodiment, an airgun image  514  and a vibrator image  516  are generated based on survey data of a given formation using the respective types of sources. As shown, a comparison of these images may correspond to difference image  520 . In the illustrated example, this difference image  520  includes both differences due to changes in the geological formation over time (represented in image form as 4D difference  530 ) and differences due to the different sources (represented in image form as source difference  540 ). Note that images of  FIG. 5  are shown for purposes of illustration and are not intended to limit the scope of calibration processing. 
     The example shown in  FIG. 5  may occur, for example, when using data from prior airgun surveys with data from subsequent vibrator surveys. If knowledge of the 4D difference is desired, the source difference  540  may contaminate this data. Note that, in ideal situations, there may not be a source difference. In real-world applications, however, this phenomenon is typically encountered when using different types of sources. 
     Therefore, in some embodiments, the transition survey operates an impulsive source and vibratory source simultaneously at approximately the same location, which allows a comparison of sensor measurements from the two sources to determine source difference data (without any 4D difference, because the formation has not changed). Knowledge of the source difference data may then allow calibration of other surveys, e.g., to generate accurate 4D difference information. 
     In some embodiments, calibration data may be generated prior to generating any images for a survey, while in other embodiments calibration data may be generated at least partially post-imaging. 
     Exemplary Techniques for Generating Calibration Data 
     In some embodiments, signals from different types of sources in a given transition survey pass may be separated as a first step in generating calibration data. In some embodiments, deconvolving the signals recorded in transition survey  420  separates the signals from the airgun source and the vibratory source based on the known modulation of the vibratory source used in survey  420 . In other embodiments, the signals from different sources may be separated based on a known signature of the airgun, by suppressing vibratory signals during an interval corresponding to the airgun impulse, etc. After separating the signals, one or more of various techniques may be used to generate calibration data.  FIGS. 6 and 7 , discussed in detail below, show two examples of such techniques. 
     In some embodiments, the signals from different types of sources are expected to have some known relationship, absent a need for calibration. Therefore, in some embodiments, generating calibration data is based on differences from the expected relationship exhibited in the data from the transition survey. 
       FIGS. 6A and 6B  illustrate one example of using an expected relationship when comparing signals measured using a vibratory source and signals measured using an impulsive source to obtain calibration data. In the illustrated embodiment, element  650  of  FIG. 6B  shows an example airgun impulse signal generated by firing an airgun. In addition, in the illustrated embodiment, element  660  of  FIG. 6B  shows an exemplary autocorrelation of a gold code that may be used to drive a vibratory source. Although gold codes are used for purposes of illustration, autocorrelation results may be generated for various types of modulation (e.g., linear sweeps, random sweeps, etc.). Comparing spikes shown in elements  650  and  660  may produce an actual relationship that is different from the expected relationship, therefore providing difference information that may be used to determine calibration information. 
     In other embodiments, signals at various levels of processing may be compared to determine a difference from an expected relationship. For example, a measured signal based on a vibratory source may be compared with airgun data without autocorrelation. Note that the expected relationship used in this example is different than the expected relationship used in other examples (e.g., different for measured signals vs. autocorrelated measured signals). 
       FIG. 6A  is a flow diagram illustrating an example method for pre-imaging calibration, according to some embodiments. The method shown in  FIG. 6A  may be used in conjunction with any of the computer circuitry, systems, devices, elements, or components disclosed herein, among other devices. In various embodiments, some of the method elements shown may be performed concurrently, in a different order than shown, or may be omitted. Additional method elements may also be performed as desired. 
     At  610 , in the illustrated embodiment, a computing system determines a signal spike in sensor measurements from an airgun source and a spike in autocorrelation results for sensor measurements from a vibratory source. 
     At  620 , in the illustrated embodiment, the signal spikes are compared to generate difference information. In some embodiments, one or both of the spikes may be scaled or processed before comparison, such that the scaled spikes are expected to be the same, in the absence of a need for calibration. 
