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`ETOCENO15628327
`World oil.
`GULF PUBLISHING CO
`1994
`Volume:
`4
`
`215
`
`Author name(s): Risch, D. L. Chowdhury, A. N. Hannan, A.
`
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`WesternGeco Ex. 1008, pg. 1
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`WesternGeco Ex. 1008, pg. 1
`
`

`

`new modern techniques
`improve seismic interpretation
`
`Part 1—Seismic interpretation has evolved from
`basic 2-D structural mapping of vague anticlines
`observed on single seismic trace paper records
`in the 19205 to sophisticated stratigraphic mapping
`with lateral resolution from 3-0 data
`
`David L. Hisch, Abu N.
`Chowdhury, Andrew E. Harman,
`Project Geophysicists, and George
`A. mm. Senior Geophysicist,
`Geco-Prakla. Houston. Texas
`
`Reflection seismology was first
`applied to hydrocarbon exploration
`in the 19208 and today is an integral
`part of oil and gas business. As tech-
`nology evolves, more information is
`derived from seismic and used in
`many interpretation modes. Inter-
`preter demands are also becoming
`greater as more data of higher quality
`is available for incorporation into
`integrated interpretation.
`Part 1 of this article describes the
`planning involved for seismic surveys,
`along with current methods and equip-
`ment used for acquisition, processing,
`display and interpretation of seismic
`data. Major points covered include:
`0 Interpretation objectives
`0 Seismic acquisition, processing
`and display
`0 Structural interpretation organi-
`zation and procedure.
`
`BACKGROUND
`
`Acquiring continuous datasets
`along traverses provides a vast
`amount of data compared to single
`data points given by well control. It
`has been used for years in exploration
`and, with the advent of 3-D seismic,
`field development use has increased.
`Seismic data was first used to map
`subsurface geologic features.
`Seismic signatures visible as direct
`hydrocarbon indicators (DHI), such
`
`ing closely-spaced, high—quality/fre—
`quency (resolving power) 3-D seis-
`mic lines, data from many wells,
`petrophysical measurements, pro-
`duction histories and extensive mod-
`eling. Geological models are essen-
`tial to begin interpretation and are
`based on literature and personal
`experience. Models are used initially
`but interpretation comes from work-
`ing with data.
`
`SEISMIC METHOD
`
`Exploration reflection seismology
`uses induced acoustic reflections of
`rock layers. Vibrations are generated
`in the earth with acoustic sources, and
`reflections are recorded with receivers.
`Reflection intensity depends on veloc-
`ity and density contrasts of rock lay-
`ers and contained pore fluids.
`
`Acquisition. Data acquisition begins
`by laying out seismic programs with
`parameters designed for geologic objec~
`tives based on study—area under-
`standing. Parameters include: seismic
`frequency for vertical and horizontal
`resolution (horizontal resoulution also
`depends on source, receiver and line
`spacing); depth to target (distance
`between source and farthest receiver,
`i.e.. far offset); rock layer dip; and envi-
`
`as bright spots and AVO (amplitude
`variation with offset), are important
`tools. Seismic sequence stratigraphy
`adds another dimension to explo-
`ration with an understanding of geo-
`logic depositional process.
`
`INTERPRETATION OBJECTIVE
`
`Geological interpretation begins
`by defining problem scope and area.
`Studies range from large to small
`scale, i.e., from regional to reservoir.
`Techniques and data types vary
`depending on the problem size. For
`example, basin and regional studies
`involve interpreting large scale fea—
`tures from a few widely spaced seis-
`mic lines, and from gravity and mag-
`netic data, often with little or no well
`control. Objectives are to obtain
`regional overviews and identify
`anomalies that warrant further study.
`In contrast, detailed reservoir
`characterization requires integrat—
`
`
`
`Fla. 1. Simplified diagram of seismic princrple used in exploration,
`
`WORLD OIL ! APRIL 1994 B5
`
`WesternGeco Ex. 1008, pg. 2
`
`WesternGeco Ex. 1008, pg. 2
`
`

