I 1111111111111111 11111 111111111111111 IIIII 1111111111 111111111111111 IIII IIII
`US007660203B2
`
`c12) United States Patent
`Barakat et al.
`
`(IO) Patent No.:
`(45) Date of Patent:
`
`US 7,660,203 B2
`Feb.9,2010
`
`(54) SYSTEMS AND METHODS FOR SEISMIC
`DATA ACQUISITION EMPLOYING
`ASYNCHRONOUS, DECOUPLED DATA
`SAMPLING AND TRANSMISSION
`
`(75)
`
`Inventors: Simon Barakat, Chilly-Mazarin (FR);
`Kambiz Iranpour, Oslo (NO); Daniel
`H. Golparian, Oslo (NO)
`
`(73) Assignee: WesternGeco L.L.C., Houston, TX (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 242 days.
`
`(21) Appl. No.: 11/683,883
`
`(22) Filed:
`
`Mar. 8, 2007
`
`(65)
`
`Prior Publication Data
`
`US 2008/0219094 Al
`
`Sep. 11, 2008
`
`(51)
`
`Int. Cl.
`(2006.01)
`GOJV 1122
`(52) U.S. Cl. ........................................... 367/76; 367/63
`(58) Field of Classification Search ................... 367/59,
`367/63, 76
`See application file for complete search history.
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`6,002,640 A * 12/1999 Harmon ....................... 367/76
`6,044,453 A
`3/2000 Paver
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`4/2000 Paver
`6,055,620 A
`4/2000 Paver et al.
`6,148,392 A
`11/2000 Liu
`2002/0193947 Al
`12/2002 Chamberlain
`2003/0174582 Al
`9/2003 Scott
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`6/2004 Chamberlain et al.
`
`3/2005
`2005/0047275 Al
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`2005/0276162 Al
`2006/0009911 Al
`1/2006
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`2006/0018196 Al
`2006/0192625 Al
`8/2006
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`2006/0203614 Al
`9/2006
`2006/0215588 Al
`10/2006
`2006/0226916 Al
`11/2006
`2006/0251081 Al
`2007 /0025484 Al * 2/2007
`
`Chamberlain
`Brinkmann et al.
`Burkholder et al.
`Chamberlain
`Sorrells et al.
`Harmon
`Yoon
`Florescu et al.
`Choksi
`Laine et al. ................. 375/355
`
`OTHER PUBLICATIONS
`
`http:// apachepersonal .miun. se/-benoel/ asynch .htm.
`http://web.mit.edu/newso ffice/2 006/batteries-02 0 8 .html.
`http://www.javvin.com/.
`http://www.networkdictionary.com/wireless/WPAN.
`php?PHPSESSID~354101c49bc9d97659791 acaecddcal6.
`http:/ /www. tutorial-reports .com/wireless/wimax/tutorial. php.
`http://www.westerngeco.com/q-technology.
`
`* cited by examiner
`
`Primary Examiner-Ian J Lobo
`(74) Attorney, Agent, or Firm-Ari Pramudji; Richard V.
`Wells; Kevin McEnaney
`
`(57)
`
`ABSTRACT
`
`Systems and methods for asynchronously acquiring seismic
`data are described, one system comprising one or more seis(cid:173)
`mic sources, a plurality of sensor modules each comprising a
`seismic sensor, an AID converter for generating digitized
`seismic data, a digital signal processor (DSP), and a sensor
`module clock; a seismic data recording station; and a seismic
`data transmission sub-system comprising a high precision
`clock, the sub-system allowing transmission of at least some
`of the digitized seismic data to the recording station, wherein
`each sensor module is configured to periodically receive from
`the sub-system an amount of the drift of its clock relative to
`the high precision clock.
`
`11 Claims, 3 Drawing Sheets
`
`10
`
`12
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 1
`
`

`

`U.S. Patent
`
`Feb.9,2010
`
`Sheet 1 of 3
`
`US 7,660,203 B2
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 2
`
`

