Patent Publication Number: US-10761227-B2

Title: Wireless exploration seismic system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority as a continuation of U.S. patent application Ser. No. 14/328,270 entitled “WIRELESS EXPLORATION SEISMIC SYSTEM WIRELESS EXPLORATION SEISMIC SYSTEM”, which claims priority as a continuation of U.S. patent application Ser. No. 13/541,928 entitled “WIRELESS EXPLORATION SEISMIC SYSTEM” filed on Jul. 5, 2012, which claims priority as a continuation of U.S. patent application Ser. No. 12/982,416 entitled “WIRELESS EXPLORATION SEISMIC SYSTEM” filed on Dec. 30, 2010, now U.S. Pat. No. 8,238,199, which claims priority as a continuation-in-part to U.S. patent application Ser. No. 12/837,177 entitled “WIRELESS EXPLORATION SEISMIC SYSTEM,” filed on Jul. 15, 2010, now U.S. Pat. No. 8,238,197, which is a continuation of U.S. patent application Ser. No. 11/538,744 entitled “WIRELESS EXPLORATION SEISMIC SYSTEM,” filed on Oct. 4, 2006, now U.S. Pat. No. 7,773,457, which claims priority to U.S. Provisional Application No. 60/724,271 filed on Oct. 7, 2005, and U.S. Provisional Application No. 60/821,217 filed on Aug. 2, 2006, all of which are hereby incorporated by reference in their entirety as if set forth in full herein. 
    
    
     FIELD 
     The present disclosure relates to systems and methods seismic survey systems where the data signals from multiple sensors are transmitted by wireless means. In particular, the present disclosure relates to systems and methods that facilitate real time read out of a seismic survey system, even in the case of rapidly repeating vibrating energy sources, without backlogging of data or delay of the survey process. 
     BACKGROUND 
     Seismic surveys are often used by natural resource exploration companies and other entities to create images of subsurface geologic structure. These images are used to determine the optimum places to drill for oil and gas and to plan and monitor enhanced resource recovery programs among other applications. Seismic surveys may also be used in a variety of contexts outside of oil exploration such as, for example, locating subterranean water and planning road construction. 
     A seismic survey is normally conducted by placing an array of vibration sensors (accelerometers or velocity sensors called “geophones”) on the ground, typically in a line or in a grid of rectangular or other geometry. Vibrations are created either by explosives or a mechanical device such as a vibrating energy source or a weight drop. Multiple energy sources may be used for some surveys. The vibrations from the energy source propagate through the earth, taking various paths, refracting and reflecting from discontinuities in the subsurface, and are detected by the array of vibration sensors. Signals from the sensors are amplified and digitized, either by separate electronics or internally in the case of “digital” sensors. The survey might also be performed passively by recording natural vibrations in the earth. 
     The digital data from a multiplicity of sensors is eventually recorded on storage media, for example magnetic tape, or magnetic or optical disks, or other memory device, along with related information pertaining to the survey and the energy source. The energy source and/or the active sensors are relocated and the process continued until a multiplicity of seismic records is obtained to comprise a seismic survey. Data from the survey are processed on computers to create the desired information about subsurface geologic structure. 
     In general, as more sensors are used, placed closer together, and/or cover a wider area, the quality of the resulting image will improve. It has become common to use thousands of sensors in a seismic survey stretching over an area measured in square kilometers. Hundreds of kilometers of cables may be laid on the ground and used to connect these sensors. Large numbers of workers, motor vehicles, and helicopters are typically used to deploy and retrieve these cables. Exploration companies would generally prefer to conduct surveys with more sensors located closer together. However, additional sensors require even more cables and further raise the cost of the survey. Economic tradeoffs between the cost of the survey and the number of sensors generally demand compromises in the quality of the survey. 
     In addition to the logistic costs, cables create reliability problems. Besides normal wear-and-tear from handling, they are often damaged by animals, vehicles, lightning strikes, and other problems. Considerable field time is expended troubleshooting cable problems. The extra logistics effort also adds to the environmental impact of the survey, which, among other things, adds to the cost of a survey or eliminates surveys in some environmentally sensitive areas. As a result, wireless acquisition units have been developed to do away with the burdensome nature of cables in such a system. For instance, U.S. Pat. No. 7,773,457, which is hereby incorporated in its entirety by reference as if reproduced herein, describes a system for performing a seismic survey using wireless acquisition units. 
     When conducting a survey using wireless data acquisition units, it may be advantageous to perform a wireless data read out from such units in real time. As such, survey verification may occur prior to the conclusion of the survey, and the cost and time associated with manual data retrieval of such units may be reduced or eliminated. However, when employing a real time data read out, the bandwidth usage of the wireless communication between modules must be carefully considered so as not to slow the speed at which the survey is conducted. 
     In turn, a multiplexing regime may be imparted to the wireless modules in the seismic survey so that multiple units within range of one another may simultaneously broadcast and receive data. Examples of such multiplexing regimes include time division, frequency division, code division, etc. For example, U.S. Pat. No. 7,773,457 describes employing multiplexing techniques in a “bucket brigade” type system to perform real time read out of data. 
     SUMMARY 
     The present invention provides systems and methods for acquiring seismic data using a wireless network and a number of individual data acquisition modules that are configured to collect seismic data and forward the data to a central recording and control system. The invention thus enables both a cable-less layout of the data acquisition modules as well as a wireless read out of the seismic data. Using a cable-less layout of data acquisition modules is advantageous over conventional systems using cable communications for several reasons. For example, a cable-less system is not as susceptible to damage caused by wear and tear, animals, lightning strikes, etc. Additionally, a cable-less system could easily be placed across rivers, highways, or other obstacles, whereas it is difficult to implement a seismic survey system that uses cables in these areas. Moreover, cable based seismic surveys require a relatively large number of vehicles and manpower to transport and lay cables spanning several square kilometers. 
     The ability to capture the data wirelessly is potentially advantageous as well. For instance, when a single cable is damaged, all data from remote data acquisition modules located on the distant end of the damaged cable may be lost. This type of problem is not present in systems where the seismic data is transmitted wirelessly. In particular, rerouting the data is much more feasible using a wireless network, which potentially provides for a more reliable system. 
     Certain proposed cable-less and/or wireless seismic acquisition systems have required manual read outs or have otherwise involved a read out procedure that resulted in delaying the seismic survey process. It has been recognized that it is important to provide for capture of data from a wireless array in a manner that avoids backlogging of data or delaying the survey process. That is, the system can operate much more efficiently and inexpensively if the data capture can be accomplished without delaying the survey process (e.g., without delaying subsequent seismic events). Conventionally, the survey process involves placing an array of vibration sensors (accelerometers or velocity sensors called geophones) on the ground, typically covering several square kilometers. Then a vibration/seismic event may be propagated through the earth and detected by these sensors. Signals from the sensors are captured and analyzed by a processing unit. Most commonly, seismic events are created by one of two methods. First, explosives may be used to generate vibrations in the earth. Alternatively, a vibrating energy source vehicle may be used. A vibrating energy source vehicle is a vehicle designed for creating vibrations in the ground. Typically, a vibrating energy source vehicle will create a vibration in one location for a short period of time (e.g., fifteen or twenty seconds). Then, the vehicle may move to another location nearby and immediately begin generating another vibration. There may only be a few seconds between each vibration event. This process may continue for several hours until the desired amount of seismic data is acquired. The survey process is very expensive in terms of the required amount of equipment and manpower, so any delays in the measurement process are extremely costly. Thus, it will be appreciated that the seismic data is preferably either stored locally for the entire survey, or otherwise transferred, for example, to a central storage system, at a rate sufficient to accommodate the operation of the vibration source without causing a delay. 
     In addition to the items noted above, there are a number of attributes that have been recognized as being useful to provide a commercially desirable wireless seismic survey system. These attributes include the following: 
     1. The system cost should be approximate to or less than that of a wired system. 
     2. The system should perform in uneven terrain and around or over obstacles, across an area measured in square kilometers. 
     3. The system should be usable worldwide without complex radio licensing issues. 
     4. Power consumption should be low enough that handling and replacing batteries does not create undue logistic problems. 
     5. Seismic data should be made available for inspection in near real-time to ensure quality and avoid the need to repeat some or all of the seismic data acquisition. 
     6. The system should be capable of scaling up to thousands of sensors. 
     7. The system should meet the technical performance specifications of contemporary wired systems. 
     Various aspects of the present invention involve a wireless transfer protocol to enable the capture of seismic data from a seismic array without delaying the seismic survey. In this regard, it will be noted that the volume of data generated by an array of vibration sensors may be significant. For instance, if each vibration sensor acquires 500 samples per second with a resolution of 24 bits per sample, the sample size for each sensor is 12,000 bits per second. For a 20 second vibration (i.e. seismic event) the resulting sample size is 240K bits per sensor. It is not uncommon for a plurality of sensors to be aligned into lines or strings that contain a hundred or more sensors. Further, multiple lines or strings may be disposed in parallel to define an array of sensors that cover a desired geographic area. For a seismic survey that utilizes 1000 sensors, 240,000,000 bits of data may be generated for each seismic event. Accordingly, the wireless transfer of all the data from the sensors to a central recording system(s) at a rate sufficient to keep up with the seismic events can be problematic due to limited bandwidth. In one implementation of the present invention, individual data acquisition modules transmit seismic data to a neighboring or near neighboring module, which, in turn, relays the received seismic data, together with its own data, on to a further module. This process of data forwarding continues until the data reaches either a line tap or a central recording and control system. This arrangement is desirable for several reasons. First, since the data acquisition modules are only transmitting data to nearby modules, rather than directly to a central location, relatively low power transmissions can be utilized. The savings in power allows the modules to operate longer without battery replacement, and/or to operate using batteries with a smaller capacity. Additionally, low power transmissions reach a smaller area, which allows fewer channels to be utilized (or available channels to accommodate a larger number of acquisition modules) because they can be reused in different areas of the array without interference. Additionally, the use of lower power signals may enhance module location information using signal strength based techniques. It will be appreciated, however, that it is problematic to reconcile the desire for such serial data transfer with the desired read out rates as described above. These potentially conflicting objectives are addressed by the invention as set forth herein. 
     In accordance with one aspect of the present invention, a seismic array is provided that can wirelessly read out an array using serial data transfer at a rate sufficient to avoid delaying operation of a vibrating energy source. As noted above, a vibrating energy source typically vibrates for a few seconds to define a seismic event, then moves to another location and immediately starts vibrating again. This cycle may be repeated in only a matter of seconds, e.g., 20-30 seconds. It is desirable to read out the resulting seismic data to a base station or the like in serial fashion such that each module needs only to communicate with a neighboring or near neighboring module, and low transmission powers can be used. However, serial data transfer generally involves increasing data transfer rate requirements, as a function of the length of the serial data transfer path, for a given overall array read out time. 
     The present aspect of the invention provides a method and apparatus (“utility”) that involves providing an array for obtaining seismic data corresponding to operation of a repeating vibrating energy source. A read out mechanism is provided in connection with the array that defines at least one serial data transfer path for reading out the seismic data. The read out mechanism is operated to read out the array at a rate sufficient to avoid delaying operation of the vibrating energy source. For example, the read out mechanism may be operative to read out a serial data transfer path involving multiple modules (e.g., more than about 10 modules) in no more than about 20 seconds. 
     In accordance with another aspect of the present invention, more than one seismic data acquisition module in a given serial data transfer path can transmit data simultaneously. As noted above, serial data transfer paths are desirable but can result in increased data transfer rate requirements. In order to address this potential issue, more than one module in a serial data transfer path can transmit at a time, for example, by employing an appropriate multiplexing mechanism to avoid interference. Thus, a utility in accordance with the present aspect of the invention involves: providing an array including at least one serial data transfer path, where data is serially transferred between multiple modules in route to a base station or other collection point; and operating the array such that at least two of the multiple modules on the serial data transfer path transmit concurrently. In this manner, higher duty cycles can be achieved for individual transmitters, resulting in enhanced read out efficiency. 