`

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`
`ronmental, cultural and geo-
`logical conditions.
`Most marine acquisition
`sound sources are air guns
`that repeatedly displace
`water volumes. On land,
`sources are explosives or
`truck mounted vibrators.
`It is common to use syn-
`chronized source arrays to
`increase, or focus, energy
`at each shot. Vibrators can
`generate more high-fre-
`quency energy, compensat-
`ing for the earth‘s natural
`attenuating frequencies.
`Geophones are land receiv-
`ers that detect slight ground
`movements. Marine receiv-
`ers are pressure sensitive
`devices called hydrophones.
`Each receiver converts pres-
`sure or ground disturbances
`to electrical impulses. The
`digitally recorded electrical
`pulses of an array or group
`of receivers are summed for
`each station and transmit-
`ted, via cable or telemetry,
`to recording computers.
`Electric response am-
`plitude is directly propor-
`tional to reflection strength.
`Increases or decreases in
`acoustic impedance (veloc-
`ity X density) across reflec-
`tors are displayed as peaks
`or troughs on seismic wave-
`form displays. Receivers
`record reflections every 2
`msec for a chosen time
`
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`
`Hg. 2. CMP gathers with hyperbolic move out (a), and velocity cor—
`rected to horizontal alignment tor stacking. (II).
`
`(commonly 4—10 sec) resulting in a
`well sampled vertical seismic record
`called a trace.
`
`In the seismic principle example,
`Fig. l. the Horizon 1 reflection
`results from an impedance contrast
`between Layers 1 and 2; likewise for
`Reflection 2 emanating from Hori-
`zon 2. Ray paths are described by
`Snell’s Law and bend at each layer
`interface (horizon). Subsurface hori-
`zons are imaged at midpoints
`between source and receiver. These
`points are imaged repeatedly by
`source-receiver pairs as shooting pro-
`gresses to each consecutive line loca-
`tion. A common midpoint (CMP)
`gather is a collection of all combina-
`tions of source-receiver pairs which
`records energy from the same mid-
`point location. therefore containing
`travel paths from near to far offset
`traces. This redundancy increases
`
`the signal to noise ratio when traces
`are processed and summed.
`
`Prue-calm. Stored reflection elec-
`trical pulses are sorted and merged
`with geographic location data to cre-
`ate CMP gathers. Reflection travel
`time increases with receiver offset
`distance and shows a hyperbolic
`geometry called move-out, Fig. 2a.
`Move-out is corrected by selecting a
`velocity that aligns reflectors hori-
`zontally so they can be summed to
`one average output trace. Layer veloc—
`ities are picked by computer algo-
`rithms and human evaluation. Veloc.
`ity analysis is critical to processing
`and may be done more than once to
`fine-tune seismic images.
`Land data frequently requires an
`additional static adjustment which
`accounts for varying travel times
`through near-surface layers. Fig. 2b
`
`WesternGeco Ex. 1008, pg. 3
`
`WesternGeco Ex. 1008, pg. 3
`
`