     At  630 , in the illustrated embodiment, airgun measurements or vibratory measurements are adjusted based on the difference information. In various embodiments, this may calibrate the adjusted measurements with other data sets from another type of source. Examples of adjustments may include scaling one or both of the spikes based on the comparison of  620  such that the results have similar amplitudes (which may be in addition to any scaling performed prior to the comparison) or shifting one or both of the spikes based on the comparison of  620  such that they occur at the same time. In other embodiments, any of various appropriate adjustments may be performed at various different stages of calibration, such as adjusting raw data, adjusting information indicating time intervals between emission of signals at a source and reception of the signals at a sensor, adjusting one or more filters, etc. 
     Note that the adjusting of element  630  may be performed on data from a transition survey or data from another survey (e.g., that uses only one type of source). In some embodiments, a system performing the adjustment may be different than a system performing the comparison. 
     At  640 , in the illustrated embodiment, the system generates one or more images using the adjusted measurements. In various embodiments, the disclosed techniques may improve imaging accuracy for 4D surveys that use different types of sources for different surveys. 
     In some embodiments, a seismic signal dataset may be processed using designature techniques before calibration. For example, measured signals may be processed to remove noise by performing a designature technique. In some embodiments, performing a designature technique on a dataset involves two components, zero phasing and de-bubbling, to prepare the data for further processing. Zero phasing and de-bubbling involve calibrating and combining two near field source datasets (e.g., signals recorded close to the signal source) for a source. In some embodiments, applying a zero-phase filter on a source signal involves applying the filter in both forward and reverse time directions. De-bubbling involves removing the signal “bubble” or “noise” that occurs when a seismic source is fired as seen in the several large oscillations in image  650 , for example. 
       FIG. 7A  is a block diagram illustrating exemplary image processing of seismic source data to generate calibration data, according to some embodiments. In the illustrated embodiment, an airgun image  714  and a vibrator image  716 , both generated from a transition survey, are processed to generate source difference image  738 . The difference between the two images may be used to calibrate other airgun and/or vibrator data sets. For example, if a geological formation is shown at a particular difference in depth in one image than in another, that region of images from other data sets may be adjusted accordingly. In this example, calibration data is determined post-imaging. 
       FIG. 7B  is a flow diagram illustrating an exemplary method for post-image generation of calibration data, according to some embodiments. The method shown in  FIG. 7B  may be used in conjunction with any of the computer circuitry, systems, devices, elements, or components disclosed herein, among other devices. In various embodiments, some of the method elements shown may be performed concurrently, in a different order than shown, or may be omitted. Additional method elements may also be performed as desired. 
     At  710 , in the illustrated embodiment, the airgun data and vibrator data are separated. This may be performed using deconvolution or other techniques, as discussed above. 
     At  720 , in the illustrated embodiment, images are created from the separated airgun and vibrator data sets. Note that the images may be identical if no calibration is necessary. In real-world situations, the images will typically vary. 
     At  730 , in the illustrated embodiment, differences between the two images are determined. 
     At  740 , in the illustrated embodiment, one or more data sets are calibrated based on the determined differences. 
     In some embodiments, the airgun  714  and vibrator  716  images are obtained after pre-processing (e.g., applying designature techniques) of the signal data. In some embodiments, vibrator image  716  is backward compatible (e.g., with previous airgun surveys) when calibrated based on the source difference image  738  or when data from the previous survey data is calibrated based on source difference image  738 . In addition, in some embodiments, vibrator image  716  is forward compatible (e.g., compatible with future vibrator surveys). In some embodiments, a calibrated vibrator image is also forward compatible with similarly-calibrated vibrator surveys. 
     Exemplary Methods 
       FIG. 8  is a flow diagram illustrating an exemplary method for a marine geophysical survey, according to some embodiments. The method shown in  FIG. 8  may be used in conjunction with any of the computer circuitry, systems, devices, elements, or components disclosed herein, among other devices. In various embodiments, some of the method elements shown may be performed concurrently, in a different order than shown, or may be omitted. Additional method elements may also be performed as desired. 
     At  810 , in the illustrated embodiment, a survey system tows an impulsive source and a vibratory source in a body of water. 