     In accordance with yet another aspect of the present invention, different modules on a given serial data transmission pathway of a seismic array use different transmission frequencies. Again, serial data transmission pathways are desirable but can raise certain issues relating to array read out rates. These issues can be addressed by using multiple frequencies in a single serial data transfer path so as to, for example, allow for frequency division multiplexing. A corresponding utility in accordance with the present aspect of the invention involves: providing an array including at least one serial data transfer path, having multiple modules; and operating the array such that a first module on the serial data transfer path transmits at a first frequency and a second module on the serial data transfer paths transmits at a second frequency different than the first frequency. For example, different frequencies can be assigned to all modules within receiving range of one another that transmit concurrently. 
     In accordance with another aspect of the present invention, multiple multiplexing mechanisms are implemented in connection with a serial data transfer path of a seismic array. For example, each multiplexing mechanism may allow for coordinated operation of the transmitters of the serial data transfer path to enable improved read out rates without undue interference. In one implementation, both time division multiplexing and frequency division multiplexing are used in connection with a serial data transfer path. For example, the time division multiplexing may be implemented such that only one-half (or one-third, one-fourth, etc.) of the transmitters in the serial data transfer path are transmitting in any defined time interval. Those transmitters that transmit at the same time (or at least those transmitting at the same time and within receiving range of a receiver in the pathway) can be assigned different frequency channels and/or can utilize another multiplexing mechanism such as code division multiplexing. Where multiple serial pathways are present in the array, such multiplexing can also be implemented so as to avoid interpathway interference. 
     In order to facilitate the capture of seismic data from an array of seismic data acquisition modules while avoiding delays in the seismic survey process, it may be desirable to provide synchronized transmission of seismic data from a number of acquisition modules that utilize a common wireless serial data transmission path for reporting seismic data. That is, along a serial data transmission path defined by a number of acquisition modules, a first subset of the acquisition modules may be operative to simultaneously transmit seismic data to a second subset of the acquisition modules. Such an arrangement may allow for simultaneous transfer of seismic data from a number of upstream acquisition modules to a number of downstream acquisition modules that are located nearer to a data collection point (e.g., line tap or base station). For instance, every other acquisition module in a serial data transfer path may be operative to transmit seismic data to an adjacent downstream acquisition module during a first time interval. Likewise, when the downstream acquisition modules receive data from an adjacent upstream module, the downstream modules may transmit the received data and/or additional seismic data further downstream during a second interval. Accordingly, by repeating the process multiple sets of seismic data may be simultaneously transferred step-by-step from a plurality of acquisition modules to one or more base stations and/or a central recording and control system. 
     In order to effect such simultaneous transfer of seismic data from a plurality of acquisition modules, one aspect of the present invention provides a utility for transmitting seismic data. The utility involves disposing multiple seismic data acquisition modules in an array where each acquisition module is operative to wirelessly communicate with at least one other acquisition module in the array. Accordingly, such communication between the acquisition modules allows for defining a data transmission path through the array. A base station is provided that is operative to receive seismic data from at least one acquisition module in the array. In this regard, the base station is in wireless communication with the data transmission path. In order to transmit seismic data from the plurality of acquisition modules, the utility further involves first transmitting first seismic data from at least a first module of a first subset of the acquisition modules to at least a first module of a second subset of the acquisition modules during a first transmission period. After the first transmission period, the first seismic data may be transmitted from the first module of the second subset of acquisition modules to a second module of the first subset of acquisition modules during a second transmission period. In this regard, the first set of seismic data may be relayed by the data transmission path towards the base station. 
     In order to transmit the data towards the base station, during each of the transmitting steps, individual acquisition modules may transmit seismic data to another seismic data acquisition module that is located nearer to the base station. In this regard, the data may be relayed from remotely located acquisition modules to acquisition modules that are located nearer to the base station. Further, one or more acquisition modules located within the transmission range of the base station may transmit the seismic data to the base station. 
     In addition to re-transmitting the first seismic data received from the first module of the first subset, the first module of the second subset may be operative to append additional seismic data (i.e., generated by the first module of the second subset) to the second module of the first subset. In this regard, during each transmitting step, additional data may be appended to the seismic data received from a remotely located data acquisition module. Accordingly, the first and second transmitting steps between the first and second subsets of acquisition modules may be repeated for additional modules of each subset until all seismic data from the plurality of acquisition modules is received by the base station. 
     In any arrangement, each seismic data acquisition module is operative to generate seismic data in response to a seismic event. In the case of a vibrating energy source, the event may last several seconds (e.g., 20 seconds). Accordingly, it may be desirable to begin data transmission prior to the end of the seismic event. That is, initiating data transmission before the seismic event ends may significantly reduce the time required to complete the transmission of the entire data set collected for the seismic event. Such transmission may be completed substantially in real-time or prior to the initiation of a second seismic event. In order to effect such transmission during the seismic event, each acquisition module may be operative to packetize seismic data as it is generated. Resulting packets (e.g., of a predetermined data size or temporal period) may include information identifying the acquisition module that generated the data as well as the acquisition time for the packet. The packet may then be relayed to the base station (e.g., via one or more relays), to a central control and storage system or another location where the packets may be stored and/or reassembled. That is, multiple packets from individual seismic acquisition modules for a single seismic event may be reassembled into a single data file at a location removed from the acquisition modules. 
     In one implementation, in order to facilitate the simultaneous transfer from the first subset of acquisition modules to the second subset of acquisition modules, each of the plurality of seismic data acquisition modules is assigned a transmission frequency for use in transmitting the seismic data. Further, due to the potentially large number of acquisition modules forming a single serial data transmission path, it may be desirable and/or necessary to assign first and second seismic data acquisition modules within a serial transmission path a common transmission frequency. That is, due to limited available transmission frequencies, it may be useful to reuse one or more frequencies. The transmission power and/or physical spacing of the modules may be selected so as to prevent interference between modules having a common transmission frequency. Moreover, when a common frequency is reused in a single serial data transmission path for a common transmission period, it may be desirable that the modules utilizing the common frequency are separated by a distance that is greater than the transmission range of the modules. It will be appreciated that modules utilizing a common transmission frequency may further be arranged such that they transmit seismic data during temporally distinct transmission periods or so that they utilize a further multiplexing mechanism, such as code division multiplexing, to reduce or avoid interference. The spacing and/or transmission power of the acquisition modules may also depend on transmission frequencies in one or more adjacent transmission paths utilized to transmit seismic data from additional arrays. 
     According to another aspect of the invention, a utility is provided for enhancing a data transfer rate between a plurality of seismic data acquisition modules defining an array and a base station that is operative to gather seismic data from the array by coordinating the operation of multiple modules that communicate directly with the base station. The utility involves disposing a plurality of seismic data acquisition modules in a physical path e.g., a line of modules. In this regard, each seismic data acquisition module in the physical path is operative to wirelessly communicate with at least one other seismic data acquisition module in the path such that the data is serially transferred from remote modules to modules located nearer to the base station. First and second serial data transfer paths are defined through the physical path. These first and second (or additional) data transfer paths may be formed by alternate or other pattern of acquisition modules along the length of the physical path. Accordingly, the first and second data transfer paths are each operative to relay data for the modules that form each path, and the base station is preferably operative to alternately receive transmission of seismic data from the first and second transmission paths. In this regard, the base station may be operative to substantially continually receive transmissions of seismic data from the first and second transmission paths. For example, the base station may be operative to alternately receive transmissions from the first and second paths so that the base station receiver has a substantially 100% duty cycle. 
     According to another aspect of the invention, a utility is provided for relaying seismic data through a series of acquisition modules where a given module uses different frequencies for receiving and transmitting data. The utility utilizes a plurality of seismic data acquisition modules that are disposed in a series and are operative to wirelessly communicate with at least one upstream acquisition module and at least one downstream acquisition module for the purposes of relaying seismic data therebetween. In this regard, the seismic data acquisition modules are typically operative to receive upstream seismic data from at least one upstream seismic data acquisition module and transmit the upstream seismic data to a downstream seismic data acquisition module. Further, to effect transmission of multiple sets of upstream data to multiple sets of downstream acquisition modules, it may be useful to assign different transmission and receiving frequencies to each of the data acquisition modules. Typically, this will require that any individual data acquisition module be operative to receive upstream data on a first transmission frequency and provide the upstream data, as well as any data generated by the receiving acquisition module, to a downstream acquisition module on a second transmission frequency. 
     Accordingly, each individual acquisition module may be tuned to a receiving frequency associated with an upstream acquisition module and tuned to a transmitting frequency that corresponds to receiving frequency of a downstream acquisition module. A single antenna or multiple antennae may be used in this regard. The acquisition module may be operative to receive transmissions from an immediately adjacent upstream acquisition module and provide data to an immediately adjacent downstream acquisition module. In another arrangement, the acquisition module may be operative to receive transmissions from a non-adjacent upstream acquisition module and transmit to a non-adjacent downstream acquisition module. In this regard, receptions and transmissions may skip adjacent acquisition modules such that first and second serial data transmission paths are interleaved within a physical path of seismic acquisition modules. 
     In accordance with another aspect of the invention, at least some of the modules are configured to be self-locating. In order for the data captured by the sensors to be processed, the geographic location of each sensor must be determined. Rather than manually measuring the location of thousands of sensors as has been previously done, the present invention may use various methods to determine the location of at least some of the modules automatically. For example, in one embodiment, each module may include a Global Positioning System (GPS) receiver. The modules may use the receiver to self-locate, and further may wirelessly transfer the location data to a central recording system. 
     In another embodiment, only some or none of the modules or base stations may be equipped with GPS receivers. The system may advantageously use RF technology to determine the location of at least some of the modules. For example, a module or base station may propagate a signal from its transmitter to several other remote modules. The other modules or base station(s) may individually record the time that the signal was received from the first module or base station and/or the strength of the signal. Using the time the signal was received at each remote module, the direction from which the signal was received, the signal strength or other parameters, the location of the module may be determined. Thus, if the location of one or more modules or base stations is known, the location of the remote module propagating the signal can be determined. It will be appreciated that this aspect is not limited to using one particular RF based method to determine the location of a remote module. A person skilled in the art will recognize that there are a number of methods that utilize RF technology that may be employed as will be discussed in more detail below. Moreover, the location of the module may be determined based on signals received at the module from other modules or base stations, or the location of the module may be determined based on signals transmitted from the module to other modules or base stations (e.g., the technology may be module-based or external). 
     In yet another embodiment, RF methods of location may be used in combination with GPS methods. For example, a fraction of the modules and/or base station may include GPS receivers. Since GPS methods give an accurate location, these modules or base stations may be used as reference locations. Then, the reference locations may be used to calculate the actual location of other modules that are not equipped with GPS receivers using the relative location calculated using RF based methods. For example, only the base stations (or a subset thereof) may include GPS receivers. In another example, all modules may include GPS receivers. Then, both RF based location methods and GPS methods may be used in tandem to ensure an accurate location measurement. 
     An advantageous feature of the present invention is the capability to record the location of the remote modules at a central recording and processing system in near real-time. That is, it is not necessary to travel to each module to extract the location data. Instead, the remote modules, or other location platforms, may be configured to wirelessly transmit the location data back to a central control and recording system. The location data may be transferred across the same network used to transfer the seismic data, or a separate network may be utilized. Moreover, different frequencies, powers, or other signal parameters may be used for the location signals (in relation to the seismic data transfer signals). It will be appreciated that the location of the modules need not be resolved prior to the initiation of seismic data acquisition. Rather, the location can be resolved contemporaneously with or subsequent to seismic data acquisition. 
     In accordance with a still further aspect of the present invention, module self-locating technologies may be employed that take advantage of the wireless seismic array context to determine location. As noted above, it has been recognized that it is important to provide for the automatic determination of location of the remote data acquisition modules, for example, using RF location techniques, embedded GPS, a combination of the two, or other suitable methods. Generally, determining the precise location of potentially thousands of data acquisition modules is a costly and time-consuming task. First, workers generally travel to each data acquisition module and determine the location using a GPS receiver or a similar device. This requires first that workers know the general location of each module so that it can be found. Once the module is found, it often takes a considerable amount of time for the GPS receiver to provide an accurate location measurement. The location of each device must be entered into a computer system where it will be used when the data is processed. The time spent locating the device, waiting for GPS data, and storing the location data can be substantial. Furthermore, this process may be repeated for thousands of sensors. Therefore, a system that provides for the automated location of the data acquisition modules substantially without contemporaneous human interaction is advantageous. The present invention provides for the automatic locating of at least some of the modules, thus reducing set-up costs. Any suitable self-locating technology can be used in this regard, including GPS location systems built into the modules and capable of wirelessly reporting location. 