`

`shows corrected traces of a CMP
`gather ready for summing. Trace
`summing process is called stacking
`and final output is a seismic section
`or line. A final process, migration, is
`used to reposition misplaced events
`associated with dipping interfaces
`and collapse diffractions caused by
`discontinuities (faults).
`
`Display. Horizontal scale of seismic
`sections is measured in shot-point
`number. corresponding to seismic
`basemaps. The vertical scale is 2-way
`seismic travel time in sec. A typical
`seismic section has a grid of vertical
`lines every 20th shot-point and hori-
`zontal lines every 0.01 sec. A sidelabel
`provides line number and informa-
`tion about acquisition, processing,
`and display parameters. Data is dis-
`played at various scales and trace
`gains depending on the exploration
`objective.
`Reflections recorded as electrical
`
`pulses are small and must be ampli-
`fied to be seen. The processor has a
`variety of gain functions to compen-
`sate for energy loss with depth. One
`gain function identifying amplitude
`or reflection anomalies preserves rel—
`ative amplitudes of all trace reflec-
`tions so largest reflections are eas-
`ily visible. Data can also be used with
`an interactive workstation. Coffeen
`(1986) offers a review of the seismic
`exploration method.1
`
`
`
`STRUCTURAL INTERPRETATION
`Structural interpretations have
`been the foundation for prospect
`development and usually precede
`stratigraphic analyses. Structural
`interpretation of seismic data involves
`organization of data, selecting hori-
`zons to map, integration of well and
`seismic data, loop-tieing seismic sec-
`tions, transferring interpretation to
`basemaps and contouring time val-
`ues. A final step sometimes involves
`depth-converting seismic time maps
`to remove false structures caused by
`abrupt lateral velocity changes.
`
`The database. First steps in seismic
`interpretation, after exploration objec—
`tives are defined, are to collect and
`organize required data. A seismic base
`map is generated and inspected for
`locations and orientations of seismic
`lines and well locations.
`Well data consists of checkshot sur-
`vey and sonic log velocity data; rock
`cores and well logs to predict litholo-
`
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`WORLD OIL / APRIL 1994 89
`
`WesternGeco Ex. 1008, pg. 4
`
`WesternGeco Ex. 1008, pg. 4
`
`

`

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`Flg. 3. Tieing well and seismic data example using a synthetic seismogram Shifting the synthetic is com-
`mon to obtain optimum character match With the seismic data. Microtossiis for dating are in italics.
`
`
`
`1 ,680
`
`ups...
`
`
`imaging, structural complexities, and
`frequency content (determines thick-
`ness of beds).
`
`Procedure. Mapping horizons are
`selected in correspondence with known
`productive reservoir rocks observed on
`well logs. Seismic characteristics
`(reflection amplitude and continuity)
`and depositional environments asso-
`ciated with reservoir rocks are noted
`for future reference as analogs.
`Well data constrains seismic inter-
`pretation, so well trajectories are pro—
`jected onto the nearest seismic line.
`Well data in depth is converted to
`seismic 2-way time and plotted along
`the seismic-line well trace using syn-
`thetic seismograms or checkshot
`velocities. Microfossil datums or
`extinctions are plotted on the seismic
`data for age dating and correlating
`from one well, or area, to another, Fig.
`3. This is important where complexi-
`ties exist. Correlated well logs pro
`vide more information to aid seismic
`interpretation where fossil data is
`lacking.
`Interpretation begins at well con—
`trol-data, and well information is
`extended by following selected seis—
`mic reflections. Horizons are distin-
`guished by color-coding chosen reflec-
`tions. The least complex structural
`path is chosen to establish a frame-
`work of interpreted lines incorpo-
`rating all available well data. The
`seismic grid of intersecting lines pro-
`vides numerous pathways minimiz-
`ing ambiguous correlations across
`faults and other features.
`Each line intersection is a tie point
`with common reflections on both
`lines. When paper sections are used,
`one seismic line is folded with a ver-
`tical crease at the intersection and
`placed on the intersecting line so sur-
`face datums match (sea level or a pre-
`determined elevation common to the
`seismic data grid). Reflections should
`match if structural dip is nearly flat,
`or compensation is made. Interpreted
`horizons are identified on the new
`line and tracked to the next inter-
`section. If done properly, a loop of
`interpretation results in horizons
`linking back to the first tied inter-
`section. Workstations (see Part 2) dis-
`play loops as a single image.
`Fault cuts are interpreted at off-
`sets in reflections and tied at grid
`intersections. Fault points are con-
`nected in a trend honoring reflec-
`tion offsets observed on the tie line,
`
`Fla. 4. Seismic lines iieing at an intersection. Seismic reflectors, horizons and faults match at the tie.
`
`gies and fluids; and rock samples with
`microscopic fossils for age dating. Well
`data, in depth, is converted to 2-way
`time using synthetic seismograms,
`checkshots or vertical seismic pro-
`files, (VSPs).
`A synthetic seismogram is a con-
`tinuous vertical record that looks like
`a seismic trace but is derived from
`sonic and density logs, Fig. 3. Check-
`shot surveys are collections of several
`
`90 WORLD OIL/APRIL 1994
`
`pairs of travel-times and known depths
`within wells. A VSP is similar to sur-
`face seismic sections but has receivers
`in the well instead of along the sur—
`face; it is the best way to convert
`between seismic and well data.
`Seismic data is sorted into dip
`lines (acquired in structural dip
`direction) and strike lines (perpen—
`dicular to dip lines). Lines are exam—
`ined for quality, depth of reliable
`
`WesternGeco Ex. 1008, pg. 5
`
`WesternGeco Ex. 1008, pg. 5
`
`