     At  820 , in the illustrated embodiment, the survey system fires the impulsive source while the vibratory source is activated. In such embodiments, signals from both sources may propagate through the water at the same time. In some embodiments, the firing the impulsive source while the vibratory source is activated is performed during a transition survey with both vibratory and impulsive sources. In these embodiments, the recorded signals may be usable to calibrate one or more prior surveys that use impulsive sources with one or more subsequent surveys that use vibratory sources. In some embodiments, the operated vibratory source is driven using one or more gold codes during a transition survey. 
     At  830 , in the illustrated embodiment, a plurality of seismic sensors record signals that are reflected from one or more geological structures in response to the firing of the impulsive source and the activation of the vibratory source. 
     At  840 , in the illustrated embodiment, the system adjusts sensor signals from at least one of a prior seismic survey with one or more impulsive sources or a subsequent seismic survey with one or more vibratory sources to calibrate one of the prior survey and the subsequent survey with the other. This calibration may include adjusting sensor data from the prior survey only, the subsequent survey only, or both. In other embodiments, a survey method may include method elements  810 - 830  and may not include element  840 , which may be performed separately. 
     In some embodiments, the impulsive source and the vibratory source are towed in approximately the same location. In various embodiments, a plurality of impulsive and vibratory sources are towed and simultaneously operated. One or more of the impulsive and vibratory sources may be towed using the same cable(s). 
     In some embodiments, a plurality of impulsive sources fire while a plurality of vibratory sources is activated. In some embodiments, signals are recorded at one seismic sensor from an impulsive source and a vibratory source that are fired and activated respectively at the same location. 
       FIG. 9  is a flow diagram illustrating an exemplary method for processing seismic source data, according to some embodiments. The method shown in  FIG. 9  may be used in conjunction with any of the computer circuitry, systems, devices, elements, or components disclosed herein, among other devices. In various embodiments, some of the method elements shown may be performed concurrently, in a different order than shown, or may be omitted. Additional method elements may also be performed as desired. 
     At  910 , in the illustrated embodiment, a computing system accesses seismic data recorded using a plurality of seismic sensors. In the illustrated embodiment, the seismic data was recorded during a transition survey based on signals reflected from one or more geological structures in response to generating seismic signals using a first type of seismic source while also generating seismic signals using a second, different type of seismic source. Accessing the data may include receiving the first data via a network, reading the data from a storage device, or reading data in real-time as it is generated, for example. In some embodiments, the first type of seismic source may be an impulsive source such as an airgun and the second type of seismic source may be a vibratory source. In other embodiments, the first type of source may be vibratory and the second type of source may be impulsive. In some embodiments, the first and second types of sources are different types of impulsive sources (e.g., different airgun models or different types of airgun arrays). In other embodiments, the first and second types of sources may be different types of vibratory sources (e.g., different vibrator models or different types of vibrator arrays). 
     At  920 , in the illustrated embodiment, the system adjusts, based on the accessed seismic data, sensor signals from at least one of a prior seismic survey with the first type of seismic source or a subsequent seismic survey with the second type of seismic source, wherein the adjustment calibrates one of the prior and the subsequent surveys with the other. In some embodiments, the computing system separates signals generated by the first type of source from signals generated by the second type of source. In some embodiments, the separation includes deconvolution based on a known modulation of the second type of source. In various embodiments, an example of means for separating the signals from a first type of source from the combined data includes such deconvolution. In some embodiments, the system generates difference information, indicating differences between signals from the first and second types of sources. One example of means for generating difference information involves comparing a signal measured from a first type of source (e.g., a vibratory source) and a signal measured from a second type of source (e.g., an impulsive source) and determining a difference between the obtained relationship and an expected relationship. Another specific example of such means involves comparing signal spikes from airgun and autocorrelated vibratory signals, as described above with reference to  FIG. 7 . Another example of such means involves generating images based on signals from the respective different types of sources (after separation) and determining differences between the images. 
     In some embodiments, the system uses the difference information to adjust sensor measurements of signals reflected from the geophysical formation during a prior or subsequent survey (or, in some embodiments, during the transition survey). The adjustment may calibrate sensor measurements from vibratory signals with sensor measurements from impulsive signals for the geophysical formation. Note that the various survey passes discussed herein may be performed as part of the same overall survey or as part of different surveys. In various embodiments, the techniques discussed above correspond to various means for calibrating survey data using difference information. In various embodiments, examples of such means include scaling, altering time of flight-values, adjustment of survey data in the time domain, etc. 