`

`U.S. Patent
`
`Feb.9,2010
`
`Sheet 2 of 3
`
`US 7,660,203 B2
`
`FIG. 2
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 3
`
`

`

`U.S. Patent
`
`Feb.9,2010
`
`Sheet 3 of 3
`
`US 7,660,203 B2
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`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 4
`
`

`

`US 7,660,203 B2
`
`1
`SYSTEMS AND METHODS FOR SEISMIC
`DATA ACQUISITION EMPLOYING
`ASYNCHRONOUS, DECOUPLED DATA
`SAMPLING AND TRANSMISSION
`
`BACKGROUND OF THE INVENTION
`
`1. Field oflnvention
`The present invention relates to the field of seismic data
`acquisition systems and methods of using same. More spe(cid:173)
`cifically, the invention relates to systems and methods for
`seismic data acquisition in which the seismic sampling is
`decoupled from data transmission using asynchronous digital
`signal processors for data sampling, and interpolation for
`synchronizing the sampling.
`2. Related Art
`Land seismic acquisition aims to capture the acoustic and
`elastic energy that has propagated through the subsurface.
`This energy may be generated by one or more surface sources
`such as vibratory sources (vibrators). The vibrators produce a
`pressure signal that propagates through the earth into the
`various subsurface layers. Here elastic waves are formed
`through interaction with the geologic structure in the subsur(cid:173)
`face layers. Elastic waves are characterized by a change in
`local stress in the subsurface layers and a particle displace- 25
`ment, which is essentially in the same plane as the wavefront.
`Acoustic and elastic waves are also known as pressure and
`shear waves. Acoustic and elastic waves are collectively
`referred to as the seismic wavefield.
`The structure in the subsurface may be characterized by 30
`physical parameters such as density, compressibility, and
`porosity. A change in the value of these parameters is referred
`to as an acoustic or elastic contrast and may be indicative of a
`change in subsurface layers, which may contain hydrocar(cid:173)
`bons. When an acoustic or elastic wave encounters an acous- 35
`tic or elastic contrast, some part of the waves will be reflected
`back to the surface and another part of the wave will be
`transmitted into deeper parts of the subsurface. The elastic
`waves that reach the land surface may be measured by motion
`sensors (measuring displacement, velocity, or acceleration, 40
`such as geophones, accelerometers, and the like) located on
`the land. The measurement of elastic waves at the land surface
`may be used to create a detailed image of the subsurface
`including a quantitative evaluation of the physical properties
`such as density, compressibility, porosity, etc. This is 45
`achieved by appropriate processing of the seismic data.
`Seismic sensor units typically also contain the electronics
`needed to digitize and record the seismic data. In one known
`embodiment, each sensor unit is connected to a land seismic
`cable, which is connected via cables to a recording instrument 50
`on a surface vehicle or other surface facility such as a plat(cid:173)
`form. The land seismic cable provides electric power and the
`means for transferring the recorded and digitized seismic
`signals to the recording instrument. In other embodiments,
`there have been efforts to reduce the use of cables in perform- 55
`ing land seismic, with movement toward wireless land seis(cid:173)
`mic systems and methods.
`Seismic sampling in a typical seismic sensor network
`(whether wired or wireless) may comprise up to tens of thou(cid:173)
`sands or more seismic sensors measuring the seismic vibra- 60
`tions for oil and gas exploration. Each sensor with an ana(cid:173)
`logue output has its output converted to a digital signal by an
`analog to digital converter (ADC) that is in turn connected to
`a digital signal processing (DSP) unit. Every sampling unit
