WO2016055826A1

WO2016055826A1 - Method and system for high productivity seismic source acquisition using time synchronized signals in combination with source location

Info

Publication number: WO2016055826A1
Application number: PCT/IB2014/002208
Authority: WO
Inventors: John Sallas
Original assignee: Cgg Services Sa
Priority date: 2014-10-06
Filing date: 2014-10-06
Publication date: 2016-04-14

Abstract

Systems and methods are provided in support of seismic prospecting. There is a method for seismic prospecting that includes: determining, based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; and actuating at least one vibratory source using the sweep to impart seismic waves into the ground.

Description

Method and System for High Productivity Seismic Source Acquisition Using Time Synchronized Signals in Combination with Source Location

BACKGROUND

TECHNICAL FIELD

[0001] Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating, acquiring and processing land seismic data.

DISCUSSION OF THE BACKGROUND

[0002] Land seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located.

[0003] Geophysical prospectors have found seismic vibrators to be useful signal sources for imaging the earth. Conventional seismic acquisition in the past generally employed multiple vibrators acting together and initiated simultaneously to form a source array. In land-based operations, the vibrators are positioned at a source location and synchronized to the same pilot sweep signal. Once activated, the vibrators generate a sweep that typically lasts between five and twenty seconds and typically spans a predetermined range of frequencies. A recording system that is connected to a plurality of receivers, typically geophones for land-based seismic exploration, is employed to receive and record the response data. For reflection seismology, the record length is typically set to equal the sweep length plus a listen time equal to the two-way travel time, which is the time required for the seismic energy to propagate from the source through the earth to the deepest reflector of interest and back to the receiver. The vibrators are then moved to a new source location and the process is repeated.

[0004] The conventional methods have a number of shortcomings some of which include: 1 ) intra-array statics because the vibrators are at different elevations or variations in the near surface that can affect source coupling to the earth; 2) spatial resolution issues due to array effects and limitation in source effort because of economic constraints; 3) control and synchronization problems associated with the use of multiple sources; and 4) mixed-phase data produced by the correlation process. Improvements in technology and reductions in the per channel cost of recording have resulted in an industry push toward using point source-point receiver methods to overcome some of the problems associated with source arrays and large receiver arrays.

[0005] Over the years a number of methods have been introduced to address shortcomings with conventional seismic survey methods. One method, titled "Method for Continuous Sweeping and Separation of Multiple Seismic Vibrators," by Krohn and Johnson (WO/2005/019865), the entire disclosure of which is incorporated herein by reference, attempts to address the data quality and data acquisition issues. This method is an extension of the High Fidelity Vibratory System ("HFVS") originally developed by MOBIL and ARCO (see U.S. Pat. Nos. 5,719,821 and 5,721 ,710). The MOBIL-ARCO alliance developed a data acquisition and data processing technique that eliminates vibrator intra-array statics problems, mitigates vibrator control errors, provides minimum phase data, and provides high spatial resolution. However, for some cases, in order to provide a cost effective method for effectively collecting point source data, a means to separate vibrators sweeping simultaneously which have asynchronous start times may be useful.

[0006] The cost of seismic surveys depends heavily on the time required to collect the data. To reduce the acquisition time a number of methods have been devised over the years. Methods for source separation disclosed vibrator sources that are operated concurrently to reduce the time required for acquiring seismic survey data. For example, two groups of vibrators shooting into the same receiver spread at different offsets can be used to form a composite record. Most of those methods involve some form of swept sine wave source signal and rely on properties of the sweeps to be separated by correlation. Some methods rearrange portions of a conventional swept sine wave to mitigate crosstalk between surveys due to cross- correlation between the sweeps employed (see U.S. Pat. No. 4, 168,485 and U.S. Pat. No. 4,982,374). Others achieve separation by using phase encoding schemes sometimes combined with up-sweeps and down-sweeps (see U.S. Pat. No. 4,823,326), others use time delays (see U.S. Pat. No. 4,953,657) and still others employ different sweep rates (see WO 2008/025986). Still others use techniques such as slip-sweep (see U.S. Pat. No. 6,603,707) that combine conventional swept sine waves, time delayed starts, and processing methods of F-T filtering, deconvolution, and migration to achieve separation (also see WO 2006/018728).

[0007] One such method, disclosed in U.S. Patent No. 7,859,945 (herein '945), the entire content of which is incorporated herein by reference, implements a seismic acquisition using vibrators operating simultaneously. In this way, the time spent for the seismic survey is further reduced. The sweeps employed by the vibrators are based upon modified pseudorandom digital sequences. These sweep modifications include: 1 ) spectral reshaping, 2) cross-correlation suppression over a time window of interest, and 3) level compression to restore a favorable root mean square ("RMS") to peak amplitude level. The composite received signal reflected from the subsurface formations is correlated with the pilot sweep signals for preliminary separation. The individual responses of subsurface formations in the transmission path between each individual source and each seismic detector may be recovered, with the source signature removed.