     In some embodiments, a survey using one or more vibratory sources is calibrated with a previous survey using one or more airgun sources. In some embodiments, a survey using one or more vibratory sources is calibrated with a previous survey using one or more vibratory sources and one or more airgun sources. In some embodiments, a survey using one or more vibratory sources and one or more airgun sources is calibrated with a previous survey using one or more airgun sources. 
     In some embodiments, the disclosed techniques may be used to calibrate data from survey passes that use different specific types of the same general type of source. For example, a first model of vibratory source may be used in a first survey and a second model of vibratory source in a second survey, and the disclosed techniques may be used to calibrate the two surveys. Although vibratory and impulsive sources are discussed herein for purposes of illustration, transition surveys with one or more of any of various types of sources are contemplated. 
     Example Computing System 
     Various operations described herein may be implemented by a computing device configured to execute program instructions that specify the operations. Similarly, various operations may be performed by circuitry designed or configured to perform the operations. In some embodiments, a non-transitory computer-readable medium has program instructions stored thereon that are capable of causing various operations described herein. As used herein, the term “processor,” “processing unit,” or “processing element” refers to various elements or combinations of elements configured to execute program instructions. Processing elements include, for example, circuits such as an ASIC (Application Specific Integrated Circuit), custom processing circuits or gate arrays, portions or circuits of individual processor cores, entire processor cores, individual processors, programmable hardware devices such as a field programmable gate array (FPGA) or the like, and/or larger portions of systems that include multiple processors, as well as any combinations thereof. 
     Turning now to  FIG. 10 , a block diagram of a computing device (which may also be referred to as a computing system)  1010  is depicted, according to some embodiments. Computing device  1010  may be used to implement various portions of this disclosure. Computing device  1010  is one example of a device that may be used as a mobile device, a server computing system, a client computing system, or any other computing system implementing portions of this disclosure. 
     Computing device  1010  may be any suitable type of device, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mobile phone, mainframe computer system, web server, workstation, or network computer. As shown, computing device  1010  includes processing unit  1050 , storage subsystem  1012 , and input/output (I/O) interface  1030  coupled via interconnect  1060  (e.g., a system bus). I/O interface  1030  may be coupled to one or more I/O devices  1040 . Computing device  1010  further includes network interface  1032 , which may be coupled to network  1020  for communications with, for example, other computing devices. 
     As described above, processing unit  1050  includes one or more processors. In some embodiments, processing unit  1050  includes one or more coprocessor units. In some embodiments, multiple instances of processing unit  1050  may be coupled to interconnect  1060 . Processing unit  1050  (or each processor within processing unit  1050 ) may contain a cache or other form of on-board memory. In some embodiments, processing unit  1050  may be implemented as a general-purpose processing unit, and in other embodiments it may be implemented as a special purpose processing unit (e.g., an ASIC). In general, computing device  1010  is not limited to any particular type of processing unit or processor subsystem. 
     Storage subsystem  1012  is usable by processing unit  1050  (e.g., to store instructions executable by and data used by processing unit  1050 ). Storage subsystem  1012  may be implemented by any suitable type of physical memory media, including hard disk storage, floppy disk storage, removable disk storage, flash memory, random access memory (RAM-SRAM, EDO RAM, SDRAM, DDR SDRAM, RDRAM, etc.), ROM (PROM, EEPROM, etc.), and so on. Storage subsystem  1012  may consist solely of volatile memory in some embodiments. Storage subsystem  1012  may store program instructions executable by computing device  1010  using processing unit  1050 , including program instructions executable to cause computing device  1010  to implement the various techniques disclosed herein. In at least some embodiments, storage subsystem  1012  may represent an example of a non-transitory computer-readable medium that may store executable instructions. 
     In the illustrated embodiment, computing device  1010  further includes non-transitory medium  1014  as a possibly distinct element from storage subsystem  1012 . For example, non-transitory medium  1014  may include persistent, tangible storage such as disk, nonvolatile memory, tape, optical media, holographic media, or other suitable types of storage. In some embodiments, non-transitory medium  1014  may be employed to store and transfer geophysical data, and may be physically separable from computing device  1010  to facilitate transport. Accordingly, in some embodiments, the non-transitory medium  1014  may constitute the geophysical data product discussed above. Although shown to be distinct from storage subsystem  1012 , in some embodiments, non-transitory medium  1014  may be integrated within storage subsystem  1012 . 
     I/O interface  1030  may represent one or more interfaces and may be any of various types of interfaces configured to couple to and communicate with other devices, according to various embodiments. In some embodiments, I/O interface  1030  is a bridge chip from a front-side to one or more back-side buses. I/O interface  1030  may be coupled to one or more I/O devices  1040  via one or more corresponding buses or other interfaces. Examples of I/O devices include storage devices (hard disk, optical drive, removable flash drive, storage array, SAN, or an associated controller), network interface devices, user interface devices or other devices (e.g., graphics, sound, etc.). In some embodiments, the geophysical data product discussed above may be embodied within one or more of I/O devices  1040 . 
     This specification includes references to various embodiments, to indicate that the present disclosure is not intended to refer to one particular implementation, but rather a range of embodiments that fall within the spirit of the present disclosure, including the appended claims. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure. 
     Within this disclosure, different entities (which may variously be referred to as “units,” “circuits,” other components, etc.) may be described or claimed as “configured” to perform one or more tasks or operations. This formulation—[entity] configured to [perform one or more tasks]—is used herein to refer to structure (i.e., something physical, such as an electronic circuit). More specifically, this formulation is used to indicate that this structure is arranged to perform the one or more tasks during operation. A structure can be said to be “configured to” perform some task even if the structure is not currently being operated. An “apparatus configured to steer a vibratory source” is intended to cover, for example, an apparatus that performs this function during operation, even if the corresponding device is not currently being used (e.g., when power is not connected to it). Thus, an entity described or recited as “configured to” perform some task refers to something physical, such as a device, circuit, memory storing program instructions executable to implement the task, etc. This phrase is not used herein to refer to something intangible. 
     The term “configured to” is not intended to mean “configurable to.” An unprogrammed mobile computing device, for example, would not be considered to be “configured to” perform some specific function, although it may be “configurable to” perform that function. After appropriate programming, the mobile computing device may then be configured to perform that function. 
     Reciting in the appended claims that a structure is “configured to” perform one or more tasks is expressly intended not to invoke  35  U.S.C. §  112 (f) for that claim element. Should Applicant wish to invoke Section  112 (f), it will recite claim elements using the “means for” [performing a function] construct. 
     As used herein, the term “based on” is used to describe one or more factors that affect a determination. This term does not foreclose the possibility that additional factors may affect the determination. That is, a determination may be solely based on specified factors or based on the specified factors as well as other, unspecified factors. Consider the phrase “determine A based on B.” This phrase specifies that B is a factor used to determine A or that affects the determination of A. This phrase does not foreclose that the determination of A may also be based on some other factor, such as C. This phrase is also intended to cover an embodiment in which A is determined based solely on B. As used herein, the phrase “based on” is synonymous with the phrase “based at least in part on.” 
     It is to be understood the present disclosure is not limited to particular devices or methods, which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” include singular and plural referents (such as “one or more” or “at least one”) unless the content clearly dictates otherwise. Furthermore, the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not in a mandatory sense (i.e., must). The term “include,” and derivations thereof, mean “including, but not limited to.” The term “coupled” means directly or indirectly connected. 
     Moreover, where flow charts or flow diagrams are used to illustrate methods of operation, it is specifically contemplated that the illustrated operations and their ordering demonstrate only possible implementations and are not intended to limit the scope of the claims. It is noted that alternative implementations that include more or fewer operations, or operations performed in a different order than shown, are possible and contemplated. 
     Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure. Although various advantages of this disclosure have been described, any particular embodiment may incorporate some, all, or even none of such advantages. 
     The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims, and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.