`has its own clock frequency that drifts over time relative to the 65
`data transmission line clock that may assumed to be the
`master clock. The digital data is typically transmitted to a
`
`2
`centralized recording unit. The individual sampling ADC/
`DSP units are traditionally phase-synchronized to the data
`transmission line clock by an electronic phase-locked loop
`(PLL).
`While these systems and methods have enjoyed some suc(cid:173)
`cess, there remains room for improvement. It is of utmost
`important in seismic acquisition to phase synchronize the
`sampling of all the sampling units. However, presently known
`systems and methods are more expensive and less flexible due
`10 to the above-mentioned individual samplingADC/DSP units
`being phase-synchronized to the data transmission line clock
`by an electronic phase-locked loop. There is a need in the
`seismic data acquisition arts for systems and methods
`wherein the transmission of data is decoupled from sampling
`15 of the data, and that eliminate the costly and inflexible elec(cid:173)
`tronic phase locking loop, while still ensuring that the output
`sampling frequency of each signal processing unit is phase
`synchronized with the data transmission line clock. The
`present invention is devoted to addressing one or more of
`20 these needs.
`
`SUMMARY OF THE INVENTION
`
`In accordance with the present invention, systems and
`methods for seismic data acquisition are described which
`reduce or overcome short-comings of previously known sys(cid:173)
`tems and methods wherein the transmission of data is coupled
`to sampling of the data. Systems and methods of seismic data
`acquisition in accordance with the invention eliminate the
`costly and inflexible electronic phase locking loop. In the
`inventive systems and methods, the drift of each clock asso(cid:173)
`ciated with a seismic sensor is periodically measured and/or
`calculated relative to the data transmission line clock (which
`may be the master clock), and interpolation techniques are
`used to adjust for the sensor clock drift. In this way the output
`sampling frequency of each signal processing unit is phase
`synchronized with the data transmission line clock without
`the use of an electronic phase locked loop circuit. Systems
`and methods of the invention allow more efficient seismic
`data acquisition, for example 2-D, 3-D and 4-D land seismic
`data acquisition, such as during exploration for underground
`hydrocarbon-bearing reservoirs, or monitoring existing res(cid:173)
`ervoirs. Electromagnetic signals may be used to transfer data
`to and/or from the sensor units, to transmit power, and/or to
`receive instructions to operate the sensor units.
`A first aspect of the invention is seismic data acquisition
`system comprising:
`one or more seismic sources (which may be land sources,
`such as vibrators, explosive charges, and the like, or
`marine sources, such as air-guns, vibrators, and the like);
`a sensor system (which may be suitable for land seismic or
`marine seismic) for acquiring and/or monitoring analog
`seismic sensor data, the sensor system comprising a
`plurality of sensor modules each configured to asyn(cid:173)
`chronously sample seismic data and comprising a seis(cid:173)
`mic sensor, anA/D converter (ADC) for generating digi(cid:173)
`tized seismic data, a digital signal processor (DSP), and
`a sensor module clock;
`a seismic data recording station, and
`a seismic data transmission sub-system comprising a high
`precision clock, the sub-system allowing the DSP to
`transmit at least some of the digitized seismic data to the
`recording station,
`wherein each sensor module receives periodically from the
`sub-system an amount of the drift of its clock relative to the
`data transmission line high precision clock.
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 5
`
`