[0008] There has also been a trend to move to using point source and point receiver acquisition systems. These types of systems can avoid issues with intra- array source statics that can complicate processing of data collected using source arrays. For example, having sources at different elevations within the same source array complicates processing, particularly when operating at higher frequencies (typically above 40 Hz) and even more so if the near surface waves have a relatively low velocity. Intra-array statics are generally not an issue for very low frequencies, below 5 Hz, since wavelengths are quite long in this region.

[0009] Additionally, there has been a desire to extend seismic acquisition bandwidth, in particular in the lower frequencies. Lower frequency content has been shown to be useful in improving acoustic impedance inversion results. The processing algorithms employed to perform acoustic impedance inversion are known to have multiple local minima, and inclusion of low frequency in seismic data acquisition activities tend to guide search procedures toward a unique, optimal solution. [0010] As currently used, when frequencies in seismic acquisition activities go below around 5 Hz in frequency, the signal level drops significantly while electronic noise in the recording equipment as well as environmental noise due to natural phenomenon like micro-tremors, tidal motion, etc., tend to increase. Another concern is that generally, the vibrators in common use cannot maintain rated output at very low frequencies and that seismic vibrators are a surface source with the vibrator ground force amplitude being controlled to be spectrally flat.

[0011] Geophones are widely used as receivers and measure particle velocity. In a homogeneous half-space the p-wave far field particle velocity will be proportional to a time-delayed version of the time derivative of the ground force signal from a surface source. Therefore, as one goes lower in frequency, there is a loss of 6 dB per octave of signal due to the differentiation effect; moreover, 10 Hz geophones are commonly used as receivers, below 10 Hz their response falls at a rate of 12 dB per octave leading to a combined signal loss of 18 dB per octave below 10 Hz.

[0012] Accordingly, there is a need to develop systems and methods to reduce the above described drawbacks.

SUMMARY OF THE INVENTION

[0013] According to an exemplary embodiment, there is a method for seismic prospecting, the method comprising: determining, based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; and actuating at least one vibratory source using the sweep to impart seismic waves into the ground.

[0014] According to another exemplary embodiment, there is a system for seismic prospecting, the system comprising: a memory configured to store a ruleset used to determine, based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; and at least one vibratory source configured to actuate a sweep to impart seismic waves into the ground.

[0015] According to still another exemplary embodiment, there is computer readable medium including computer executable instructions, wherein the instructions, when executed by a processor, implement a method for seismic prospecting, the medium including instructions for: determining, based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; and actuating at least one vibratory source using the sweep to impart seismic waves into the ground.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0017] Figure 1 is a schematic diagram of electronics of a vibratory source according to an embodiment;

[0018] Figures 2-3 illustrate a subdivided seismic survey according to an embodiment;

[0019] Figure 4 shows a portion of a 3 dimensional (3D) survey according to an embodiment;

[0020] Figure 5 depicts a flowchart for performing a seismic vibratory sweep according to an embodiment;

[0021] Figure 6 illustrates a portion of a three dimensional seismic survey in which low frequency emission properties can be used according to an embodiment;

[0022] Figures 7(a)-(c) show graphs of frequency emissions associated with a vibratory source according to an embodiment;

[0023] Figure 8 is a flowchart of a method for separating data into shot records according to an embodiment;

[0024] Figure 9 is a flowchart of a method for seismic prospecting according to an embodiment; and

[0025] Figure 10 is a schematic diagram of a computing device configured to implement one or more of the methods discussed in the embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0026] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a land seismic system.

[0027] Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0028] As described in the Background, there has been a trend to move to using point source and point receiver acquisition systems as these types of systems can avoid intra-array source statics and there has been a desire to extend seismic acquisition bandwidth, particularly in the lower frequencies. According to embodiments, methods and systems can determine distances between operating vibratory sources and execute a desired sweep plan which can reduce the above described challenges and improve operations in various frequency bandwidths. Furthermore, rules can be developed to have the vibrator controller (or remote management system) determine a desirable sweep to be used.

[0029] As described above in the Background, there are challenges when performing seismic data acquisition activities which use lower frequencies even though intra-array statics are not generally an issue at very low frequencies. According to embodiments, it can be useful to have sources, e.g., vibrators, acting in an array as the signal to ambient noise is improved by six dB when two sources are operating together. For four sources operating together there can be a twelve dB boost in the signal to noise ratio. Additionally, the spatial sampling requirements for low frequency acquisition are less when compared to the spatial sampling requirements for high frequency acquisition to remove coherent noise, e.g., ground roll, or for imaging purposes.

[0030] According to an embodiment, it can be desirable to boost very low frequencies, e.g., signals below 5 Hz. For non-limiting descriptive purposes, one can consider a midrange frequency to be 15-40 Hz and a high frequency to be 60 Hz and above. Spatial sampling requirements and spatial aliasing are both related to wavelength. A 3 Hz signal will have a wavelength an order of magnitude greater than a signal at 30 Hz and so on. So the spatial sampling requirements for 30 Hz would be an order of magnitude greater than for a 3 Hz signal in general. Another consideration is with Fresnel zones that describe the horizontal resolution.