`

`locations
`
`Seismic shot
`
`Fig. 5. Seismic structure map 0! interpreted horizon. Three faulted anticlines (highs) are potential
`hydrocarbon traps.
`
`Fig. 4. Loop-tieing is done repeat-
`edly until the seismic grid is inter-
`preted, working from least to the
`most complex areas.
`Mapping follows completion of
`horizon interpretation on the seis-
`mic grid of data. Seismic 2-way
`times measured from surface, or ref-
`erence datum, to interpreted hori-
`zons are transferred to seismic base
`maps by hand, or computer if digi~
`tized. Fault cuts are also posted and
`mapped according to their trends
`determined from interpretation.
`Structural highs (anticlines) and
`lows (synclines) are marked on maps
`to guide contouring. Equal seismic
`time values are connected with a
`curved line (isotime contour) to
`depict subsurface structural config-
`uration, Fig. 5. Contour lines ter~
`minate at faults but contour trends
`continue on the other side of faults.
`Workstations post data to maps
`automatically.
`Seismic time-structure maps are
`
`92 wow on. I APRIL 1994
`
`usually depth converted because
`abrupt lateral velocity variations
`will give false time structures that
`may look like promising traps.
`Depth conversion is performed using
`velocity control (preferably grids)
`from surface down to horizons being
`converted. If structural closures,
`highs against faults or anticlines,
`remain after depth conversion, then
`prospects are high graded; particu-
`larly if they have seismic attributes
`similar to those observed at a pro-
`ductive well.
`
`COMING NEXT
`The second half of this seismic
`
`interpretation overview will discuss
`and illustrate additional modern
`methods used in seismic data inter-
`
`pretation, including direct hydro-
`carbon indicators; sequence strati-
`graphic interpretation models,
`techniques and maps; and interac-
`tive workstation techniques, aids
`and data access.
`no
`
`
`
`
`The authors
`David L. Risch, a project
`geophysicisr with Good
`Prakla's Exploration Ser-
`vices Group in Houston,
`has a BS in geology and
`physics from University of
`Wisconsin at Eau Claire
`and an MS in geophysical
`oceanography from Texas A&M University.
`He worked for Phillips for six years in data
`processing and interpretation of dalasels
`from the GOM, South China Sea and Nor-
`wegian See. Since joining Geco~PraI<Ia In
`1988. he has worked offshore Chile and
`GOM as a structural interpreter and
`sequence stratigrapher on 2-D and 3-D seis-
`mic datasers.
`
`Abu N. Chowdhury joined
`Geco-Prak/a, Houston, as
`a project geophysicist in
`1991, He has BS and MS
`degrees in geology from
`I.I.'lZ (India) and an MS in
`geophysics from the Uni-
`versity of Houston. He has
`15 years of worldwide
`industry experience with
`The Analysts, Fairfield. Sohio and T63. He is
`currently doing regional sequence strati-
`graphic interpretations on non-exclusive seis-
`mic grids ln the GOM.
`
`
`
`
`
`Andrew E. Harman, pro-
`ject geophysicisl. Geco-
`Prakla, Houston, has 85
`and MS degrees in geoI.
`agy from the University of
`Cincinnati. He has 20
`years in the petroleum
`industry in the GOM hav-
`ing worked With Cities Ser~
`vice. Kerr-McGee and
`Mark Producing. Hannan joined Geco-
`Prakla, Houston.
`in 1989 and works pri-
`marily on GOM structural and sequence
`stratigraphic interpretations.
`George A. Jamieson,
`senior geophysicist, Geco-
`Prakla. has a BS in geol—
`ogy from Sheffield Unl-
`vorslty, England, and an _
`MS In petroleum geology g
`from Imperial College.
`London. He was a wellsite
`geologist In the North Sea
`and Persian Gulf, and per-
`formed geophysical studies in Angola,
`Ghana and Gabon. Joining Geco-Prskle in
`London in 1986, he worked with 2.0 and 3-
`D seismic data in the North Sea, Egypt.
`Cameroon and, since 1991, has been
`assigned to GOM and Venezuelan projects
`in Houston.
`
`
`
`ACKNOWLEDGMENTS
`The authors thank Geco-Prakla for per—
`mission to show seismic data, and Karen El-
`T‘awil and Jerry Watson for their support and
`review of the paper. We also thank Jack Cald»
`well and Mark Chang for the paper review,
`Kathy Lee for figure drafting and Sandy
`Abrams for typing.
`
`LITERATURE CITED
`' (‘ofl‘wnnl A..S-Imniu Explumlum Fundunwnlnls. l‘mlnwvll
`l’uhlishinu Cu. “iii-a. Okla, l986, p. 347.
`
`WesternGeco Ex. 1008, pg. 6
`
`WesternGeco Ex. 1008, pg. 6
`
`