`

`US 7,660,203 B2
`
`15
`
`3
`Alternatively, in certain system embodiments of the inven(cid:173)
`tion, rather than each sensor module periodically receiving an
`amount of drift from the data transmission sub-system, each
`DSP may periodically receive information from outside the
`system (for example via GPS) to calculate its clock drift. In 5
`yet other system embodiments, both techniques may be
`employed.
`Systems of the invention may comprise each DSP in the
`sensor system up sampling data at a particular fixed sampling
`rate relative to the high precision clock. The data is 10
`upsampled using a linear or nonlinear interpolation tech(cid:173)
`nique, based on the amount of drift of each sensor module
`clock relative to the data transmission line high precision
`clock, to increase its effective sampling rate. The data may
`then be decimated (downsampled to a fixed sampling fre-
`quency) relative to the high precision clock. The period
`between intermittent adjustments of the sampling frequency
`to the high frequency clock may be determined based on the
`nominal drift of the sensor module clocks, for example 50
`parts per million (ppm), and the level of noise allowed in the 20
`system.
`Optionally, the data transmission sub-system allows trans(cid:173)
`mission of data to one or more base stations, which in turn
`transmit at least some of the data they receive to the recording
`station, which may be advantageous in wireless systems and 25
`methods of the invention. Wireless versions of systems of the
`invention may be characterized as comprising a wireless data
`network, wherein the wireless data network comprises the
`seismic sensors transmitting at least a portion of the data to
`one or more base stations via first wireless links which in turn 30
`transmit at least some data they receive to the recording
`station via second wireless links (for a completely wireless
`system), or through cables, wires, or optical fibers in other
`embodiments (partially wireless). Also as further explained
`herein, the recording station need not be on land, and need not 35
`be immobile. For example, the recording station may be
`selected from a stationary land vehicle, a moving land
`vehicle, a stationary marine vessel, a moving marine vessel,
`and a moving airborne vessel, such as a helicopter, dirigible,
`or airplane.
`Base stations, if used in wireless or partially wireless sys(cid:173)
`tems, may be located strategically to cover predefined groups
`of sensor modules. In these embodiments, each group of
`sensor modules may relay data wirelessly via a mesh topol(cid:173)
`ogy and/or in a hop to hop fashion (also referred to herein as 45
`multi-hopping). Star topologies and other topologies may
`also be used, but mesh topology will produce the greatest
`redundancy. Between each base station and the data recording
`station (for example recording truck), seismic data may be
`transferred directly from base station to recording station. 50
`Sensor modules may be spaced relatively close together in
`systems of the invention, for example a distance ranging from
`1 meter up to about 10 meters. Because of the relatively short
`distance between sensor modules, multi-hopping may cir(cid:173)
`cumvent the potential wireless communication (RF, micro- 55
`wave, infra-red) problems in uneven terrain, or terrain includ(cid:173)
`ing man-made obstacles. It is known that for transmitting data
`wirelessly between points A and B separated by a large dis(cid:173)
`tance, relaying between multiple spots between A and B will
`consume less energy compared to direct wireless communi- 60
`cation between points A and B.
`Systems within the invention include those comprising a
`first wireless link that wirelessly transmits seismic data
`sampled from a seismic sensor to a base station (which may
`be a mobile or non-mobile communication device), the base 65
`station having a second wireless link that receives the seismic