[0031] Continuing with understanding lower frequency operations, as operating frequencies in vibratory surveys go lower in frequency more and more traces contribute to the image. Therefore, a greater number of reflection signals from larger source offsets contribute to an image for low frequencies than for high frequency signals. This implies that, according to an embodiment, more widely distributed low frequency sweep locations can be used than in conventional shooting. Therefore, positioning determination to be able to optimally use arrays of sources in various acquisition systems and methods can be desirable, embodiments of which are described below in more detail. However, other operations other than just low frequency operation can benefit from various embodiments described herein.

[0032] According to an embodiment, an exemplary vibrator 100 for use in seismic surveying, which can be used in various embodiments described herein, will now be described. Each vibrator 100 can include memory 102, a Global Positioning System (GPS) receiver 104, an electronics suite 106, a controller 108, a recorder 1 10, a central processing unit (CPU) 1 12, a telemetry unit 1 14 and a proximity detector 1 16. General use of the vibrator 100 will now be described.

[0033] The starting address of the sweep in the memory 102 of the vibrator 100 is computed (e.g., a pointer) using a universal clock such as the one provided by the GPS receiver 104. Alternatively, or additionally, a GPS receiver 104 may be installed in each vehicle carrying the vibrator 100. The vibrator electronics 106 executes the stored pilot signal from the calculated starting address up to the predetermined sweep length (SL) and simultaneously the vibrator controller 108 uses the pilot signal as a reference signal. The vibrator controller 108 uses the pilot signal and feedback signals from the vibratory system to create a drive signal suitable for operating the servo valve so that vibratory energy is produced by the hydraulic system.

[0034] Typically, the drive signal is computed to compensate for the system dynamics so that the ground force signal produced is proportional to the pilot signal. When the vibrator electronics 106 gets to the end of the table storing the pilot signal, it moves to the first entry and continues down until the excitation signal is stopped when that pointer has moved to the sweep entry address that immediately precedes the starting address. If only one sweep is to be executed per source point location, the vibrator 100 then advances to the next source point location. If more than one sweep is to be executed per source point location, the vibrator 100 remains in the same location and the vibrator electronics 106 can be programmed to execute the desired number of sweeps with a listen time interval between each repetition.

[0035] All the pilot signals are also recorded in the recorder 1 10 and these too are synchronized to the GPS time as well. As part of processing steps, when daughter records are extracted the recorded pilot signals need to be tapered in a way that agrees with what is happening in the vibrators. Also, the processor 1 12 can be used for calculating the pilot sweep index pointer position from the GPS time when the vibrator 100 is in position and the operator initiates the sweep. The telemetry system 1 14 can be used for receiving commands from the recording system 1 10 or for transmitting source performance data to the recording system 1 10. The proximity detector 1 16 can be used for determining locations of other vibrators.

[0036] According to an embodiment a seismic survey can be subdivided into sectors or quadrants as shown in Figure 2. Figure 2 has a survey area 200 which is divided into four sectors 202, 204, 206, 208 and also shows a pair of vibrators 210, 212 located in sector 202. When a vibrator(s) 210, 212 moves into a new sweep position, as shown in Figure 3 by the new location of the vibrators 210 and 212 in sector 206 as compared to the vibrators' position shown in Figure 2, the vibrator electronics 106 with the GPS receiver 104 to obtain the vibrator coordinates and the universal time. Using the current GPS location and time, the vibrator electronics 106 selects a sweep table that has been assigned to that sector while using the time to compute a sweep table index pointer to locate the starting point for the sweep. This embodiment may be desirable to implement in cases where there are a pair of vibrators working together in a particular sector which are located relatively close to one another. It relaxes, to some extent, a typical need to for both vibrators to be ready at the same time to begin sweeping, e.g., providing a leeway between sweep starts of a few seconds. Additionally, it may be desirable to keep the vibrator pair within a close enough range of each other so as to not, or to relatively not, compromise high frequency data.

[0037] According to one embodiment, the vibrator pair would be no farther apart than one-third the wavelength of the p-wave in the near surface for the highest frequency in the very low frequency range to be used which embodiments can enhance. For example, if the highest operating frequency is 5 Hz on dry sand with a velocity of 500 m/s, the wavelength is 100 m so the vibrator pair should be no farther apart than approximately 33 m. For another example, if the vibrator pair was operating on limestone with a near surface p-wave velocity of 6000 m/s then the vibratory sources should all be within 400 m of one another. It is too be understood that with different frequencies and operating conditions, e.g., make-up of the underlying ground through which the seismic signals of interest propagate through, other separation distances can be used.

[0038] According to an embodiment, as described above, each vibrator can be provided with a proximity detector 1 16 for determining if one or more other vibrators are nearby and/or assisting with determining its position with respect to other vibrator units. Examples of this proximity detector include a device (or devices) which may use, but is not limited to, infrared, a low power telecommunication device for detecting other vibrators, other position detecting equipment, or some combination thereof. Additionally, for cases where the GPS position of the vibrators is known, e.g., when each vibrator has a GPS receiver, the vibrators can share, via various known communication methods, their respective GPS positions from which a relative position may also be determined.