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`DEPARTMENTS
`22 Meetings
`117 People in industry
`123 Companies
`127 New equipment
`138 New literature
`145 Classified advertising
`146 Advertisers' index
`
`About the cover: HZ-26 oil field
`in the South China Sea, dis(cid:173)
`covered by the ACT Group
`(Agip, Chevron, Texaco), cur(cid:173)
`rently produces 40,000 bopd
`from 15 wel/s. The platform,
`shown here on a hazy day, is
`located in 395 ft of water. Photo
`courtesy Agip.
`
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`vided that they are in a position to recom(cid:173)
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`tions at the following rates. Subscriptions in
`United States, Canada and Latin Amerlca,
`one year $24; a/l other countries, one year
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`WOfl/dOi/®
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`__ r;;n1
`
`APRIL 1994 • VOL. 215 NO.4
`
`SPECIAL FEATURE
`39 OlC '94
`A preview of the technical program for the 26th Annual Offshore Technology
`Conference, to be held in Houston, May 2-5 .
`
`\
`
`.
`
`FEATURES
`45 Gullfaks development provides challenges
`S. Tollefsen, E. Graue and S. Svinddai
`.... ,,~ft~~ Iftl!li-' • ifI-' -mi.,n
`Part 1-History, future perspective and proposed fie
`of 570,OOO-bopd field with complicated reservoir geolog
`H
`57 Advanced composites enhance coiled tubing capabilities
`A. Sas-Jaworsky and J. G. Williams
`How Conoco's development of continuous pipe made from high-performance
`fibers improves on performance of steels under certain load conditions
`71 Subsea intervention modules offer diverless solutions
`D. Langrock
`New diverless ROV intervention system operating on the farm-tractor
`and implement concept performs wellhead/tree work in 3,OOO-ft water
`75 Blowout control: Response, intervention and management
`P. Smestad, O. B. Rygg and J. W. Wright
`Part 5-Hydraulic modeling. Matching known downhole well information
`with blowout surface flow helps select most efficient kill method
`85 How modern techniques improve seismic interpretation
`D. L. Risch, A. N. Chowdhury, A. E. Hannan and G. A. Jamieson
`Part 1-Planning for seismic surveys, plus overview of current methods
`for acquisition, processing, display and data interpretation
`95 Colombia prospects still hot
`J. Roeiofsen
`Status of petroleum activity and exploration opportunities, including
`areas that hold potential for discovering another giant field
`107 What's new in artificial lift
`J. F. Lea and H. W. Winkler
`Part 2-Twenty-two new surface and downhole developments for beam
`pumping systems, progressive cavity pumping and a joint industry study
`
`INDUSTRY IN FOCUS
`9 Editorial comment
`Global what (?) and cold winters; Billy Jeff, wheat and the Alamo
`11 011 country hotline
`Four big new fields off Brazil; IPAA petitions for oil import relief
`15 Looking ahead
`Modest U,S, demand growth seen; Drillers cautious during first half
`19 Industry at a glance
`Twelve-month U,S. gas price steady; March rig count up from '93
`25 What's happening in exploration
`Re-exploration in developed areas; Oryx's 3-D seismic applications
`27 What's happening in drilling
`Extended reach drilling applications from the IADC/SPE Conference
`29 What's happening in production
`Some key meetings coming up; New Sundowner rig for the Caspian Sea
`31 What's happening offshore
`Shell selects FPSOs in North Sea; U.S. Gulf 1994 hurricane prospects
`33 A world of oil
`,Comments and thoughts on Viet Nam; New study on subsea/multiphase flow
`35 Oil and gas in Washington
`How plans to upgrade EPA to a more oppressive agency were waylaid
`37 Management issues
`Advice on managing investment risk while protecting cash flow
`
`WORLD OIL 1 APRIL 1994 3
`
`WesternGeco Ex. 1008, pg. 9
`
`

`

`How modern techniques
`improve· seismic interpretation
`
`. Part 1-Seismic interpretation has evolved from
`basic 2-D structural mapping of vague anticlines
`observed on single seismic trace paper records
`in the 1920s to sophisticated stratigraphic mapping
`with lateral resolution from 3-~ data
`
`David L. Risch, Abu N.
`Chowdhury, Andrew E. Hannan,
`Project Geophysicists, and George
`A. Jamieson, Senior Geophysicist,
`Geco-Prakla, Houston, Texas
`
`Reflection seismology was first
`applied to hydrocarbon exploration
`in the 1920s and today is an integral
`part of oil and gas business. As tech(cid:173)
`nology evolves, more information is
`derived from seismic and used in
`many interpretation modes. Inter(cid:173)
`preter demands are also becoming
`greater as more data of higher quality
`is available for incorporation into
`integrated interpretation.
`Part 1 of this article describes the
`planning involved for seismic surveys,
`along with current methods and equip(cid:173)
`ment used for acquisition, processing,
`display and interpretation of seismic
`data. Major points covered include:
`• Interpretation objectives
`• Seismic acquisition, processing
`and display
`• Structural interpretation organi(cid:173)
`zation and procedure.
`
`as bright spots and AVO (amplitude
`variation with offset), are important
`tools. Seismic sequence stratigraphy
`adds another dimension to explo(cid:173)
`ration with an understanding of geo(cid:173)
`logic depositional process.
`
`INTERPRETATION OBJECTIVE
`Geological interpretation begins
`by defining problem scope and area.
`Studies range from large to small
`scale, i.e., from regional to reservoir.
`Techniques and data types vary
`depending on the problem size. For
`example, basin and regional studies
`involve interpreting large scale fea(cid:173)
`tures from a few widely spaced seis(cid:173)
`mic lines, and from gravity and mag(cid:173)
`netic data, often with little or no well
`control. Objectives are to obtain
`regional overviews and identify
`anomalies that warrant further study.
`In contrast, detailed reservoir
`characterization requires integrat-
`
`ing closely-spaced, high-quality/fre(cid:173)
`quency (resolving power) 3-D seis(cid:173)
`mic lines, data from many wells,
`petrophysical measurements, pro(cid:173)
`duction histories and extensive mod(cid:173)
`eling. Geological models are essen(cid:173)
`tial to begin interpretation and are
`based on literature and personal
`experience. Models are used initially
`but interpretation comes from work(cid:173)
`ing with data.
`
`SEISMIC METHOD
`Exploration reflection seismology
`uses induced acoustic reflections of
`rock layers. Vibrations are generated
`in the earth with acoustic sources, and
`reflections are recorded with receivers.
`Reflection intensity depends on veloc(cid:173)
`ity and density contrasts of rock lay(cid:173)
`ers and contained pore fluids.
`
`Acquisition. Data acquisition begins
`by laying out seismic programs with
`parameters designed for geologic objec(cid:173)
`tives based on study-area under(cid:173)
`standing. Parameters include: seismic
`frequency for vertical and horizontal
`resolution (horizontal resoulution also
`depends on source, receiver and line
`spacing); depth to target (distance
`between source and farthest receiver,
`i.e., far offset); rock layer dip; and envi-
`
`Layer 1
`
`BACKGROUND
`Acquiring continuous datasets
`along traverses provides a vast
`amount of data compared to single
`data points given by well control. It
`has been used for years in exploration
`and, with the advent of 3-D seismic,
`field development use has increased.
`Seismic data was first used to map
`subsurface geologic features.
`Seismic signatures visible as direct
`hydrocarbon indicators (DHI), such Fig. 1. Simplified diagram of seismic principle used in exploration.
`
`Layer 2
`
`Layer a
`
`WORLD OIL I APRIL 1994 85
`
`WesternGeco Ex. 1008, pg. 10
`
`