`data from the sensor modules and wirelessly transmits the
`
`40
`
`4
`seismic data to the land seismic data recording station, the one
`or more vibrators having a third wireless link that receives
`commands from the land seismic data recording station and
`wirelessly transmits vibrator data (such as status information)
`to the land seismic data recording station. As used herein the
`term "mobile", when used to describe a device, includes
`hand-held devices and devices that may be worn on the body
`of a person, for example on a belt, in a pocket, in a purse, and
`the like. It is not meant to include objects that may in fact be
`moved, but only with great effort, such as a building or shed,
`or with less effort a desk top computer.
`In certain system embodiments the first wireless link may
`be selected from any wireless personal area network (WPAN)
`communication protocol. The second and third wireless links
`may be individually selected from any wireless communica(cid:173)
`tion protocol that supports point to multi-point (PMP) broad(cid:173)
`band wireless access. These protocols may include, but are
`not limited to IEEE standard 802 .16 ( sometimes referred to as
`the WiMax (Worldwide Interoperability for Microwave
`Access) standard), IEEE standard 802.20, and the like. The
`second and third wireless links may use the same or different
`protocols.
`Certain land seismic data acquisition systems of the inven(cid:173)
`tion may utilize wireless links and equipment allowing broad(cid:173)
`casting of messages ( audio, video, alphanumeric, digital, ana(cid:173)
`log, and combinations thereof) between sensor modules,
`vibrators, base stations, and the recording station, or simply
`between the sensor modules. The messages may be time
`tagged and used for distance measure and clock calibration.
`The communication network may also be used for transmis(cid:173)
`sion of status information and/or quality control (QC).
`A second aspect of the invention comprises methods of
`acquiring seismic data during a seismic survey, including
`time-lapse (4-D) seismic data acquisition, one method com(cid:173)
`prising:
`a) initiating one or more seismic sources;
`b) asynchronously acquiring reflected analog seismic data
`using a sensor system, the sensor system comprising a
`plurality of sensor modules each comprising a seismic
`sensor, anA/D converter (ADC) for generating digitized
`seismic data, a digital signal processor (DSP), and a
`sensor module clock;
`c) transmitting at least some of the digitized seismic data to
`a data recording station via a data transmission sub(cid:173)
`system comprising a high precision clock having a high
`precision clock frequency; and
`d) correcting clock frequency drift of each sensor module
`clock relative to the data transmission line high precision
`clock frequency, and each sensor module receiving peri(cid:173)
`odically from the sub-system an amount of the drift ofits
`clock relative to the data transmission line high precision
`clock.
`Other methods of the invention include passive listening
`surveys (where no vibratory source is used) and electromag(cid:173)
`netic (EM) surveys, where one or more of the sensor units
`comprises one or more EM sensors.
`As used herein, "survey" refers to a single continuous
`period of seismic data acquisition (which may occur simul(cid:173)
`taneously, sequentially, or with some degree of time overlap),
`over a defined survey area; multiple surveys means a survey
`repeated over the same or a same portion of a survey area but
`separated in time (time-lapse, sometimes referred to herein as
`4-D seismic). In the context of the present invention a single
`seismic survey may also refer to a defined period of seismic
`data acquisition in which no controlled seismic sources are
`active (which also may be referred to alternatively as passive
`seismic listening or micro seismic measurements).
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 6
`
`