     However, it has been recognized that the wireless nature of the acquisition modules provides additional self-location possibilities. In particular, a number of RF-based location technologies have been developed in connection with wireless telephony and mesh networks. Generally, these technologies involve multilateration (such as triangulation) algorithms using ranging and/or directionality information derived from multiple signals, such as time difference of arrival (TDOA), angle of arrival (AOA), signal strength, etc. 
     In particular, it has been recognized that these technologies can be adapted to the present wireless data acquisition context. The existing technologies are based on a paradigm involving many mobile handsets and a smaller number of fixed network structures such as wireless network base stations. The goal of these systems is generally to locate the mobile handsets and the locations of the fixed network structures are known. Oftentimes, however, the handsets have limited contact with the fixed structures due to distance, buildings or other obstructions and local topology, resulting in the inability to accurately and timely locate the handsets. Additionally, as the handsets are mobile, the location signals must generally be obtained substantially simultaneously. As a result, in many cases, handsets cannot be located using such technologies or the location is subject to large uncertainties. 
     This would seem to make such technologies unattractive for locating acquisition modules in the present context, particularly given the accuracy required. However, a number of factors render these technologies viable in this context if properly adapted. First, many RF transmitters can be made available in a close proximity to an acquisition module to be located, thereby allowing for high spatial resolution multilateration algorithms. In addition, if desired, a number of these RF transmitters can be accurately located using GPS or other technologies and can thereby serve as fiducial references to anchor the multilateration calculations. Moreover, the acquisition modules are stationary, allowing for execution of statistical processing over time to reduce uncertainties in the multilateration techniques. The fiducial references can also be located to ensure uninterrupted communication of the RF location signals. It will be appreciated that the location calculations can be performed at the acquisition modules (e.g., based on dedicated location signals or other signals received from multiple transmitters) or at another location (e.g., based on dedicated location signals or other signals transmitted from a module and received at multiple receivers). 
     Another aspect of the present invention involves a utility for designing a wireless seismic survey using parameters that are unique to wireless seismic survey systems. For example, the layout of the remote data acquisition modules is not constrained by a physical connection between each module, as is the case in a conventional cable-based system. This may enable the system designer to have more freedom when choosing the layout of the modules. For instance, there may be a need to place the modules in a configuration so as to avoid an obstruction. Additionally, parameters relating to the characteristics of wireless transfer may be used to design the wireless seismic survey (e.g., transmit power, number of channels, bandwidth, etc.). As an example, the transmit power and/or the distance between the modules may be increased or decreased to meet the particular survey&#39;s objectives. Furthermore, the number of wireless channels required to transfer the seismic data may depend on the spacing of the remote data acquisition modules. Moreover, the number and spacing (density) of acquisition modules may differ, in relation to conventional wired arrays, due to economies associated with the wireless implementation that affect the economic tradeoffs between the cost of the survey and number of sensors. Accordingly, the utility according to the present aspect of the invention involves: obtaining parameter information regarding potential array configurations, wherein the array configurations are a function of the characteristics of wireless data transfer systems; obtaining survey objective information regarding a seismic survey under consideration; and determining an array configuration based on the parameter information and survey objective information. 
     Yet another aspect of the present invention involves receiving seismic data from a rapid read out (e.g., read out between seismic events) wireless seismic survey system and processing the data into a form that is useful for analyzing the characteristics of one or more subsurface geologic structures. The receiving step may include, for example, transferring the seismic data to a computing system that is capable of processing the data. The processing step may include a number of methods for manipulating the seismic data (e.g., filtering, summing, synchronizing, displaying, normal move out, executing a common mid-point or other gather etc.). It will be appreciated that this may further involve particular processing unique to the wireless context such as deinterleaving of data from the different modules and accounting for unique wireless array geometry. Additionally or alternatively, the processing step may include interpreting the seismic data to identify the characteristics of one or more subsurface geologic structures. 
     It has been recognized that a dynamic multiplexing technique, wherein the multiplexing signature used to multiplex a signal is varied at different time periods, may be provided for use in systems such as those described in U.S. Pat. No. 7,775,457. For example, when using frequency division multiplexing, regulatory bodies such as the Federal Communication Commission in the United States may enforce regulations requiring that no single frequency may be used by a module for longer than a set duration. As such, a dynamic multiplexing technique may allow for operation in such cases where static multiplexing is not feasible. When employing such a dynamic multiplexing technique, a random or pseudo-random multiplexing regime may be employed wherein each unit uses a different random multiplexing signature during successive transmission periods. However, with multiple units using a random or pseudo-random multiplexing signature, two units may simultaneously attempt to use the same multiplexing signature to transmit data. In such instances, a data collision may occur and the units may each be required to retransmit the data which was subject to the collision. These retransmissions may reduce the bandwidth of the system, which may slow the survey. This may add additional time and cost to the survey, and is thus preferably avoided. For example, in the case of two units operating within a common collision domain, it has been found that the bandwidth of the system may decrease by between 3%-5%. This may add additional time and cost to the survey, and is thus preferably avoided. As the number of units within a collision domain increases, the problem is exacerbated and the bandwidth can deteriorate much more than the amount listed above. 
     Accordingly, another aspect includes a method for use in seismic data acquisition. The method includes disposing, in series, a plurality of seismic data acquisition modules that are operative to wirelessly communicate acquired seismic data. The acquisition modules define a wireless serial data transfer path for relaying data from upstream acquisition modules to downstream acquisition modules and a data collection unit. The method further includes assigning a first acquisition module in said serial data transfer path a first multiplexing signature sequence. In successive transmission periods, the first acquisition module advances sequentially through a first plurality of multiplexing signatures of the first multiplexing sequence. The method further includes assigning a second acquisition module in said serial data transfer path a second multiplexing signature sequence. In successive transmission periods, the second acquisition module advances sequentially through a second plurality of multiplexing signatures of the second multiplexing sequence. 
     A number of feature refinements and additional features are applicable to this aspect. These feature refinements and additional features may be used individually or in any combination. As such, each of the following features that will be discussed may be, but are not required to be, used with any other feature or combination of features of this aspect. 
     For example, in one embodiment, the method may further include first transmitting, using a first multiplexing signature of the first plurality of multiplexing signatures, seismic data from the first acquisition module to at least one downstream acquisition module and second transmitting, using a second multiplexing signature from the second plurality of multiplexing signatures, seismic data from the second acquisition module to at least one downstream acquisition module. 
     In an embodiment, the first multiplexing signature sequence and the second multiplexing signature sequence may be a random sequence of multiplexing signatures. In turn, the method may further include detecting when the first multiplexing signature used in the first transmitting is the same as the second multiplexing signature used in the second transmitting. The method may also include first retransmitting, using the next multiplexing signature from the first multiplexing signature sequence, seismic data from the first acquisition module to at least one downstream acquisition module and second retransmitting, using the next multiplexing signature from the second multiplexing sequence, seismic data from the second acquisition module to at least one downstream acquisition module. 
     In one embodiment, the first multiplexing signature may be different than the second multiplexing signature. Additionally, at least a portion of the first transmitting and at least a portion of the second transmitting may occur during a common transmission period. In an embodiment, the first multiplexing signature sequence and the second multiplexing signature sequence may be the same pseudo-random sequence of multiplexing signatures. Furthermore, the first acquisition module and the second acquisition module may operate out of phase with respect to the shared pseudo-random sequence of multiplexing signatures. In one embodiment, the multiplexing signatures comprise a plurality of different frequencies. 
     In yet another embodiment, the method includes assigning the first multiplexing signature sequence to a third data acquisition module. The first and third acquisition modules may be positioned in the serial transmission path at first and second locations. A distance between the first and second locations may be greater than a transmission range of the first and third acquisition modules. 
     Another aspect includes a method for use in seismic data acquisition. The method includes disposing a plurality of seismic data acquisition modules in an array. Each seismic data acquisition module is operative to wirelessly communicate with at least one other seismic data acquisition module in said array to define at least a first serial data transmission path. Additionally, the method includes assigning each of said plurality of seismic data acquisition modules in said first serial data transmission path a multiplexing signature sequence for use in transmitting said seismic data. At least one multiplexing signature is assigned to first and second seismic data acquisition modules in said first serial data transmission path during a first common transmission period. The method also includes, during the first common transmission period, transmitting seismic data from said first and second seismic data acquisition units using said at least one multiplexing signature. Also, the method includes advancing the first and second seismic data acquisition modules in the multiplexing signature sequence such that the first and second seismic data acquisition modules are assigned at least one different multiplexing signature during a second common transmission period. 
     A number of feature refinements and additional features are applicable to this aspect. These feature refinements and additional features may be used individually or in any combination. As such, each of the following features that will be discussed may be, but are not required to be, used with any other feature or combination of features of this aspect. 
     For instance, in one embodiment the multiplexing signature sequence may be a random sequence of multiplexing signatures. Additionally, the method may also include detecting a data collision between the first and second seismic data acquisition unit during the first common transmission period and retransmitting the seismic data during the second common transmission period using the at least one different multiplexing signature. 
     In an additional embodiment, the multiplexing signature sequence may be a pseudo-random sequence. The multiplexing signature sequence for the first seismic data module and the second seismic data module may be the same. As such, the first seismic data module and the second seismic data module may be out of phase with respect to the multiplexing signature sequence. 
     Yet another aspect includes a system for use in seismic data acquisition. The system includes a plurality of seismic data acquisition modules, disposed in series, that are operable to wirelessly communicate acquired seismic data. The acquisition modules define a wireless serial data transfer path for relaying data from upstream acquisition modules to downstream acquisition modules and a data collection unit. The system further includes a first acquisition module in said serial data transfer path for transmitting seismic data using a first multiplexing signature from a first multiplexing signature sequence during a first transmission period. The first acquisition module advances through the first multiplexing signature sequence in successive transmission periods. Additionally, the system includes a second acquisition module in the serial data transfer path for transmitting seismic data using a second multiplexing signature from a second multiplexing signature sequence during the first transmission period. The second acquisition module advances through said second multiplexing signature sequence in successive transmission periods. The first multiplexing signature and said second multiplexing signature are different during the first transmission period. 
     A number of feature refinements and additional features are applicable to this aspect. These feature refinements and additional features may be used individually or in any combination. As such, each of the following features that will be discussed may be, but are not required to be, used with any other feature or combination of features of this aspect. 
     In one embodiment, the first multiplexing signature sequence and said second multiplexing signature sequence may be a common pseudo-random sequence of a plurality of frequencies. The first acquisition module and said second acquisition module may be out of phase with respect to the common pseudo-random sequence. 
     In another embodiment, the system may include a third acquisition module in said serial data transfer path for transmitting seismic data using the first multiplexing signature from the first multiplexing signature sequence during the first transmission period. The third acquisition module may advance through said first multiplexing signature sequence in successive transmission periods. Additionally, the first acquisition module and third acquisition module may be at a first location and a second location. The first location and second location may be separated by a distance greater than the transmission range of the first and third acquisition module. In this regard, the first and third module may be in phase with respect to the common pseudo-random sequence. 
     Furthermore, a number of feature refinements and additional features are applicable to any of the foregoing aspects. These feature refinements and additional features may be used individually or in any combination. As such, each of the following features may be, but are not required to be, used with any other feature or combination of features of the foregoing aspects. 
     For example, the multiplexing sequence for the one or more modules described above may reside in non-volatile storage at each unit. That is, the multiplexing sequence may be stored locally in a memory of the module. Alternatively or additionally, the multiplexing sequence may be transmitted among units (e.g., from a base unit to individual units disposed in the array). In this regard, the multiplexing sequence may be updated or modified during a survey. The transmitted sequence may in turn be stored in memory once received at the module. 