[0039] According to an embodiment, this location information for more than one vibrator can be used in support of sweep selection for each vibrator unit. For example, if a vibrator is less than a pre-determined distance apart from another vibrator, the vibrators can execute sweeps from the same sweep table. If the vibrators are beyond the pre-determined distance apart from one another, the vibrators can execute sweeps from different sweep tables. Sweep starting times for each vibrator can be independent of each other or not as desired, but each vibrator can use the GPS time to compute their respective starting point in the sweep table.

[0040] According to an embodiment, there is a so-called "frequency partitioned" embodiment in which two sets of sweep tables can be used. The first sweep table can be used when vibrators are to operate at a greater than (or equal) to a predetermined distance from each other. The second sweep table can be used when vibrators are to operate less than (or equal) to a predetermined distance from each other. A first sweep table set {A, B, C, D} which includes pseudorandom sweeps that are designed to be uncorrelated (weakly correlated) with respect to one another over a specified time interval of interest and a second sweep table set {E, F, G, H} designed to be companion sweeps with the first sweep table set. For example sweep table E would be the companion for sweep table A, with sweep table F being the companion for sweep table B, etc.

[0041] According to an embodiment, table E can be designed such that the low frequency portion of table E has approximately the same phase and amplitude spectrum as that of sweep table A, while the high frequency portion of sweep table E can be designed to be weakly correlated to the high frequency portion of sweep table A. According to an embodiment, frequencies that fall below about 10Hz could be the crossover frequency from going from high frequency to low frequency, however other crossover frequencies could be used as desired.

[0042] For example, consider a case where there are two vibrators that have downloaded their GPS position and have also determined their relative position with respect to each other. The two vibrators have determined that they are both in a same operating sector and within the predetermined threshold distance. Therefore, the first vibrator will execute a sweep from table A and the second vibrator will execute a sweep from table E. In this way, the signal to noise ratio will be improved for the low frequencies, while the ability to separate the high frequency contributions will be retained while avoiding intra-array statics and/or spatial sampling issues.

[0043] According to an embodiment, a reverse time sweep can be used by a vibrator instead of sweeps which are weakly uncorrelated over a time window of interest. This embodiment may use a single or a combination of two filtered pseudorandom signals. To do this, begin with a pseudorandom signal (parent) and perform both a low pass filter and a high pass filter on the pseudorandom signal. One can then, for example, flip in time the high frequency filtered version and add the result to the low frequency filtered sweep to produce a new daughter pseudorandom sweep signal whose low frequency phase and amplitude matches that of its parent signal but its high frequency content will not be highly correlated. [0044] According to an embodiment, instead of using point sources, several small vibrator arrays each comprised of two or three trucks in a survey could be used. In the case where there are small arrays working together for all frequencies and then when the plurality of small arrays get relatively close to a different small vibrator group, all of the vibrators can switch to using sweeps that are synchronized for the low frequencies.

[0045] According to embodiments, various embodiments as described herein can be combined with other blended acquisition schemes, for example, acquisition schemes that employ distance separation. According to some embodiments, there may not be a need for a high number of different uncorrelated sweeps. Additionally, when the sectors used in a seismic survey are large enough, distance separation can provide a secondary level of separation (for the signals) in addition to being weakly correlated, so that possibly as few as only four sweep tables are required for, e.g., some embodiments which use a pair or an array of vibrators which are relatively close to one another operating within a same sector.

[0046] Embodiments described herein have generally been described having sweeps which use pseudorandom signals. However, some embodiments can use swept sine waves or a combination of swept sine waves and pseudorandom signals as desired. For example, in some cases it may be easier to synchronize swept sine waves, particularly when the vibrators are in relatively close proximity and when they are to operate together as a source array. Alternatively, when the vibrators are farther apart it may be more desirable to use pseudorandom signals.

[0047] According to an embodiment, other sweep options can also be used. For example, in one instance, it may be desirable to concatenate two subsweeps. The first subsweep could be a chirp covering a range of frequencies and the second subsweep using a different range of frequencies. The second subsweep could be pseudorandom. According to an embodiment, another type of sweep that can be used is a so-called "serpentine" sweep. A serpentine sweep can describe a family of sweeps which may be a linear or non-linear swept sine wave type of sweep in which the frequency versus time function is perturbed to create several sweeps whose mutual cross correlation properties have been improved. In other words, the serpentine sweep scheme allows sweeps that have a similar power spectra, but have slightly different frequency versus time functions that allow users to more easily separate the sweeps' contributions in composite seismic recordings in which vibratory sources operate simultaneously.