`

`Near
`offset
`
`3.0
`
`Near
`Far
`offset offset
`
`Far
`offset
`
`3.5
`
`4.0
`
`~4.5 i
`
`5.0
`
`5.5
`
`6.0
`
`ronmental, cultural and geo(cid:173)
`logical conditions.
`Most marine acquisition
`sound sources are air guns
`that repeatedly displace
`water volumes. On land,
`sources are explosives or
`truck mounted vibrators.
`It is common to use syn(cid:173)
`chronized source arrays to
`increase, or focus, energy
`at each shot. Vibrators can
`generate more high-fre(cid:173)
`quency energy, compensat(cid:173)
`ing for the earth's natural
`attenuating frequencies.
`Geophones are land receiv(cid:173)
`ers that detect slight ground
`movements. Marine receiv(cid:173)
`ers are pressure sensitive
`devices called hydrophones.
`Each receiver converts pres(cid:173)
`sure or ground disturbances
`to electrical impulses. The
`digitally recorded electrical
`pulses of an array or group
`of receivers are summed for
`each station and transmit(cid:173)
`ted, via cable or telemetry,
`to recording computers.
`Electric response am(cid:173)
`plitude is directly propor(cid:173)
`tional to reflection strength.
`Increases or decreases in
`acoustic impedance (veloc(cid:173)
`ity x density) across reflec(cid:173)
`tors are displayed as peaks
`or troughs on seismic wave(cid:173)
`form displays. Receivers Fig. 2. CMP gathers with hyperbolic move-out, (a), and velocity cor(cid:173)
`record reflections every 2
`rected to horizontal alignment for stacking. (b).
`msec for a chosen time
`(commonly 4-10 sec) resulting in a
`the signal to noise ratio when traces
`well sampled vertical seismic record are processed and summed.
`called a trace.
`In the seismic principle example,
`Fig. 1, the Horizon 1 reflection
`results from an impedance contrast
`between Layers 1 and 2; likewise for
`Reflection 2 emanating from Hori(cid:173)
`zon 2. Ray paths are described by
`Snell's Law and bend at each layer
`interface (horizon). Subsurface hori(cid:173)
`zo