     In addition, the modules may comprise a seismic array. The seismic array may be of any appropriate configuration. For example, the modules of any of the foregoing aspects may be arranged in lines to communicate to a base station. The modules may transmit to an adjacent module in the line or the modules may transmit to a non-adjacent module. As such, each module may communicate sequentially down the line to the base station or each module may jump adjacent modules and transmit to non-adjacent downstream modules. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and further advantages thereof, reference is now made to the following detailed description taken in conjunction with the drawings, which are briefly described below. 
         FIG. 1  shows a typical layout of sensors and cables in a wired seismic system. 
         FIG. 2  is a block diagram of the remote data acquisition and relay module in accordance with an embodiment of the present invention. 
         FIG. 3  is a typical layout of modules and sensors in the wireless system in accordance with an embodiment of the present invention. 
         FIG. 4  is a detailed drawing of a portion of the wireless system use to discuss frequency allocation and operation in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates a system for automatically determining the location of data acquisition modules. 
         FIGS. 6A-6C  illustrate the operation of various technologies that can be used to automatically locate the data acquisition modules. 
         FIG. 7  illustrates a layout of modules in the wireless seismic survey system in accordance with an embodiment of the present invention. 
         FIG. 8  illustrates an alternative layout of the seismic array and an associated read out protocol in accordance with the present invention. 
         FIGS. 9A-9C  illustrate certain self-locating structure and functionality in accordance with the present invention. 
         FIG. 10  is a flowchart illustrating a process for designing a wireless seismic array in accordance with the present invention. 
         FIG. 11  is a flowchart illustrating a process for processing data from a wireless seismic in accordance with the present invention. 
         FIGS. 12A-C  illustrate a process for transferring data along a serial data transfer path of a wireless seismic array in accordance with the present invention. 
         FIGS. 13A and 13B  are schematic view of a first and a second wireless acquisition unit pair employing frequency division multiplexing to transmit seismic data. 
         FIGS. 14-17  are charts graphically depicting the respective relative portion of a first and second acquisition unit pair at different transmission periods. 
         FIG. 18  is a flow chart depicting an embodiment of a process for avoiding data collisions. 
         FIG. 19  is a flow chart depicting another embodiment of a process for avoiding data collisions. 
         FIG. 20  is a schematic view of another embodiment of a wireless array. 
         FIG. 21  is a schematic view of a portion of an embodiment of a wireless array shown in three different instances of operation. 
         FIG. 22  is a schematic view of yet another embodiment of a wireless array. 
         FIG. 23  is a schematic view of a wireless array shown at various instances of operation. 
         FIG. 24  is a schematic view of still another embodiment of a wireless array. 
     
    
    
     DETAILED DESCRIPTION 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that it is not intended to limit the invention to the particular form disclosed, but rather, the invention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the claims. 
     In certain implementations as described below, the present invention combines the data acquisition function and radio relay function into a single module, thus solving a number of problems. Each module typically relays the data to another module en route to the central control and recording system. Some of the advantages of one embodiment of the present invention include:
         (a) Because the space between modules is small, typically no more than about 50 meters, relatively low power radios can be used to relay the information to the next module. The savings in power allows the modules to operate for several days on flashlight batteries or even longer on solar cells. Additionally, batteries could be replaced or recharged during scheduled relocation of the modules.   (b) Low-cost, integrated-circuit radio chips are available allowing a design that costs less to manufacture than systems with cables running between modules in a wired system.   (c) Because the array of acquisition modules is laid on the terrain as a mesh, it automatically adapts to uneven terrain. If an obstruction exists, a spare module can be used to pass the signal over or around the obstruction.   (d) If the 2.4 GHz frequency band is chosen, it will be legal in most areas of the world to operate at these low power levels. Because seismic surveys are normally conducted in remote areas, interference from external sources should not be a problem.   (e) By operating the modules on different frequencies within this band, the modules may continuously pass data down a seismic array like a “bucket brigade,” providing real time data acquisition and relay to the central control and recording system.   (f) The network may be expanded in scale by adding modules and extending the array to accommodate thousands of modules.       

       FIG. 1  depicts a common physical layout of a conventional seismic survey. A number of remote data acquisition modules  101  are connected by cables in a line and arrayed on the ground. Connected to each of the modules is one or more sensors configured as individual sensors, multi-component sensors, or strings of sensors wired into groups. Each of the modules may contain electronics to amplify, digitize, and store the signals from the sensors, or in the case of digital sensors, collect and store the data. The remote data acquisition modules may contain additional circuitry to test the sensors and/or the acquisition circuitry to ensure proper function and performance. 
     The remote modules are connected together in a line by electrical or fiber optic cables, and the line is connected to a second device called a “line tap” or “crossline unit”  102 . The line taps are then connected together in a string, and eventually to a central control and recording system  103 . 
     Seismic data is generally acquired and passed down the cables from the remote data acquisition modules to the line taps, and thence to the central control and recording system. Instructions and timing signals are passed up the cables from the central control and recording system to the line taps and thence to the remote data acquisition modules. Other geometries may be used, including just a linear array. Redundant lines or a ring topology may be used to provide alternate data and control paths in the event of failures or obstructions. The number of sensors deployed may vary considerably depending on the requirements of the survey. If one of the lines must be discontinuous because of some obstruction, such as a river, a radio frequency communication system may be inserted to carry the data and instructions across the gap. 
     The central control and recording system usually consists of a computer with a display, keyboard, interface to the line tap string, and digital storage system. In one implementation, the central control and recording system might consist of a standard notebook computer with an Ethernet, USB, or wireless interface to connect to a line tap string or to an interface device that connects to the line tap string. Data may be stored on the computer&#39;s internal hard disk. For larger systems, the central control and recording system might consist of a larger computer with separate display and keyboard and separate storage device such as a tape drive, one or more hard disks, or some other storage device consistent with storing relatively large amounts of data. 
     In the present invention, wireless data acquisition and relay modules replace the conventional wired units. The positions of the remote modules might be the same as in a wired system, or the array might be adapted to exploit the flexibility of a wireless system. In the following discussion, a generalized example is first provided to illustrate the flexibility of the present system. That is, the acquisition modules can be arranged in substantially any pattern and serial communications of seismic data can occur along substantially any route in order to report information to a central control and recording system. Thereafter, specific examples are provided that illustrate advantageous array configurations and read out protocols. 
       FIG. 7  shows a layout of a wireless seismic survey system  700  in accordance with one embodiment of the present invention. The system  700  includes data acquisition modules  701  which are distributed throughout a seismic survey site. The data acquisition modules  701  are configured to communicate with surrounding modules through wireless links  707 . Generally, seismic data is wirelessly forwarded from data acquisition modules  701  that are more remotely located to a central control and recording system  703  to those that are less remote until the data reaches the central control and recording system  703 . As shown, the data may be forwarded by the data acquisition modules  701  until it reaches a base station  705 . This base station may be capable of transferring and receiving data between the central control and recording system  703  by any suitable method (e.g., Ethernet, USB, fiber-optic link, some computer compatible wireless interface such as IEEE 802.11, etc.). Additionally the base stations  705  may simply be data acquisition modules that are configured to communicate directly with the central control and recording system  703 . 
       FIG. 7  illustrates that the layout of the data acquisition modules need not be a linear array, or any regular geometric configuration. This is advantageous because it allows the survey system to operate around obstructions, and allows freedom for survey designers to choose a layout that will optimize the performance of the system. 
     It will be appreciated that the data path of the seismic data between the data acquisition modules may be manually or automatically configured. In the former case, each module may be programmed, for example, to communicate with predetermined modules that will be placed immediately or closely adjacent to each other. Alternatively, the modules may be configured to automatically detect and select an optimal path for the seismic data to be transferred. In this latter case, the modules may be positioned without needing to place particular modules in particular locations. Then, the modules may select an optimal data path based on various factors such as obstructions, signal strength, transfer rates, etc. 
     An important aspect of the present invention is the capability to provide for rapid capture of data from a wireless array. That is, the system can operate more efficiently and inexpensively if the data capture can be accomplished without delaying the survey process, as described hereinabove. In order to achieve this goal, it will be appreciated that the seismic data needs to either be stored locally (e.g., at a module base station or other collection station), in whole or in part, for the entire survey, or otherwise transferred to a central control and recording system in a manner which does not interfere with or delay the operation of the vibration source device. The present invention provides a system for wireless transfer of seismic data which achieves this goal. In one embodiment of the present invention, all of the seismic data for a seismic event is transferred from the data acquisition modules to the central control and recording system, or other storage system, in no more than about 20 seconds, from the end of the vibration event. This is achieved by selecting data transfer rates, the number of data acquisition modules in the array and in each serial data transfer line, and other factors. An example of a preferred embodiment is described in detail below. 
       FIG. 2  shows a block diagram of a wireless remote acquisition and relay module  200  in accordance with an embodiment of the present invention. A vibration sensor  201  converts vibrations into electrical signals which are fed through switch  210  to preamplifier  202  and thence to the analog to digital (A/D) converter  203 . The digital data from the A/D converter  203  is fed into the Central Processor  204  or directly into a digital memory  205 . Alternately, in the case of a sensor  201  with direct digital output, the signals may flow directly to the processor  204  or memory  205 . 
     In addition to controlling the system and storing the data in the memory, the processor  204  may perform some calculations on the data including decimation, filtering, stacking repetitive records, correlation, timing, etc. The remote module  200  may also receive information through the transceiver  206 , for example: timing information, cross-correlation reference signals, acquisition parameters, test and programming instructions, location information, seismic data from upstream modules and updates to the software among other commands. The transmit and receive signals couple through antenna  207 . 
     The processor  204  can control the transceiver  206 , including transmit/receive status, frequencies, power output, and data flow as well as other functions required for operation. The remote module  200  can also receive data and commands from another remote module or base station, store them in the memory, and then transmit them again for reception by another remote module up or down the line. 
     A digital-to-analog (D/A) converter  208  may be included in the system which can accept digital data from the processor  204  to apply signals through a switch  210  to the input circuitry. These signals, which may for example consist of DC voltages, currents, or sine waves, can be digitized and analyzed to determine if the system is functioning properly and meeting its performance specifications. Typical analysis might include input noise, harmonic distortion, dynamic range, DC offset, and other tests or measurements. Signals may also be fed to the sensor  201  to determine such parameters as resistance, leakage, sensitivity, damping and natural frequency. The power supply voltage may also be connected through the switch  210  to the A/D converter  203  to monitor battery charge and/or system power. The preamplifier  202  may have adjustable gain set by the processor  204  or other means to adjust for input signal levels. The vibration sensor  201  may be a separate generic unit external to the remote module  200  and connected by cables, or the sensor  201  might be integral to the remote module package. 
     If the remote module  200  is to be used as a base station, equivalent to a “line-tap” or interface to the central recording system, it will also have a digital input/output function  211  which may be, for example, an Ethernet, USB, fiber-optic link, or some computer compatible wireless interface (e.g., one of the IEEE 802.11 standards) or another means of communication through a wired or radio link. It may be acceptable to use larger battery packs for the line tap wireless data acquisition and relay modules because they will normally be relatively few in number and may communicate over greater distances using a high speed data communication protocol. 
     The remote module  200  is constructed of common integrated circuits available from a number of vendors. The Transmit/Receive integrated circuit  206  could be a digital data transceiver with programmable functions including power output, timing, frequency of operation, bandwidth, and other necessary functions. The operating frequency band may preferably be a frequency range which allows for unlicensed operation worldwide, for example, the 2.4 GHz range. The Central Processor  204 , Memory  205 , and switch  210  can include any of a number of generic parts widely available. The A/D converter  203  could preferably be a 24-bit sigma delta converter such as those available from a number of vendors. The preamplifier  202  should preferably be a low-noise, differential input amplifier available from a number of sources, or alternatively integrated with the A/D converter  203 . The D/A converter  208  should preferably be a very low distortion unit which is capable of producing low-distortion sine waves which can be used by the system to conduct harmonic distortion tests. The module  200  may include a number of other components not shown in  FIG. 2 , such as a directional antennae for AOA signal measurements, separate transmit and receive antennae, separate antennae for location signals and seismic data transfer signals, GPS receivers, batteries, etc. 