[0048] According to an embodiment, vibrators operating to survey a sector could have a so-called "close proximity sweep" which includes a common first subsweep that is concatenated with an orthogonal second subsweep. In other words, when any two vibrators are close to one another, their starts could be synchronized so that for the first subsweep portion of the sweep the two vibrators act as a source array and for the second subsweep the two vibrators use separable orthogonal subsweeps. According to an embodiment, this close proximity sweep can further be performed with more than two vibrators as long as, depending upon operating conditions, all of the vibrators are within a desired range of each other.

[0049] According to an embodiment, it can be desirable to implement a set of rules for how vibratory sources are to be used in a seismic survey. This can allow for a more automated survey. Prior to describing a set of rules for a seismic survey, a map 300 which illustrates a portion of a 3-D seismic survey is shown in Figure 4 for which the set of rules can be applied. In this example, section 302 of the survey has been designated as being of special interest. Based upon prior surveys and possibly other criterion, e.g., environmental concerns, a different sweep frequency range, sweep type, and/or sweep length is to be used by the sources operating in section 302 than for sources operating in other locations. Source lines 304 are shown vertically and receiver lines 306 are shown horizontally. For this example, it is assumed that continuous recording is used, and that the vibrators can sweep shortly after they reach their positions and have their baseplates down, e.g., in contact with the ground. Four vibrators 308, 310, 312 and 314 are shown operating at various locations within the map 300. For the source positions shown in this example, vibrator 308 is to be operated as an independent/orthogonal source. Vibrator 310 is operating in section 302 and will use a special sweep. Vibrators 312 and 314 are in relatively close proximity to each other with overlapping proximity of circles with radii 316 and 318 respectively.

[0050] According to an embodiment, a rule set can be implemented within each vibrator's sweep controller. In this example, each vibrator is equipped with a GPS receiver 104 (or other global satellite position system recorder) and some form of local communication equipment which allows for communication between vibrators and an acquisition management system. An example of a flowchart 400 which incorporates an algorithm and rule set is shown in Figure 5 and explained below.

[0051] According to an embodiment, the flowchart 400 can be implemented by one or more vibrators, e.g., vibrators 308, 310, 312 and/or 314, in performance of seismic vibratory sweeps. Instructions for performing the steps shown in the flowchart 400 can be stored in memory 102 and executed by the controller 108 in a vibrator. Initially, in step 402, a vibrator moves to a new shot point location. After arriving at the new shot point location, in step 404, the vibrator acquires location, time and proximity information with respect to other vibrators in the survey sector. In step 406, the vibrator then applies a rule set. The choice of rule set and/or final outcome of the rule set can affect how the vibrator performs the sweep. Factors that impact the rule set include information described above for the various embodiments. For example, relative location of vibrators, previous sweep information, environmental sensitivity of the survey area (or a portion of the survey area), terrain types or changes in terrain from previous surveys, lower frequency issues and the like can, at least in part, impact this decision.

[0052] Based on the outcome of the application of the rule set, a determination is made, in step 408, as to whether the vibrator will act as a single source or as a part of an array of sources. When the decision is for the vibrator to be a part of a source array, the vibrator selects source array sweep in step 410, performs sweep synchronization in step 412 and then performs the sweep in step 414. When the decision is for the vibrator to be a single source, the vibrator selects an orthogonal sweep in step 416, computes a sweep starting point in step 418 and then performs the sweep in 420. Upon completion of the sweep, the vibrator determines, or is informed, as to whether or not the survey is complete as shown in step 422. If the survey is complete, the process is finished. If the survey is not complete, the process can be repeated with the vibrator again moving to a new shot point location, as shown in step 402. This process can be iterated numerous times as desired.

[0053] According to another embodiment, it is possible to exploit properties of low frequency emissions by vibratory sources which are as follows: (1 ) that low frequency emissions create numerous surface waves that attenuate with distance; (2) that low frequencies are not attenuated as quickly with an increase of depth as are higher frequencies; and (3) that at long offsets there is a lower content of a high frequency portion of signal(s) as compared to the low frequency portion of the signal(s). Therefore, by generating the low frequency signals at the longer offsets, there will be less surface wave noise hitting the central part of the seismic survey that is centered over the subterranean region of interest that is to be imaged.

[0054] At the nearer offsets, techniques that are designed to remove surface wave noise, e.g., F-K filtering, will tend to also remove useful p-wave low frequency energy at the lower end. As one goes to longer offsets, the p-wave and surface waves are spaced farther apart and so the process is less prone to remove the useful p-wave low frequency energy when the noise is filtered out or otherwise mitigated. As conventional vibrators are not very efficient at generating very low frequencies, it can take a significant amount of sweep time to build up the low frequency content. From an efficiency standpoint it can be desirable to generate most of the low frequency content at the longer offsets and then generate more of the higher frequencies in a region which is centered around the subterranean target that is being imaged.

[0055] An example of a 3-D survey where the above described exploitation low frequency emission properties will now be described with respect to Figure 6. The 3-D seismic survey 500 shows an outer region 502 and an inner region (or central zone) 503 in which it can be desirable to generate signals that have a steady or increased high frequency content. The outer region 502 includes all of the shown survey area which is not within the inner region 503. Figure 6 also shows source lines 304 (or shot lines) and receiver lines 306 as well as a plurality of vibratory sources 504, 505 and 506.