`

`US 7,660,203 B2
`
`5
`Systems and methods of using systems of the invention
`allow more efficient data acquisition (including time-lapse)
`than previously known systems and methods. These and other
`features will become more apparent upon review of the brief
`description of the drawings, the detailed description of the
`invention, and the claims that follow.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The manner in which the objectives of the invention and
`other desirable characteristics may be obtained is explained
`in the following description and attached drawings in which:
`FIG. 1 illustrates a simplified plan view of a system of the
`invention;
`FIG. 2 illustrates schematically wireless communication
`between components of systems of the invention;
`FI GS. 3-4 illustrate schematically prior art communication
`topologies useful in practicing systems and methods of the
`invention; and
`FIG. 5 illustrates the protocol structure of the IEEE 802.16
`Broadband wireless MAN standard.
`It is to be noted, however, that the appended drawings are
`not to scale and illustrate only typical embodiments of this
`invention, and are therefore not to be considered limiting of
`its scope, for the invention may admit to other equally effec(cid:173)
`tive embodiments.
`
`DETAILED DESCRIPTION
`
`In the following description, numerous details are set forth
`to provide an understanding of the present invention. How(cid:173)
`ever, it will be understood by those skilled in the art that the
`present invention may be practiced without these details and
`that numerous variations or modifications from the described
`embodiments may be possible.
`As noted in the literature and on the Internet (see for
`example, the website of Prof. Bengt Oelmann, Mid-Sweden
`University, Sundsvall, Sweden, http://apachepersonal.mi(cid:173)
`un.se/-benoel/asynch.htm, accessed Dec. 10, 2006) digital
`designs can be divided into synchronous and asynchronous 40
`circuits. The common timing reference called clock signal
`defines the synchronous designs. Consequently, asynchro(cid:173)
`nous designs are those without a common timing reference. In
`the early days of digital design, design methodologies were
`not established and combinations of synchronous and asyn(cid:173)
`chronous techniques were used. From the 1960s to the
`present, the usage and the development of synchronous cir(cid:173)
`cuits and methods achieved almost total dominance. When
`computers were first constructed, a few of them were fully
`asynchronous. Two examples are ORDVAC from the Univer- 50
`sity of Illinois (19 51-52) and later, MU 5 from their University
`of Manchester (1969-7 4 ). But asynchronous techniques have
`later found their applications in places where synchronous
`techniques are not feasible. A typical example is high-speed
`communication over long distances, such in computer bus 55
`systems. UNIBUS in PDP-11 (1969) and VMEBUS (1980)
`are examples of such asynchronous buses.
`The growing complexity ofICs makes clock distribution in
`synchronous designs more costly to design in terms of power
`consumption, area, and design effort. The clock distribution 60
`problem has made asynchronous design techniques a viable
`alternative. Some of the research carried out in the field of
`asynchronous design searches for ways to utilize these advan(cid:173)
`tages for solving real-world problems. Most of the work in
`this area is carried out at universities, but there is also some 65
`research in industry. For example, Sun MicroSystems Labs
`proposed a new processor architecture known under the trade
`
`5
`
`6
`designation "Counterflow Pipeline Processor", and Philips
`Research Labs has focused on designing low-power I Cs using
`automatic synthesis of asynchronous circuits. In recent years
`a mixed synchronous/asynchronous approach, commonly
`referred to as Globally Asynchronous-Locally Synchronous
`(GALS), has been advocated (according to Oelmann). The
`basic idea is to have a local clock for each module on the chip
`and to have asynchronous communication between the syn(cid:173)
`chronous modules. When considering other complex digital
`10 systems based on multiple ICs and PCBs, this seems to be a
`natural development for very complex I Cs.
`The systems and methods of the present invention offer one
`or more of the possible advantages discussed by Oelmann,
`and, as noted by at least this reference, asynchronous design
`15 methods differ significantly from the methods that are cur(cid:173)
`rently used. Some of the possible advantageous of asynchro(cid:173)
`nous systems are the following:
`Average case performance: In synchronous systems, the
`slowest combinational path defines the maximum clock fre-
`20 quency. This leads to worst-case performance for all opera(cid:173)
`tions independently on the data. Asynchronous data-paths are
`designed to indicate when computation is completed. The
`computation time for many operations is very data dependent
`and this property can be exploited in cases where the worst-
`25 case delay is much larger than the average delay.
`No or reduced clock skew problems: In a synchronous
`system, the differences in arrival time of the clock signal to
`different parts of the system must be controlled. Clock skew
`affects speed performance and may also cause malfunction-
`30 ing due to race conditions. The cost of maintaining low clock
`skew becomes higher when the complexity of the IC
`increases. Asynchronous circuits do not have a global clock
`signal and clock skew is therefore not a problem.
`Low power consumption: Only active parts of a CMOS
`35 design dissipates power in CMOS. In a synchronous system,
`the clock signal is still active in the idle parts. The event(cid:173)
`driven nature of asynchronous designs leads to the fact that
`only the parts of the design that actually take part in the
`computation are dissipating power.
`Low noise: Simultaneous switching in CMOS leads to high
`current transitions in the power lines. In a synchronous sys(cid:173)
`tem, the charge and discharge of the clock net is a large
`contributor to the current transitions. Most of the switching in
`the gates occurs shortly after the active clock edge. This
`45 makes the total current concentrated to the time of the active
`clock edge. The fast current transitions cause fluctuations on
`the power supply lines that may cause lowered speed perfor(cid:173)
`mance or malfunctioning of the digital logic. In a mixed
`analog/digital system, the digital noise may affect sensitive
`analog circuits. Asynchronous circuits are not synchronized
`and the current is more uniformly distributed in time.
`Modularity: In asynchronous modules, both timing and
`functionality may be located inside the module. From the
`user's point of view, only the sequence of operations is impor(cid:173)
`tant when using the module. Incremental upgrading of the
`performance of the asynchronous system only requires the
`replacement of the module that is limiting the performance,
`without having to change or retime the system in any other
`way.
`Scalability: In general, a digital system consists of different
`parts implemented in different technologies and these may be
`communicating over different types of media. Different types
`of design techniques are then used for different types of
`implementation technologies. A typical scenario is as fol(cid:173)
`lows: Inside the IC, a high-speed global clock signal is used
`that is generated from a phase-locked loop (PLL), which is
`synchronized to a slower external clock signal. Communica-
`
`Petitioners Microsoft Corporation and HP Inc. - Ex. 1037, p. 7
`
`