     The following example depicts how the system can acquire seismic data continuously. Assume that every module is sampling the vibration signals at 500 samples per second with a resolution of 24 bits per sample. The seismic data from the vibration sensor is digitized and stored in memory. While this is taking place, the transceiver  206  is receiving data from the next module more remotely located to the central recording system. After some amount of data is collected from the sensor  201  and the other modules, the module switches to transmit mode and sends some packets of data collected from the sensor  201  and the other modules on towards a module closer to the central recording system. Each packet of data is also annotated with some identification as to the original source sensor and the time acquired. The module continues to acquire and store data during the transmit phase so there are no gaps in the record. 
     The time stamp annotation may come from a clock in the microprocessor or the radio. The clocks in all the modules may be periodically adjusted and synchronized with a signal from the central recording system or other source. 
       FIG. 3  shows one possible configuration of a wireless seismic system in accordance with an embodiment of the present invention. A number of remote modules  301  may be arranged in lines as is done with a wired system as shown in  FIG. 1 , except that there is no physical connection between the remote modules. Replacing the line-tap modules are base station modules  302  which may be connected to a central control and recording system  303  by Ethernet, fiber optic, or other digital data link or a wireless substitute. Example radio links operating on frequencies F 1  to F 12  are indicated by arrows. Note that for improved data rate, each radio link in the illustrated embodiment leaps past the nearest remote module to the next module closer to the base station. Other radio transmission paths are possible, including direct to the nearest remote module, leaping multiple modules, or in the case of an obstruction or equipment fault, past a defective remote module or even across to another line or any other logical path that establishes a communication flow. The central control and recording system may be a notebook computer or larger equivalent system. 
       FIG. 4  is an exploded view of a portion of the line representative of an array of wireless remote modules Rn and base stations BSn connected together by an Ethernet or Fiber Optic or radio frequency wireless or other link in accordance with an embodiment of the present invention. During the data acquisition phase, signals from the sensors will be digitized and stored in memory. Some of the remote modules will then transmit digital data from the memory through the RF link. Others will be receiving digital data from nearby modules, temporarily storing data in memory. At maximum data transmission rate, half the remote modules may be transmitting and half would be receiving at any time. So that the modules don&#39;t interfere with one another, different frequencies may be assigned to each remote module within range of any other remote module. For example, remote module R 11  will transmit on frequency F 1  to remote module R 13 . Remote module R 12  can simultaneously transmit on frequency F 2  to remote module R 14 . Remote modules R 15 , R 16 , R 19  and R 20  are also transmitting on their assigned frequencies F 3 , F 4 , F 5 , and F 6 , respectively. Remote module R 20  sends its data packet to its base station BS 1 . After the data packets are passed between the remote modules, the transmit/receive states of the remote modules is reversed, as illustrated in the drawing by the next string of remotes. Remote module R 23  transmits its data packet on frequency F 7  and the process continues, with remote module R 31  transmitting its packet to base station BS 2 . Data is further transmitted down the backbone data link to the central recording and control system. 
     It will thus be appreciated that the signals used by the modules are time-division multiplexed and frequency division multiplexed. That is, as noted, modules within receiving range of one another that transmit during the same time intervals may be assigned different transmission frequency bands to avoid interference. Thus, different modules within receiving range of one another and even within a single serial data transfer path (e.g., the path defined by R 11 , R 13 , R 15 , R 17 , R 19 , R 21 , BS 1  in  FIG. 4 ) can transmit concurrently, thereby enhancing read out rates. 
     In addition, as noted above, any particular module may transmit only half of the time or less. For example, a module may transmit data substantially half of the time and receive data from an upstream module or modules substantially half of the time. Thus, within a given serial data transfer path, adjacent modules will typically transmit in opposite (i.e., alternating) time intervals. Signals from adjacent modules will therefore generally be time division multiplexed. As such, a given frequency band can be re-used by modules within receiving range of one another or even within a given serial data transfer path, thereby resulting in better use of the available data channels, which may allow for greater density of the array (closer spacing of the modules) as may be desired. 
     Moreover, it will be noted that each physical line of modules in  FIG. 4  is, in effect, defined by two (in the example of  FIG. 4 ) or more interleaved serial data transfer paths. That is, the even number modules associated with base station BS 1  (R 12  . . . R 20 ) define one serial data transfer path and the odd number modules (R 11  . . . R 21 ) define another. These two serial data transfer paths may be coordinated so as to improve data read out. For example, these serial data transfer paths may be synchronized so that modules R 20  and R 21  alternately transmit to base station BS 1 . In this manner, resources are used more effectively as a single antenna at base station BS 1 , operating with a substantially 100% duty cycle, can receive the signals from R 20  and R 21 . Alternatively, base station BS 1  can receive signals from spatially separated lines to make a similarly effective use of resources. Although not shown in  FIG. 4 , the base stations BS 1 , BS 2 , etc., may receive data from separate lines, e.g., on opposite sides of the base stations. 
     It will be appreciated that the advantages of multiple multiplexing techniques (e.g., time and frequency multiplexing) may be realized in a variety of ways. For example, rather than having the operation of a given module be divided into two substantially equal intervals defining a cycle period (e.g., a receive interval followed immediately by a transmit interval), the cycle period may be divided into more than two intervals. For example, a module may receive data from a first upstream module during a first interval, receive data from a second upstream module in a second interval and transmit data (obtained at the module and/or received from the first and/or second upstream modules) in a third interval. For example, such a three interval cycle may be used in connection with a nonlinear, serial data transfer layout. Alternatively, a three interval cycle may be used in connection with interleaving three serial data transfer paths in a single physical line of modules as may be desired. 
     Moreover, in some cases, modules may operate at less than a 100% duty cycle with respect to wireless data reception and transmission, i.e., with quiet intervals within a cycle period where the module is neither transmitting nor receiving. For example, the cycle may be divided into four intervals, and odd numbered modules may receive in the first interval, transmit in the third interval and neither transmit nor receive in the second and fourth intervals. Even numbered modules may receive in the second interval, transmit in the fourth interval and neither transmit nor receive in the first and third intervals. This provides a further dimension of time division multiplexing as between the interleaved serial data transmission lines as may be desired, for example, where efficient use of bandwidth is more critical than full-duty-cycle usage of any individual antenna. 
     Multiplexing technologies other than time-division and frequency division multiplexing may also be used, for example, code division multiplexing. In code division multiplexing, a transmitter-receiver pair are assigned a digital code that enables a signal of interest to be distinguished from other signals even where the other signals overlap the signal of interest in time and frequency. The codes of potentially interfering signals may be selected to be mathematically orthogonal so as to reduce interference. In the case of applications involving many potentially interfering signals, long codes may be utilized, thereby potentially complicating processing and increasing overhead. In the present context, where low power transmissions and a well-defined array geometry are employed, shorter codes may suffice. Moreover, code division multiplexing may be combined with time division and/or frequency division multiplexing as discussed above to further shorten codes and optimize processing. Additionally, depending on economic considerations amount other things, multiple antennae, for example, separate transmit and receive antennae, may be employed for a given module. 
     In the example of  FIG. 4 , which involves a sampling rate of 500 samples per second and a sampling resolution of 24 bits, remote module R 11 , transmitting half the time, must transmit data at 24000 bits per second to remote module R 13 , which is currently acquiring data from its own sensor at the 12000 bits per second rate. After some elapsed time, the remote modules all switch between transmitting and receiving status, and pass the data further down toward the base station. Now, remote module R 13  will pass the data previously received from R 11 , plus its own accumulated data to remote module R 15 . Since it now needs to pass data from two sensors, the data rate will have to be 48000 samples per second in order to prevent a backlog of data at the sensors. It will be appreciated that data originating from different modules, even though it is subsequently transferred in a single transmission interval, may have a slightly different reference time or time stamp as will be discussed in more detail below. Appropriate header or metadata may be associated with the data to not only identify the source module/location, but also to identify the acquisition time. 
     The required data rate will increase in a linear fashion as the line of remote modules grows longer. When the number of stations in a line multiplied by the sample rate times two exceeds the maximum data rate of the wireless data acquisition and relay modules, the line will no longer be able to do keep up with the data flow. At that point, it will be necessary to add another line of base stations, or to allow delays in the data transmission process, or allow “wait periods.” In the case of a vibrating energy source, this would mean expanding the system components or stopping the energy source for the necessary delay. That is, in some cases, data may be stored at one or more of the modules or base stations for read out during repositioning of the energy source or at another time without delaying the survey process. Another option would be to correlate and/or stack the pilot signal from the vibrating energy source in the remote modules using the central processor, which greatly reduces the amount of data required. Yet another option would be to use data compression to reduce the number of data bits required to carry the information, which would allow the system to have more remote modules per base station. When an explosive source is used to generate the vibration, the amount of seismic data collected in a time period is much less, so arrays could be much larger than surveys that use a vibrating energy source vehicle. 
     Each data packet from each remote module may contain information on the time the data was collected, acquisition parameters, index number and serial number of the remote module, station coordinates, etc. Periodically, commands and information may be sent up or down the line to the remote modules (e.g., time synchronization, acquisition parameters, self-test instructions, etc.). The base station modules may contain circuitry from multiple modules to allow data transmission to two or more arrays in the same or different directions. 
       FIGS. 12A-12C  graphically illustrate the serial transfer of seismic data packets from a series of modules R 1 -Rn to a base station BS. As illustrated, each module R 1 -Rn generates a data packet P 1 R (1-n)  in response to at least a portion of a seismic event. The data in each packet P 1 R (1-n)  is collected for a first time period T 1 . The first packet for each unit is then transmitted to another module disposed along the serial data transfer path nearer to the base station BS. The process is repeated for a second time period T 2 . However, it will be noted that the second module R 2 , in addition to generating a second data packet P 2 R 2  for the second time period T 2 , also is in receipt of the first packet P 1 R 1  for the first time period as received from the first module. See  FIG. 12B . The first data packet P 1 R 1  may be appended to the second data packet P 2 R 2  for transfer to the third module R 3  during the next transmission period. In this regard, the data file transferred by the second module R 2  to R 3  will include packets P 1 R 1  and P 2 R 2  from different modules (i.e., modules R 1  and R 2 ). Further, these packets P 1 R 1  and P 2 R 2  will contain seismic data that is collected during two separate time periods T 1  and T 2 . 
     It will be appreciated that as the number of time periods increases, the number of data packets transferred by the last remote unit Rn to the base unit BS may equal the number of remote units in the serial transfer path. For instance, the last module Rn may include a data packet from each module unit R( 1 - n ). Further, each of these data packets may include data for a different time period T( 1 - n ). Accordingly, prior to utilizing the packets from the module R( 1 - n ), those packets will be correlated and reassembled. For instance the packets associated with the first module R 1  (e.g., P 1 R 1   T1 -PnR 1   Tn ) may be collated from multiple data files that were transmitted to the base station BS. The collated packets may be reassembled in temporal order to define a response of the first module R 1  to a seismic event. Of note, such collation and/or reassembly may be performed by the base station, by the central control or at a location remote from the seismic survey. In the latter regard, such collation and/or reassembly may be performed after the seismic survey is completed. 
     Frequencies are assigned to the modules in such a manner as to avoid interference with other modules. Such assignment may depend the on known location and separation of the modules, or may be based on automatic field tests where actual experiments are conducted manually or automatically by the central computer. Alternatively, the individual modules might be instructed to conduct their own tests to determine the best frequency allocation. In the case of weak signal strength, the modules might adjust their power output to the level necessary consistent with minimum use of battery power. In the event of an obstruction such as a ridge line, structure, or other problem with radio communication, one or more extra modules may be placed to maintain data flow by acting as a radio relay. The extra module may or may not contain a vibration sensor. 
     Data acquisition, the digitization of data from the sensors, and radio transmission of previously acquired data will occur simultaneously so there will be a small delay between acquisition and transmission. Accordingly, each packet of data may include information on the source and time of acquisition. The data packets will be reassembled into a file with records from all the sensors comprising the active portion of the array. 