[0056] According to an embodiment, a variety of signal options can be used. For a first option, pseudorandom signals can be used, where the GPS coordinates of the vibratory source can be used to select the sweep (and sweep table) to be performed optionally in conjunction with the proximity of another vibratory source. The GPS time can then be used to calculate a sweep table pointer to give the starting position within a sweep.

[0057] For a second option, swept sine waves can be used as in High Productivity Vibroseis Acquisition (HPVA), HPVA-V1 mode and/or slip sweep. For these uses of swept sine waves, the GPS coordinates of the vibratory source can be used to determine which sweep option to use. Additionally, if a vibrator operator pushes a sweep start button, the GPS time delays the sweep start until the appropriate slip sweep time window. For a third option, which is based on the second option, the proximity of another vibrator (or possibly plurality of vibrators) to determine whether to synchronize the sweep starting time so that the plurality of vibratory sources operate as either a source array or not as desired. For swept sine waves, the low frequency content of the sweep can be boosted by dwelling at the lower frequencies for longer periods of time such that the instantaneous sweep rate is lower as one traverses the lower frequency portion of the sweep.

[0058] Regarding the processing associated with the example for exploiting low frequency emissions by vibratory sources the following is proposed. According to an embodiment, sweeps in all regions that cover a same frequency range, e.g., 2- 100 Hz, can be used such that a same source wavelet can be used in later process. In this example, sweeps in zone 502 can have a larger low frequency content than sweeps in zone 503 which can have a more balanced frequency content or have a boosted high frequency content. While the example associated with Figure 6 only shows two zones 502 and 503, it is to be understood that more zones could be used as desired.

[0059] Considering Figures 7(a)-(c), it may be desirable to cover a same range of frequencies with there being a beginning frequency (Fb) and an ending frequency (Fe), where Fb = 2 Hz and Fe = 100 Hz. Note that other values can be used for Fb and Fe. Figure 7(a) shows a flat low boost or source spectral energy density (ESD). Figure 7(b) shows a high boost which tapers down and Figure 15(c) shows an initial low boost which increases. The ESD would correspond to for example, the Fast Fourier Transform (FFT) of the pilot signal that is used by the vibrator for control of the ground force output. Dotted line 602 represents the ESD level in dB of the pilot signal with a flat spectral output over the seep frequency range with graph 604 being a graph of the actual ESD.

[0060] According to an embodiment, in zone 502 of Figure 6 it can be desirable to use a sweep or source signal, which can be swept sine wave or pseudorandom, that has an ESD represented by graph 606 in Figure 15(b) where the ESD is weighted more to the lower frequencies, so that over the range 608 the ESD level is greater than the dB level represented by dotted line 602. In zone 503 of Figure 6, it can be desirable for the vibratory sources to have an output ESD similar to that shown in Figure 15(a) or Figure 15(c) where the high frequency range 610 is boosted.

[0061] According to an embodiment, separating and recombining data sets from seismic surveys can be performed. Prior to describing a method for such, an environment is first described. For this purely illustrative 3-D survey, data is acquired. The target spectrum is from 1 -100 Hz, the pilot signal type is pseudorandom and alternate pilot sweeps were used when more than one vibrator was in close proximity to one or more other vibrators. The acquisition method can be an acquisition method described in the above incorporated by reference US Patent 8,773,950. However, other acquisition methods can also be used, both here and in other embodiments. When the vibrators are more widely separated, the 1 - 100 Hz pilot sweeps can be pseudo-orthogonal. The alternate pilot sweeps cover the same 1 -100 Hz frequency band, but are constructed so that the component of the pilot signal that falls within, for example, the 1 -5 Hz frequency range is in phase while the 5-100 Hz components are designed to be pseudo-orthogonal. Alternatively, the pilot sweeps can be designed such that there is not an abrupt transition at 5 Hz, e.g., a different frequency could be used other than 5 Hz.

[0062] Operating in the above described environment, according to an embodiment a process 700 for separating the continuous or semi-continuous record into shot records is represented by the flowchart in Figure 8 and will now be described. In step 704, the continuous or semi-continuous recorded data 701 , source data set 702 and survey geometry information 703 is combined and stored in the processing system memory. Recorded data 701 contains the receiver (e.g., geophone) recordings, pilot signals, GPS time stamps and may also contain information associated with the recording system configuration, filters, geophone response, sample rate, etc. Source data 702 includes the measured source signals, e.g. ground force and/or vibrator accelerometer signals, GPS time stamps and GPS position information. Alternatively, for surveys equipped with source wide-bandwidth telemetry systems, source data 702 may be included in recorded data 701 .