`

`US 7,660,203 B2
`
`7
`tion between the ICs on the same PCB is synchronized to the
`slower clock. Board-to-board communication is handled by
`an asynchronous standard bus system (such as VMEBUS).
`Crossing the boarders of implementation technologies makes
`it necessary to introduce new design techniques. By using 5
`asynchronous circuits from the beginning, it is possible to
`keep the same design technique throughout the system
`design.
`As discussed in U.S. Pat. No. 6,049,882, "synchronous"
`systems apply a fixed time step signal (i.e., a clock signal) to 10
`the functional units to ensure synchronized execution. Thus,
`in synchronous systems, all the functional units require a
`clock signal. However, not all functional units need be in
`operation for a given instruction type. Since the functional
`units can be activated even when unnecessary for a given 15
`instruction execution, synchronous systems can be ineffi(cid:173)
`cient.
`The use of a fixed time clock signal (i.e., a clock cycle) in
`synchronous systems also restricts the design of the func(cid:173)
`tional units. Each functional unit must be designed to perform 20
`its worst case operation within the clock cycle even though
`the worst case operation may be rare. Worst case operational
`design reduces performance of synchronous systems, espe(cid:173)
`cially where the typical case operation executes much faster
`than that of the worst case criteria. Accordingly, synchronous 25
`systems attempt to reduce the clock cycle to minimize the
`performance penalties caused by worst case operation crite(cid:173)
`ria. Reducing the clock cycle below worst case criteria
`requires increasingly complex control systems or increas(cid:173)
`ingly complex functional units. These more complex syn- 30
`chronous systems reduce efficiency in terms of area and
`power consumption to meet a given performance criteria such
`as reduced clock cycles.
`In asynchronous seismic data acquisition systems and
`methods of the invention, performance penalties only occur in 35
`an actual (rare) worst case operation, and the inventive sys(cid:173)
`tems and methods may be tailored for typical case perfor(cid:173)
`mance, which can result in decreased complexity for proces(cid:173)
`sor
`implementations
`that achieve
`the performance
`requirements. Further, because asynchronous systems only 40
`activate functional units when required for the given instruc(cid:173)
`tion type, efficiency is increased. Thus, the asynchronous
`seismic data acquisition systems and methods of the inven(cid:173)
`tion may provide increased efficiency in terms of integration
`and power consumption.
`A first aspect of the invention is seismic data acquisition
`system comprising:
`one or more seismic sources (which may be land sources,
`such as vibrators, explosive charges, and the like, or
`marine sources, such as air-guns, vibrators, and the like);
`a sensor system (which may be suitable for land seismic or
`marine seismic) for acquiring and/or monitoring analog
`seismic sensor data, the sensor system comprising a
`plurality of sensor modules each configured to asyn-
`chronously sample seismic data and comprising a seis(cid:173)
`mic sensor, anA/D converter (ADC) for generating digi(cid:173)
`tized seismic data, a digital signal processor (DSP), and
`a sensor module clock;
`a seismic data recording station, and
`a seismic data transmission sub-system comprising a high
`precision clock, the sub-system allowing the digital sig(cid:173)
`nal processor to transmit at least some of the digitized
`seismic data to the recording station,
`wherein each sensor module clock has a clock frequency
`that drifts over time relative to a data transmission line high
`precision clock frequency, and each sensor module receives
`
`8
`periodically from the sub-system an amount of the drift of its
`clock relative to the data transmission line high precision
`clock.
`Alternatively, in certain system embodiments of the inven(cid:173)
`tion, rather than each sensor module periodically rece