     It will be appreciated that many variations of the systems of  FIGS. 3 and 4  are possible. One such system  800  is illustrated in  FIG. 8 . The illustrated system  800  includes a generally rectilinear array of acquisition modules  801  generally similar to that of the systems of  FIGS. 3 and 4 . However, the illustrated system  800  includes multiple rows of base station modules, including a row of first base station modules  802  and a row of second base station modules  803 . As discussed above, a column of acquisition modules  801  transfers data in serial fashion to a base station  802  or  803 . In this case, alternate columns of acquisition modules  801  transfer data to opposite base stations  802  or  803 , as indicated by arrows  805 . That is, if a given column transfers data to a first base station  802 , adjacent columns on either side thereof transfer data to a second base station  803 . There are various array design reasons why the array might be implemented in this fashion. For example, if a given data acquisition unit  801 A is defective or otherwise off-line, data may be transferred around that module  801 A by using a module  801 B of an adjacent column, as indicated by dashed lines  806 . For example, this may be implemented by appropriately adjusting the transmit and/or frequencies of the relevant modules  801 . In this regard, having adjacent columns read out in opposite directions is convenient so as to avoid the need to transfer undue amounts of data. That is, as noted above, the amount of data transferred increases as you approach a base station in any column due to the serial data transfer protocol. In the illustrated case, modules  801  close to a base station are proximate to modules  801  in an adjacent column that are remote from their respective base stations. Accordingly, it will generally be possible to use a module  801 B from an adjacent column as illustrated to bypass a defective module  801 A without overloading the bypass module  801 B. In this case, each module receiving data from bypass module  801 B may only re-transmit data relevant to its serial data transmission line. 
     The illustrated base stations  802  and  803  transfer data to a central control and recording system  804  generally as described above. Also, as illustrated in  FIG. 8 , the base stations  802  and  803  may receive information from modules  801  on either side thereof. For example, each of the base stations  802  or  803  may have a pair of receivers for receiving data from modules  801  on opposite sides thereof. 
     Turning now to  FIG. 5 , the wireless survey system  500  may provide for the automatic determination of the location of the data acquisition modules. As shown, the system may utilize one or more Location Finding Technology (LFT) systems  507 ,  509 ,  511  to determine the location of a data acquisition module  501 . These LFT systems may employ a number of location finding technologies such as AOA, TDOA, GPS, signal strength or other methods. Examples of such systems include infrastructure-based systems such as AOA, TDOA and signal strength, external systems such as GPS, and hybrid systems such as infrastructure assisted GPS. Generally, an infrastructure-based system may determine the location of a data acquisition module based on communications between the data acquisition module and other wireless location units (WLUs) (e.g., dedicated LFT units, other data acquisition modules, etc.). For example, and as will be described in more detail below, such systems may receive information concerning a directional bearing of the data acquisition module or a distance of the module relative to one or more other WLUs. Based on such information, the location of the data acquisition module may be determined by triangulation or similar geometric/mathematic techniques. External systems such as GPS systems, typically determine the location of the data acquisition module relative to an external system (e.g., a GPS satellite constellation). This is accomplished by equipping the data acquisition module with a GPS receiver. 
     Normally, the output from the LFT systems will be transferred to a central control and processing system  505 . The nature of the output and the method of data transfer are described in more detail below. Generally, the output will include the location of one or more data acquisition modules, and will be used by a data processor to process the seismic data into a format that can be analyzed. 
     For purposes of illustration, a number of different location finding technologies are depicted in  FIGS. 6A-6C .  FIG. 6A  depicts time of arrival (TOA) based LFT  600 . In this case, the range between a first data acquisition module  601  and another WLU  603  is determined, based on time of signal arrival or signal transmit time of a signal from a data acquisition module to another WLU. Once the range between the data acquisition module and at least three other WLUs  603 ,  605 ,  607  is known, the relative position of the first data acquisition module  601  can be determined by resolving the intersection of the ranges. 
       FIG. 6B  generally illustrates an AOA based LFT system  610 . AOA based LFT systems may determine the location of a first data acquisition module  611  based on the angle of arrival of signals  618 ,  619 , generally indicated by dashed lines  617 , from the first data acquisition module as measured by two or more WLUs  613  and  615  which are equipped with antennas capable of resolving the angle of arrival of a signal, such as multiple directional antennas (not shown). It will be appreciated that modules may therefore be equipped with specialized antennae for this purpose. The various angles of arrival can be used to calculate the location of the first data acquisition module  611  based on the intersection of angles from two or more WLUs. 
       FIG. 6C  illustrates a TDOA based LFT system  620 . In TDOA systems, multiple WLUs  623 ,  625 ,  627  measure the time of arrival of signals from a first data acquisition module  621 . Based on such measurements, the difference of the time of arrival between two WLUs can provide information regarding the first data acquisition module&#39;s location in terms of a hyperbola  629 . The intersection of three or more of the hyperbolas  629  can be used to determine the location of the first data acquisition module  621 . 
     It will be appreciated that some of the methods described above provide a relative location of a data acquisition module, that is, a location relative to one or more WLUs. In order to translate the relative location of the data acquisition module into an absolute one, the absolute location of at least one WLU may be predetermined. This can be accomplished using several methods. For example, one or more of the WLUs may be equipped with a GPS receiver. As another example, the location of one or more WLUs may be determined by any suitable method, and the location of the one or more WLUs may be used to translate the relative location into an absolute location. 
     Additionally, it is noted that the WLUs may be provided as any of a number of suitable devices. As an example, in a preferred embodiment the WLUs may include other data acquisition modules and/or base stations. In this embodiment, the data acquisition modules would be configured to receive signals from (or transmit signals to) surrounding data acquisition modules (or base stations) for the purpose of automatic location. Alternatively, the WLUs may include receivers that are dedicated to the function of locating the data acquisition modules. 
     It will be appreciated that the control and processing of the automatic location system may be performed at the central control and recording system, or remotely. In one embodiment, the central control and recording system may send a command to a first data acquisition module, directing it to start the automatic location procedure. This command may be transferred in the same manner that the seismic data is transferred, or alternatively, may use a separate data transfer method. In other words, the data transfer structure for the automatic location system may or may not be the same as is used for the transfer of seismic data. Next the first data acquisition module, or a separate control platform, may wirelessly send a command to surrounding data acquisition modules indicating that they should prepare to receive a signal. Then the first data acquisition module may transmit a signal, and the surrounding modules would receive it. The surrounding modules may then transfer the raw data (which may include identification, timing information, angles of arrival, geographic coordinates, etc.) or processed information back to the central control and recording system using a suitable data transfer method. In the case of raw data, the central control and recording system may then use the information from the surrounding modules to calculate the location of the first data acquisition module using one or more of the methods described herein. Alternatively, the logic to perform the location calculations may be included in the WLUs, or another suitable system. In the latter case, the WLUs may send processed data that includes the location of one or more data acquisition modules back to the central control and recording system. 
     As noted above, the system of the present invention can utilize a variety of multilateration technologies that have been developed in connection with wireless telephony and mesh networks. However, the system of the present invention takes advantage of the seismic array context to optimize these positioning technologies. In this regard, an exemplary seismic array  900  is illustrated in  FIG. 9A . The array  900  includes a number of acquisition modules  901 . At least one of the acquisition modules  901 A is self-locating, in this case, using an RF technology such as TDOA or signal strength. Such multilateration technologies involve communication between the module to be located  901 A and multiple reference structures. For example, the multiple reference structures may transmit signals to the module to be located  901 A and/or the module  901 A may communicate signals to the reference structures. In this case, the reference structures may be other acquisition modules  901 B or dedicated reference structures  901 C. For example, it may be convenient to have some or all of the base stations  902  function as the reference structures  901 C. Similarly, the signals used for location purposes may be dedicated location signals or may be data transfer signals including encoded information that can be used for positioning purposes. 
     In the illustrated example, the module  901 A receives positioning signals from a number of other acquisition modules  901 B. In this regard, the module  901 A may receive positioning signals from more than the minimum number of modules  901 B required for three-dimensional positioning in order to enhance positioning accuracy. Because the module  901 A is stationary, it is not necessary that the various positioning signals be received at the same time. Thus, for example, the module  901 A may include an antenna that can be tuned to different frequencies at different times so as to receive positioning signals from different modules  901 B. Alternatively, the modules  901 B may transmit positioning signals at a designated frequency for module  901 A. The transmitting structures  901 C, which may be other modules or base stations, may use a higher transmission power for location signals than for seismic data transfer signals so as to provide a longer transmission range. As less location signals than seismic data transfer signals will generally be used, this can be done without undue depletion of batteries. It will thus be appreciated that the illustrated positioning system has a number of advantages in relation to wireless telephony positioning systems due to the array configuration and stationary nature of the array. 
     Further advantages in this regard are illustrated by  FIGS. 9B and 9C . As noted above, certain multilateration technologies such as TDOA and signal strength involve calculating ranges between the acquisition module to be located and multiple external references. For purposes of illustration above, such technologies were illustrated as involving simultaneously solving equations representing certain curves. In reality, each of these ranging equations has an uncertainty, for example, relating to uncertainties in time measurements or signal strength measurements. This is graphically illustrated in  FIG. 9B . Specifically, the uncertainties in these measurements correspond to a finite thickness envelope around each of the curves. As a result, the location of the acquisition module is defined by a shaded uncertainty region  900 , representing the overlap of the thickness envelopes. Thus, in the illustrated example, there may be considerable uncertainty based on resolving three range curves. Although the uncertainty region  900  is illustrated in two dimensions, it will be appreciated that the uncertainty region extends in three dimensions, all of which are of interest in relation to processing seismic data. 
     Due to the stationary nature of the seismic array, certain statistical processing techniques can be used to reduce this uncertainty. This is illustrated in  FIG. 9C . Specifically,  FIG. 9C  illustrates a plot of a series of range determinations between an acquisition module to be located and a given reference structure. The samples  902  may be taken over a period of time. It may be determined, theoretically or empirically, that these samples  902  define a Gaussian distribution or other defined distribution. Accordingly, statistical processes may be used to determine a projected actual range  903  based on the samples  902 . This range is further depicted in  FIG. 9B . By performing such statistical processes in relation to multiple reference structures, the location of the acquisition module of interest can be determined to the required accuracy. 
     Another feature of the present invention involves a method for designing a seismic survey based on parameters that are unique to wireless seismic survey systems. Designing a wireless seismic survey system such as the present invention involves considerations that are different from those present when designing a conventional wired system. For instance, various parameters for the wireless transfer protocol may be selected to achieve the desired performance (e.g., transmit power, antenna sensitivity, number of channels to use, data transfer rate, etc.). As an example, suppose a seismic survey is planned for a specific geographic area. A designer may choose the spacing of the data acquisition modules so as to achieve the desired resolution of the resulting seismic data. Then the transmit power and the number of wireless frequencies needed may be selected. Furthermore, the data transfer rate may be chosen based on the operating characteristics of the vibration source device, as well as transmission duty cycle of the module and length of the serial data transmission lines among other things. It will be appreciated that the examples provided includes only a few of a number of various parameters that may be considered when designing a wireless seismic survey system. 
     Additionally, choosing the layout of the data acquisition modules may involve unique considerations when designing a wireless system. First, the layout of the modules is not constrained by the physical connection of cables between each module. This may enable the designer to have more flexibility when choosing a particular layout. For instance, the distances between modules may be varied or randomized for various reasons (e.g., to prevent aliasing of signals). Alternatively, the layout may need to be an irregular pattern in order to avoid obstructions such as roads, bridges, rivers, buildings, etc. 
     Another feature of the present invention involves receiving seismic data from a wireless seismic survey system and processing and/or analyzing the data into a form that is useful for resolving the characteristics of one or more subsurface geologic structures. 
     The receiving step may include, for example, transferring the seismic data to a computing system that is capable of processing the data. In one embodiment, the computing system is the central control and recording system. In another embodiment, the computing system is a system other than the central control and recording system. In the latter case, the seismic data may be transferred from the central control and recording system to the computing system by any suitable method (e.g., Ethernet, 802.11 wireless protocol, USB, Fire Wire, CD-ROM, hard disk drive, etc.). Additionally, it will be appreciated that the computing system may be geographically distant from the seismic survey site. For example, in one embodiment, the seismic data may be processed by a computing system in a different country from that which the seismic survey was carried out. 
     The processing step may include a number of methods for manipulating the seismic data (e.g., filtering, summing, synchronizing, displaying, etc.). Generally, the processing step involves manipulating the raw seismic data by a computing system into a form that is useful for analysis. As an example, the output of the processing step may display a 3D image of a subsurface geologic structure on a suitable display device. As another example, the processing step may output frequency data, such as data formatted for spectral analysis. Those skilled in the art will recognize that there are a number of algorithms that may be used to process seismic data into a useful form. Additionally, the processing step may include interpreting the seismic data obtained from a system such as the present invention to identify the characteristics of one or more subsurface geologic structures. This portion of the processing step may be implemented by a computing system, or by a person qualified to interpret such data. 
     In practice, a seismic survey may be designed with reference to the characteristics of the wireless system. An associated process  1000  is illustrated in  FIG. 10 . In this regard, it is noted that there is an interplay between the available bandwidth, the number of channels utilized, the multiplexing technology or technologies used and associated antenna duty cycle, and transmitter power. Thus, for example, based on the objectives of the survey, the geographical extent of the survey may be determined ( 1002 ). It will be appreciated that there may be some flexibility regarding the exact geographical extent of the survey, as well as the number and spacing of the data acquisition modules. In this regard, the designer may determine ( 1004 ) the total bandwidth that is available for use by the acquisition modules and determine ( 1006 ) the desired channel width for the communication channels used by the acquisition modules. It may be desired to provide some buffer between adjacent channels to avoid interference. 
     Based on the total bandwidth available and the desired channel width, the total number of channels that are available for the system can be determined ( 1008 ). In addition, the designer can determine ( 1009 ) the multiplexing technology or technologies to be used, for example, time division and frequency division multiplexing as described above. The designer can also determine ( 1010 ) the read out time that is desired for the array. For example, in the case of a vibrating energy source, it may be desired to complete read out of the array in approximately 20 seconds so as to avoid delaying operation of the source and completion of the survey. 
     Based on all of this information, as well as the technical specification of the modules and other array equipment, the designer can perform a number of calculations to determine possible array configurations. For example, the designer can calculate ( 1012 ) how many modules can be in a serial data transfer path. Thus, in the example described above, the modules were sampled at a rate of 500 samples per second with a resolution of 24 bits per sample. Further, each module was assumed to transmit half of the time. As a result, the first module transmits data at a rate of n (in this case, 24000 bits per second), the second module transmits at 2n (in this case, 48000 bits per second), the third module transmits at 3n (in this case, 72000 bits per second), etc., in order to prevent a backlog of data at the modules. Accordingly, there will be some maximum serial data transfer path length at which the data transfer rate required will equal the maximum data transfer rate specification of the modules. For example, if the module specification is 1 Mbit/sec., the maximum length of a serial data transfer path may be about 40 modules. The designer may use this parameter to calculate the maximum number of modules in a serial data transfer path and, hence, how frequently “backbones” of base stations or other storage/transfer units will be required. In the example, where two serial data transfer paths are interleaved in a single physical line of modules associated with a given base station, the length of the physical line, in terms of number of modules, will be twice the length of a serial data transfer path. 
     However, using the assumption, for ease of illustration, that data transfer rates are independent of module spacing, the configuration of the array can still vary considerably. Specifically, it is still necessary for the designer to determine the spacing between modules in a physical line, the spacing between lines, and the total number of modules to be used in the array. Conventionally, for a broad range of seismic surveys, the spacing between modules in a line may be on the order of 25-100 m, and the spacing between lines may be on the order of 100-400 m. The spacings selected are typically based on weighting the desire for improved imaging resolution against the increased survey expense associated with using a longer number of modules. 
     In the case of the wireless system described herein, it is anticipated that survey expense may be reduced and designers may, therefore, desire to use a denser array of modules. However, this requires that such increased density be accommodated without unacceptable signal interference between the modules. Accordingly, the designer may calculate ( 1014 ) the array density that can be achieved without unacceptable interference. For example, this may be calculated as a function of how many communication channels are available, how often they can be reused as a function of geometric space and how often they can be reused as a function of any multiplexing technology employed beyond frequency division multiplexing. 
     It will be appreciated that these parameters will be affected by a variety of factors such as external sources of interference, the topology of the survey landscape including any obstructions, and the like. Moreover, one of the advantages of the present invention is that laborers are not required to precisely position the modules, so actual positions may vary. Additionally, the transmit powers can be varied, before or after array deployment, and the array can be self-configuring based on expected or actual signal strength. Accordingly, some uncertainty may be accounted for in array design. 
     In a simple example, however, a designer may assume that channels may be reused by modules no closer than 400 m from one another (transmit powers may be tuned after deployment to match this specification). Moreover, based on the available bandwidth spectrum and the channel width required by the array equipment, the total number of channels available may be determined as discussed above. Based on this information, the designer can calculate ( 1016 ) intra and inter line spacing of modules. In practice, this may be affected by any directionality of the antennae and other factors. 
     For purposes of illustration, it may be assumed that 100 channels are available and that the antennae have no directional selectivity. Moreover, because only half of the modules are transmitting at any given time, the 100 channels may be reused by modules within 400 m of one another, provided that the channels are not used at the same time. Accordingly, 200 channel/time slots are available within a circle of 400 m radius (assuming no interference from adjacent regions of the array). Accordingly, if an interline spacing is desired that is four times the intraline spacing, it may be determined that an intraline spacing of about 25 m and an interline spacing of 100 m can be accommodated. Assuming that the survey objectives can at least be satisfied by the wireless array, the designer can then select ( 1018 ) a layout that is at least sufficient for the survey objectives and within the interference limits of the wireless array and/or can select ( 1020 ) a transmitter power for the acquisition units to achieve the survey objectives without undue interference. Once the general parameters of the array are thus established, the designer can determine ( 1022 ) the array configuration (addressing, for example, topography and obstructions) and can assign channels to the various acquisition units. 
     It will be appreciated that the processing of the data from the wireless array will also take into account the nature of the wireless system. Such processing may be executed locally at the survey location and/or remotely. For ease of reference, the following discussion references a processor, though multiple machines at multiple locations may be involved in such processing. An associated process  1100  is summarized by reference to the flow chart of  FIG. 11 . In the illustrated example, the processor receives ( 1102 ) the read out data from the array. It will be appreciated that, due to the read out process described above, this data will not be received in proper time sequence. That is, a first, or most remote, module in a serial data transfer path transmits data for time period n to an adjacent second module. That module then retransmits that data from the first module for time period n together with data acquired by the second module for time period n+1. This process continues down the serial data transfer path such that packets for different modules that form a single data stream have different time stamps. 
     Accordingly, the processor sorts ( 1104 ) the data by acquisition time. The processor then collects ( 1106 ) data from the modules associated with a common time. These steps of receiving, sorting and collecting may be repeated until data from the array is obtained corresponding to the time period of a detected seismic event. The data from any one of the modules for this time period thus defines a trace. 
     Processing of these traces generally requires knowledge of the time and location parameters of the data. As discussed above, the data packets will have time stamps that allow for correlation of data obtained at different modules at the same time. In this regard, reference time signals can be provided from a common system clock (or other time sources(s)) and synchronization, as between modules, is maintained via control signals transmitted serially across the array as discussed above. Accordingly, the processor stores ( 1108 ) time parameter information for the data by way of the time stamps or a packet-by-packet basis in the context of the present invention. 
     The process also receives ( 1110 ) location parameter information for the data. This information and the processing thereof is also a function of the location finding system context of the array. That is, as noted above, this information may be based on a multilateration process and may be statistically processed for improved accuracy. Accordingly, the information may be developed over time and may be dependent on knowledge of the location of other modules or position references, which knowledge may also be developed over time. Accordingly, at least with respect to initially acquired data, the location information may not be concurrently available. Rather, the header information or other metadata may simply identify a module, such that location information corresponding to that module can be later associated with the data. 
     Accordingly, the processor receives data with time and location parameter information associated therewith in a manner unique to the wireless context of the array. It will further be appreciated that the nature of the data may be a function of this wireless context, e.g., the array may be denser than normally utilized for wired arrays. Moreover, the associated processing may involve demultiplexing and noise reduction filtering as a function of the wireless context. The processor then processes ( 1112 ) the resulting data in conventional fashion to obtain information regarding subterranean structure. For example, such processing may involve normal moveout, trace stacking, etc. 
     With additional reference to  FIG. 20 , each module  101  may communicate with an adjacent module such that no modules are skipped.  FIG. 21  shows a similar configuration as shown in  FIG. 20 , wherein each module  101  transmits to an adjacent module.  FIG. 21  shows a portion of a line segment in various instances in time (T 1 , T 2 , and T 3 ). At each successive time interval, different modules may transmit data (e.g., appending acquired seismic data to the transmission) such that the data is in turn passed down the line segment to a base station  102 . A full array of this sort is shown in  FIG. 22 . Note that either side of a line segment may each comprise a subnet of modules. The base stations  102  may receive a transmission from a respective one of the different subnets in different transmission cycles. In this regard, and as illustrated in  FIG. 23 , a number of subnets may be provided that may transmit data back to a base station  102 . The number of subnets may correspond on the number of cycles required for each subnet to subsequently transmit data to the base station  102 . Alternatively, as shown in  FIG. 24 , base stations  102  may be provided with multiple radios to receive transmissions from more than one subnet at the same time. In this regard, for an embodiment wherein the base station unit interrupts the line segment and is positioned therein, the base station unit may have multiple radios operative to receive the data multiple subnets at the same time. Alternatively, the subnets may transmit to the base station in alternating time periods such that each subnet transmits data to the base station at different periods as shown in  FIGS. 21-23 . 
     In this regard, any given one of the remote modules  101  may be positioned within transmission range of one or more other remote modules. Accordingly, collision domains (e.g., areas having the potential for interference or crosstalk between modules) are introduced into the system. Each module may have a unique collision domain wherein potential interference with other modules may occur. One or more multiplexing techniques may be used to reduce the potential for collisions of transmitted seismic data in the data acquisition system. For instance, each of the remote modules  101  transmitting data may do so on one of a plurality of different frequency ranging from F 1  to F 12  as represented in  FIG. 3 . In this regard, even if all the transmissions occur during a common transmission period, use of the different frequencies allow for the simultaneous transfer of seismic data with reduced potential for data collisions. Accordingly, modules receiving data may only listen for the particular frequency such that other seismic data transferred at a different frequency is not received. 
     Other multiplexing techniques may also be used. For example, each of the remote modules  101  may use a different code in a code division multiplexing technique. Further still, each module  101  may transmit in a different discrete time period in a time division multiplexing technique. 
     In this regard, different multiplexing regimes may be applied to a wireless acquisition unit system such as the one shown in  FIG. 3 . As used herein, a multiplexing regime refers to the type of multiplexing used. As such, a multiplexing regime may be, but is not limited to, frequency division multiplexing, code division multiplexing, time division multiplexing or other appropriate multiplexing techniques. A multiplexing regime may employ a plurality of unique multiplexing signatures. The multiplexing signature is a unique parameter assigned to each of the remote modules. As such, in the example provided in  FIG. 3 , the plurality of frequencies F 1 -F 12  each represent a unique multiplexing signature. Other multiplexing signatures may also be provided, such as different codes for use in code division multiplexing or different discrete time periods for use in time division multiplexing. 
     In addition to the static multiplexing technique depicted in  FIG. 3  wherein each module in a collision domain employs a static multiplexing signature for transmission of data, a dynamic multiplexing regime may be employed in such a system  100 . This may allow for use of the system in jurisdictions where regulatory bodies prevent the use of a single constant frequency by any particular device for longer than a set duration. As such, these units may frequency hop (i.e., change frequencies) after the unit operates at a particular frequency for a predefined duration. As such, these units may include a random or pseudo-random sequence of multiplexing signatures. The unit may randomly hop frequencies in successive transmission periods such that no single frequency is used longer than the predefined duration. This type of random frequency hopping may be employed by each of the modules in a system. 