[0063] In step 705, using the time stamp information, for example, the combined source data and receiver data sets are parsed into shot records using but not limited to, for example, methods outlined in U.S. Patent 8,773,950. Using the source position information, in step 706, the parsed records are separated into two groups with one group 707 which includes all of the shot records where the selected vibratory source operates independently and with another group 708 which includes all of the shot records where the selected vibratory sources, e.g., an array of vibratory sources, operate in tandem in phase at low frequencies with one or more nearby vibratory sources.

[0064] Following the process steps after there was a determination in step 706 that there was no array of vibratory sources, the first group 707 then is de-noised in step 710. De-noising can include the removal of high line pickup, burst noise, noise spikes and the like. The de-noised data can then optionally be processed to correct the receiver data by removing the receiver impulse response. In step 718, the received de-noised data from step 710 is received and then decoded/separated out the contribution to the receiver data due to a particular source. This can be performed, for example but not limited to, by using source separation matrices as described in U.S. Patent 8,773,950.

[0065] Following the process steps after there was a determination in step 706 that there was an array of vibratory sources, the second group 708 then is de-noised in step 709. De-noising can include the removal of high line pickup, burst noise, noise spikes and the like. The de-noised data can then optionally be processed to correct the receiver data by removing the receiver impulse response. In step 71 1 , the received de-noised data from step 709 is received and then the shot records corresponding to data acquired using a plurality of sources with synchronized low frequency content are separated again into two data sets, for example: group 712 containing 1 -5 Hz data (or other lower frequency ranges as desired) and group 713 containing 5-100 Hz data.

[0066] According to an embodiment, the separation, which can be a spectral separation, performed in step 71 1 can be in the form of low-cut and high cut filters or the separation can be performed using different correlation operators where one operator contains only the 1 -5 Hz component of the pilot sweep, while the other two correlation operators contain the 5-100 Hz pseudo-orthogonal sweep components for the plurality of sources in relatively close proximity to each other. In steps 714 and 715, the received de-noised data is decoded/separated out the contribution to the receiver data due to a particular source. This can be performed, for example but not limited to, by using source separation matrices as described in U.S. Patent 8,773,950. For the decode step 714, only the earth impulse response covering the range of 1 -5 Hz would be computed and for the decode step 715, only the earth impulse response covering the range of 5-100 Hz would be computed. The output from steps 714 and 715 are then recombined in step 716 to create a full bandwidth, e.g., a 1 -100 Hz impulse response.

[0067] According to an embodiment, it may be desirable to take the output of the recombination step 716 and regularize the recombined output so that the recombined output can be put into a format for merging with the decoded separated data from step 718. This regularizing of the recombined output is shown in step 717 and may include some form of spectral interpolation to address frequencies near the transition frequency of 5 Hz, or steps to remove or reduce other processing noise artifacts which are deemed to be undesirable.

[0068] In step 719, the final outputs from the single vibratory source steps and the array of vibratory source steps are merged. For example, the separated shot gathers that contain the earth impulse are merged into a common data set that then progresses to a follow noise removal step 721 . Noise removal step 721 may include various filtering methods to remove coherent noise, e.g., ground roll and airblast noise, by using, for example, F-K filtering. The output from step 721 then undergoes a model building step 723 that can be iterative in nature. In some association with step 723, normal move out (NMO) and static corrections are applied using an initial velocity model 722 that is iteratively adjusted as part of the velocity model building of step 723 to create and align first arrivals to a seismic datum. In step 724, the correct shot gathers are output/passed on to other image processing steps to generate an image (or images) for display associated with the seismic survey.

[0069] The terms "orthogonal" and "pseudo-orthogonal" have been used herein associated with sweeps and signals and will now be described in more detail for clarification purposes. In theory, it is preferable for to use sweeps whose signals were perfectly orthogonal to one another with zero cross-correlation properties. In practice, this is generally not realizable for sweeps of finite length where simultaneous or semi-simultaneous source emissions overlap. So pseudo- orthogonal sweeps that can be, for example, pseudo-random signals or swept sine wave signals that are encoded by some sort of phase, frequency or time coding scheme or by other coding schemes can be created with a property that they are only weakly correlated over a time interval of interest. The time interval for which the cross-correlation noise is minimized corresponds to the time after correlation when we would expect reflection data to be arriving from targets of interest. Therefore, the set of pseudo-orthogonal signals or sweeps can include orthogonal signals or sweeps.

[0070] According to an embodiment, a method for seismic prospecting will now be described with respect to Figure 9. Initially, at step 802, determining, based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; at step 804, actuating at least one vibratory source using the sweep to impart seismic waves into the ground.

[0071] The above method and others may be implemented in a computing system specifically configured to calculate the source separation matrix. An example of a representative computing system capable of carrying out operations in accordance with the exemplary embodiments is illustrated in FIG. 10. Hardware, firmware, software or a combination thereof may be used to perform the various steps and operations described herein.