     For instance, the system shown in  FIG. 3  may employ such a random frequency hopping technique. As such, each of the frequencies shown F 1 -F 12  may actually be a random or pseudo-randomly generated frequency. These random or pseudo-random frequencies may be bounded at upper and lower ends and may include a number of discrete frequencies that are defined within a range of operating frequencies. In successive transmission periods, each remote module  101  may randomly hop to another random frequency. 
     One such embodiment of a frequency hopping technique may be Frequency Hopping Spread Spectrum (FHSS). In an FHSS system, the initiating party sends a request via a predefined frequency or control channel to the receiving party. The receiving party may return an index number to the initiating party. The initiating party may uses the index number to determine one or more frequencies (e.g., a pseudo-random sequence of frequencies to use). In turn, the initiating party may send a synchronization signal on the first frequency in a predetermined sequence. This synchronization signal may synchronize the transmission periods of the module and serve as an acknowledgement to the receiving party that the initiating party has correctly determined the frequency sequence. In turn, data may be transmitted with both the initiating and receiving party hopping frequencies in the predefined sequence in successive transmission periods. 
     However, this random frequency hopping introduces a collision domain into the system because two modules may randomly employ the same multiplexing signature such that a data collision occurs. It has been estimated that the occurrence of two modules hopping to the same random frequency may reduce the bandwidth of a system by 3% to 5%. This is because once a data collision occurs, each module may hop to another random multiplexing signature in the multiplexing regime and retransmit the data, effectively losing one duty cycle for each of the colliding modules. 
     Accordingly, another embodiment of a dynamic multiplexing regime may include assigning a shared pseudo-random multiplexing signature sequence to each of the modules in a system. In this regard, each module in the system may be assigned an identical multiplexing signature sequence. This shared pseudo-random multiplexing signature sequence may include a sequence of frequencies, codes, or other different multiplexing signatures. By using an identical multiplexing signature sequence, the potential for two modules randomly hopping to the same multiplexing signature may be reduced by controlling the phase of each module with respect to the multiplexing signature sequence. Stated differently, the modules within a collision domain may operate out of phase from each other with respect to the multiplexing signature sequence. As each of the modules may advance through the shared multiplexing signature sequence at the same rate (e.g., one multiplexing signature per transmission cycle), once modules within a given collision domain become out of phase from one another, the occurrence of data collisions may be minimized while still achieving a dynamic multiplexing environment. 
     This concept is further demonstrated with reference to  FIG. 13A , which depicts a first seismic acquisition unit pair  1300  and a second seismic acquisition unit pair  1350 . The first seismic acquisition unit pair  1300  shares a collision domain with the second seismic acquisition unit pair  1350  such that without a multiplexing technique, transmission between the first seismic acquisition unit pair  1300  and the second seismic acquisition unit pair  1350  may result in a data collision or crosstalk between the pairs. In this regard, the first seismic acquisition unit pair  1300  may be within transmission range of the second seismic data acquisition unit pair  1350 . However, as shown in  FIG. 13A , a first seismic acquisition unit  1310  transmits to a second seismic acquisition unit  1320  using a first frequency F 1 . At the same time, a third seismic acquisition unit  1360  may transmit to a fourth seismic acquisition unit  1370  using a second frequency F 2 . As such, crosstalk and/or data collisions are avoided and each seismic acquisition unit pair transmits data without interference using a unique multiplexing signature. 
       FIG. 13B  depicts the same first seismic acquisition unit pair  1300  and second seismic acquisition unit pair  1350  shown in  FIG. 13A . However, as shown in  FIG. 13B , the transmission between the first seismic acquisition unit  1310  and the second seismic acquisition unit  1320  as well as the transmission between the third seismic acquisition unit  1360  and the fourth seismic acquisition unit  1370  uses a common frequency F 3 . The result would be a data collision such that the data transmitted between each of the unit pairs  1300  and  1350  may be subject to a collision. In turn, neither the second seismic acquisition unit  1320  nor the fourth seismic acquisition unit  1370  may accurately receive the data intended for the respective modules. This is an example of a situation where in prior systems, each unit pair would in turn randomly hop within a random sequence and retransmit the data attempted to be transmitted in  FIG. 13B . 
     Such an instance is also depicted in  FIG. 14 .  FIG. 14  includes a chart  1400  depicting the frequency at which the first unit pair  1300  and a second unit pair  1350  are operating. A sequence of pseudo-random frequencies  1410  are represented in the middle column of the table  1400  and include frequencies f 0  through f n . Any number of unique multiplexing signatures may be used. In addition, while f 0 -f n  are numbered consecutively, the pseudo-random frequencies may not be consecutively arranged and may jump to different (i.e., higher and lower frequencies) in the spectrum without limitation. That is, the order of f 0 -f n  does not necessarily indicate an increase or decrease in frequencies, but rather each subsequent frequency or multiplexing signature in the sequence may be random. As shown, both the first unit pair  1300  and the second unit pair  1350  are operating using a common frequency f 0 . This corresponds to the situation shown in  FIG. 13B  where a collision may occur. 
     In one particular embodiment, a total of sixty-four different multiplexing signatures may be employed in order to transmit data between modules. For example, these sixty-four multiplexing signatures may comprise different frequency signatures residing in the 2.4 GHz band. Additionally, fifteen multiplexing signatures may be used that are dedicated to deployment, formation, and wake-up system functions. In this regard, these fifteen multiplexing signatures may be used in a configuration mode in order to order and establish the array. 
     In accordance with the present invention, rather than having each of the first unit pair  1300  and second unit pair  1350  randomly sequence among the pseudo-random frequency signature sequence upon occurrence of a collision, the second unit pair  1350  is advanced out of turn in the frequency sequence  1410 . The first unit pair  1300  may maintain its position in the frequency sequence  1400  while the second unit pair  1350  is advanced. As such, the modules may operate as depicted in  FIG. 15 , wherein the first unit pair  1300  operates at f 0  while the second unit pair  1350  operates at f 1 . Alternatively, the first unit pair  1300  may be advanced by a single step in the sequence and the second unit pair  1350  may be advanced further out of turn (e.g., by two frequencies). As such, a retransmission of data may occur such that no data collision occurs. Additionally, the first unit pair  1300  is now out of phase from the second unit pair  1350  with respect to the frequency sequence  1410 . 
     Furthermore, upon successive transmission periods, such as the one shown in  FIG. 16 , the first unit pair  1300  and second unit pair  1350  may advance at the same rate through the frequency sequence so as to stay out of phase with respect to the common frequency sequence  1410  shared between the first unit pair  1300  and the second unit pair  1350 . This may continue until the time period shown in  FIG. 17  wherein the first unit pair  1300  remains on the last frequency f n  in the frequency sequence  1410  while the second unit pair  1350  recommences from the beginning of the frequency sequence  1410 . In turn, once one of the unit pairs is advanced out of sequence different than the other unit pair, the unit pair will remain on different multiplexing signatures in the pseudorandom sequence and thus avoid data collisions. 
     In this regard, rather than the ongoing potential of data collisions when using a purely random multiplexing signature hopping technique, the foregoing allows the wireless acquisition units to hop to different multiplexing signatures without the potential for data collisions so long as the units in a given collision domain remain out of phase from one another. The manner in which unit pairs are set to be out of phase from one another may vary in different embodiments. For instance, the collision domain for each individual module may be manually analyzed and respective collision domains may be determined for each acquisition unit. In turn, acquisition units identified as belonging to a common collision domain may be set such that each unit in the collision domain initiates operation out of phase with respect to each other unit in the collision domain. Based on this information, the modules are manually set to be out of phase from one another. This manual setting of the modules out of phase from one another may occur at the deployment or set up of the modules. 
     Alternatively, a collision detection method may be employed such that the modules begin communicating according to the sequence. Only when a collision is detected do the two colliding modules act in accordance with the foregoing such that one of the modules is advanced out of phase from the other. This out of phase advancement of one module may result in the module advancing a single point or may randomly advance through the sequence to a different point than the other colliding module. As such, a learning protocol may be employed such that the modules in the collision domain identify a collision and learn to settle into the multiplexing signature sequence such that each module in the collision domain is out of phase. 
     One example of a process  1800  for performing such a dynamic multiplexing technique is shown in  FIG. 18 . The process  1800  may include assigning  1810  a multiplexing signature sequence to a first and a second module. The process  1800  may further include transmitting  1820  seismic data from the first and the second module using at least one multiplexing signature at each one of the first and second module. In this regard, the first and second module may operate using a common multiplexing signature. In this regard, a collision may be detected  1830 . Upon detection  1830  of a collision, the process  1800  may include offsetting  1840  one of the first or second modules such that the first and second module are out of phase from each other with respect to the multiplexing signature sequence as discussed above with regard to  FIGS. 15-17 . 
     After offsetting  1840  at least one of the first and second modules within the multiplexing signature sequence, the offset module may transmit  1850  and index number to a corresponding receiving unit. In this regard, the transmitting  1850  of the index number to the corresponding receiving unit may allow the corresponding receiving unit to similarly advance in the multiplexing signature sequence such that the receiving unit corresponding to offset module is in sync with the offset module. In this regard, the process  1800  may include modifying  1816  the receiving unit to operate at the same multiplexing signature sequence as the offset module. 
     In this regard, the process  1800  may include retransmitting  1870  data subject to the data collision with the first and second modules being out of phase within the multiplexing signature sequence. The process  1800  may include continuing to transmit  1880  data from the first and second module when the first and second modules are out of phase with respect to the multiplexing signature sequence. 
     An alternative embodiment of a process  1900  from deploying a dynamic multiplexing technique as shown in  FIG. 19 . The process  1900  may include manually offsetting a first and second module in an identified collision domain to be out of phase prior to the occurrence of a new transmission. In this regard, the process  1900  may occur at the start up or the deployment of the units such that there is no data transmission prior to the modules being manually set to be out of phase. In this regard, the process  1900  may include assigning  1910  a multiplexing signature sequence to the first and second module. The process  1900  may further include offsetting  1920  at least one of the first and second modules within the multiplexing signature sequence such that the first and second modules are out of phase from one another with respect to the multiplexing signature sequence. Again, the offsetting  1920  of the modules may be manually achieved based at least partially on identified collision domains within the array. 
     The process  1900  may further include transmitting  1930  and indexing number from a respective one of the first and second modules to corresponding receiving units. The transmitting  1930  of an indexing number may allow the transmitting and receiving unit to become synchronized such that corresponding receiving units are in synch with the first and second module with respect to the multiplexing signature sequence. The method  1900  may further include transmitting  1940  seismic data from the first module and the second module using a multiplexing signature sequence such that the first module and the second module are out of phase with respect to the shared multiplexing signature sequence. The process  1900  may continue transmitting  1950  from the first and second module such that the first and second modules remain out of phase and the occurrence of data collisions is reduced. 
     Based on the foregoing, it will be understood that for a given number of modules in a collision domain, there may be at least a corresponding number of unique multiplexing signatures such that each of the modules may in turn occupy a point in the sequence out of phase from the other modules. Based on this, the design of the survey may be modified such that on appropriate number of multiplexing signatures may be made available based on the number of modules in a given collision domain. 
     While described herein as using frequencies, the same principle may also be applied wherein the multiplexing signature sequence includes different codes applied to the data and transmitted using code division multiplexing. In this regard, the sequence may be pseudorandom may include a random code generator. 
     In addition, the wireless modules described herein may be location aware. In this regard, and as described in U.S. Pat. No. 7,773,457, the modules may have components that are operative to obtain location data for the module. A module which has obtained location data may transmit its location data to another module in the array which has yet to locate itself. In this regard, the location data transmitted from the location aware module to the locating module may be used by the locating module to acquire a location. For instance, when a GPS module is provided to obtain location data, ephemeris data of a wireless module which is used to acquire a GPS location may be provided to another module which is yet to acquire a location data from a satellite. In this regard, the ephemeris data may be used by the locating wireless module to obtain location data. This may improve the locating modules ability to require satellite location data. For instance, the time required to acquire a satellite fix may be reduced such that the modules may be deployed and located faster. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character. For example, certain embodiments described hereinabove may be combinable with other described embodiments and/or arranged in other ways (e.g., process elements may be performed in other sequences). Accordingly, it should be understood that only the preferred embodiment and variants thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.