[0072] The exemplary computing system 900 suitable for performing the activities described in the exemplary embodiments may include server 901 . Such a server 901 may include a central processor (CPU) 902 coupled to a random access memory (RAM) 904 and to a read-only memory (ROM) 906. The ROM 906 may also be other types of storage media to store programs, such as programmable ROM (PROM), erasable PROM (EPROM), etc. The processor 902 may communicate with other internal and external components through input/output (I/O) circuitry 908 and bussing 910, to provide control signals and the like. The processor 902 carries out a variety of functions as is known in the art, as dictated by software and/or firmware instructions.

[0073] The server 901 may also include one or more data storage devices, including a hard drive 912, CD-ROM drives 914, and other hardware capable of reading and/or storing information such as DVD, etc. In one embodiment, software for carrying out the above discussed steps may be stored and distributed on a CD- ROM 916, removable memory device 918 or other form of media capable of portably storing information. These storage media may be inserted into, and read by, devices such as the CD-ROM drive 914, the disk drive 912, etc. The server 901 may be coupled to a display 920, which may be any type of known display or presentation screen, such as LCD displays, LED displays, plasma display, cathode ray tubes (CRT), etc. A user input interface 922 is provided, including one or more user interface mechanisms such as a mouse, keyboard, microphone, touch pad, touch screen, voice-recognition system, etc.

[0074] The server 901 may be coupled to other computing devices, such as the landline and/or wireless terminals via a network. The server may be part of a larger network configuration as in a global area network (GAN) such as the Internet 924, which allows ultimate connection to the various landline and/or mobile client devices. The computing device may be implemented on a vehicle that performs a land seismic survey or as a portion of the vibrator or as an attachment to the vibrator, e.g., a vehicle which moves the vibrator.

[0075] The disclosed exemplary embodiments provide a system and a method for actuating sources asynchronously. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0076] Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0077] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

Claims

WHAT IS CLAIMED IS:

1 . A method for seismic prospecting, the method comprising:

determining (802), based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; and

actuating (804) at least one vibratory source using the sweep to impart seismic waves into the ground.

2. The method of claim 1 , wherein the ruleset is based at least in part on vibratory source location in a survey area, time and proximity to other vibratory sources.

3. The method of claim 1 , wherein the step of determining further comprises: determining if the at least one vibratory source is a single vibratory source or an array of vibratory sources.

4. The method of claim 3, wherein the set of rules determines a sweep table to be used for the at least one vibratory source.

5. The method of claim 1 , further comprising:

actuating a plurality of vibratory sources, based on the ruleset, such that the plurality of vibratory sources operates as a low frequency source array by having a portion of a low frequency content of the plurality of vibratory sources' emissions be synchronous and coherent.

6. The method of claim 3, wherein when a single vibratory source is determined to be used, the method further comprises:

selecting a pseudo-orthogonal sweep; and

computing a sweep starting pointer.

7. The method of claim 3, wherein when an array of vibratory sources is determined to be used, the method further comprises:

selecting a source array sweep; and

synchronizing sweeps for the source arrays.

8. The method of claim 1 , wherein the imparted seismic waves are at least one of pseudorandom signals, swept sine waves, or a combination of both pseudorandom signals and swept sine waves.

9. The method of claim 1 , wherein an output of the ruleset uses shot point location to select a spectral content of the at least one vibratory source emission.

10. The method of claim 1 , further comprising:

storing in memory a plurality of sweep tables for use, wherein a first sweep table is used when two or more vibratory sources operate simultaneously at greater than a predetermined distance and a second sweep table is used when the two or more vibratory sources operate together at less than a predetermined distance.

1 1 . A system for seismic prospecting, the system comprising:

a memory (102) configured to store a ruleset used to determine, based at least in part by a ruleset, a sweep to be emitted by at least one vibratory source; and at least one vibratory source (100) configured to actuate a sweep to impart seismic waves into the ground.

12. The system of claim 1 1 , wherein the ruleset is based at least in part on vibratory source location in a survey area, time and proximity to other vibratory sources.

13. The system of claim 1 1 , further comprising:

a process configured to determining if the at least one vibratory source is a single vibratory source or an array of vibratory sources.

14. The system of claim 13, wherein the set of rules determines a sweep table to be used for the at least one vibratory source.

15. The system of claim 1 1 , further comprising: the system configured to actuate a plurality of vibratory sources, based on the ruleset, such that the plurality of vibratory sources operate as a low frequency source array by having a portion of a low frequency content of the plurality of vibratory sources' emissions be synchronous and coherent.

16. The system of claim 13, wherein when a single vibratory source is determined to be used, a pseudo-orthogonal sweep is selected and a sweep starting pointer is computed.

17. The system of claim 13, wherein when an array of vibratory sources is determined to be used, a source array sweep is selected and sweeps for the source arrays are synchronized.

18. The system of claim 1 1 , wherein the imparted seismic waves are at least one of pseudorandom signals, swept sine waves, or a combination of both pseudorandom signals and swept sine waves.

19. The system of claim 1 1 , wherein an output of the ruleset uses shot point location to select a spectral content of the at least one vibratory source emission.

20. A computer readable medium including computer executable instructions, wherein the instructions, when executed by a processor, implement a method for seismic prospecting, the medium including instructions for: