GEOPHYSICAL MGNOGRAPH SERIES
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`David V. Fittezrman, Series Editor
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`William H. Dragoset I12, Volume Editor
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`NUMBER '7
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`A HANDBOOK FOR SEISMIC DATA
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`ACQUISITION IN EXPLORATION
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`By Brian J. Evans
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`SOCIETY OF EXPLORATION GEOPHYSICISTS
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`iIC§
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`1
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`ION ‘I038
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`ION 1038
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`1
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`4
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`9
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`T
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`7
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`sszsmc DATA acgozsmon
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`included several technical innovations that furthered the development of
`seismic data acquisition equipment and the'intez'pretatlon of seismic data.
`Beginning in the early 1930s seismic explorafion activity in the United
`States surged for 20 years as reiated -technology was being developed and
`refined [Figure 2}. For the next 20 years, seismic activity, as measured by the
`US. crew count, deciined. During this period, however, the so-called digital
`revolution ushered in what some historians now are calling the Information
`Age. This had a tsemendous impact on the seismic exploration industry. The
`ability to record digitized seismic data on magnetic tape, then process that
`data in a coxnputen not oniy greatly improved the productivity of seismic
`crews but also greatly improved the fidelity with which the processed data
`imaged earth structure. Modem seismic data acquisition as we know it could
`not have evolved without the digital computer.
`During the past 20 years, the degree of seismic exploration activity has
`become related to the price of a barrel of oil, both in the United States
`(figure 3) and worldwide. in 1990, US$2.195 billion was spent worldwide in
`geophysical exploration activity {Goodfellowg 1991). More than 96% of this
`{US$2.110 billion) was spent on petroleum exploration.
`Despite the recent deciine in the Seismic crew count, innovation has con-
`tinued. The late 1970s saw the development of the 3-D seismic survey, in
`which the data imaged not just 2: Venice} cross-section of earth but an entire
`volume of earth. The technology improved during the 1980s, leading to more
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`Crew Coon’:
`700
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`TDTI’-‘oi. LAND AND |"‘iI5iRiNE CREWS
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`6 O0
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`500
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`200 $00
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`400
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`300
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`0
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`MARINE ONLY
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`1930
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`1940
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`i950
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`1950
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`1970
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`1930
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`1950
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`F1‘g. 2. 11.8. seismic crew count {Goodfeliow, 1991).
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`2
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`1, Seismic Explomticm
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`9
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`caily monitored by radio navigation so that shots (or ”pops”) can be fired at
`the desired locations.
`
`lust as with land records, marine shot records also are recorded and dis-
`played in time (Figure 7). Instead of traces showing stations versus time, they
`are referred to as channels versus time. The shot records in Figure 7 have the
`ship and energy—source position to the left of the streamer. Seismic events
`such as A arrive first at channels on the left which are nearest to the source,
`then spread to the right in a curved manner. Event B is the direct arrival. The
`area of a marine shot record of greatest interest to the geophysicist is win~
`dowed on the right—hand record. A comparison of the land shot record (Fig-
`ure 5) with the marine records shows that the marine events appear more
`continuous across the record. Although some reflection events are visible on
`the land. record, most of that record is obscured by surfacegenerated noise.
`The marine record—being relatively noise freewis said to have a high signal-
`to—noise ratio, While the land record has a low signal—to—noise ratio. Reasons
`for this are discussed in greater detail in Chapter 3.
`_
`Consider again the land and marine acquisition schemes (Figures 4 and 6).
`After each land shot, the line of receivers may be moved along to another
`appropriate location and the shot fired again. This is the so—called 1‘oIl—1zlong
`method of seismic recording, the parameters of the roll—along being governed
`by both the geology and how the data are to be processed. Alternatively, the
`geophones may be left in place while the shot position is moved several
`times. To record an extensive number of lines on land is clearly time consum-
`ing because of the need to reposition the geophones manually. In marine
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`Seismic ship
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`Sea 3 LI rface
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`Streamer
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`Fig. 6. Marine recording technique.
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`SEISMIC DATA ACQUISITION
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`grains are generally only used in special circumstances {such as in transition
`zone or erratic coverage areas).
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`1.5 Survey Design and Pianning
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`If we take a vertical cut through a geologic section, the direction Where the
`geologic units are horizontal is known as the strike direction. A geologic sec-
`tion perpendicular to this direction is cut in the dip direction (see Figure 31).
`The geology of beds is easier to understand if a 2-D profile through them is
`made in the dip direction rather than in the strike direction. Also, data tend to
`be of better quality in the dip direction. Hence, dip lines are more important
`than strike lines in 2-D recording. In 3-D surveying, the situation is somewhat
`different (see Chapter 7). In 2-D recording, lines shot in any direction other
`than the dip direction can be confusing to interpret. Consequently, a general
`idea of basin shape, orientation, or structure initially must be appreciated in
`order to position lines correctiy. In addition, advanced 2—D migration process-
`ing is more effective with dip lines and thus a knowledge of the steepest dip
`direction is of extreme importance in line layout. In a new area to be mapped,
`seismic lines ideally should be recorded in both the- dip and strike directions.
`The strike lines, in conjunction with the dip lines, help the interpreter form a
`coherent picture of an area's geology.
`Line spacing is determined by the type of survey and the nature of the
`structure under examination, For reconnaissance work, iarge line spacing
`(50 km+) may give a regional picture, and in—fill lines with small spacing
`(500 rn+) may be added later. If an interpreter cannot follow the geologic hori~
`zone from one line to the next during his interpretation of the data, the lines
`are too far apart. In 3-D surveying, the line spacing is required to be as little as
`25 In in many cases to provide as detailed a geologic image as possible. Apart
`from geologic considerations, survey planning cannot proceed until the logis-
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`Fig. 31. Dip and strike directions.
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`4
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`SEISMIC DATA ACQUISITIO
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`the advent of the IBM personal computer (PC). The PC reduced the cost
`'=e
`T :processing but was frequently too slow or had inadequate software to p
`'2 form much more than the simplest of input] output functions. UNlX»base
`workstations were then developed to be more powerful than the PCS. Today-._;
`' "many field crews have data processing workstations to provide quick-Iooléfii
`general data processing support dining field acquisition,
`Interactive field computers are considered necessary during land crew-j;
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`' startup when test lines and source tests need to be evaluated. Processing costs _‘
`and time at the computer center can be saved using a field computer system‘
`that can demultiplex field records. Threedimensional data acquisition, both"
`land and marine, would be almost irnpossible today without some form of
`field computing«~—even if it were only to locate the position of common mid-
`points during the recording operations——to ensure that the fold of coverage is i
`adequate and within specified tolerances.
`Fieid computers have blossomed on marine vessels during the upsurge in
`3-D data recording. When four streamers are collecting data from four source
`arrays, the amount of positioning information for recording increases sub-
`stantially. Networked workstations are becoming the norm for recording and
`processing the navigation sensor data in near real time- For example, a
`streamers depth, feathering angle, and x,y1ocation can be updated every sec-
`ond using ship—rnonitoring computers. The collected data also can be
`inspected to ensure that the quality of recording is acceptable.
`Many recording systems have computers able to perform on-line phone
`tests and analyses as well as cabie tests prior to each shot. This is useful in
`checking receiver integrity before recording commencesjt number of instru-
`ments are able to perform limited signal processing as a quiclc—I0ok data pro-
`cessing package. The advantage of having a system that can do some form of
`field processing is that interpretation of field stacks may identify interesting
`formafions that could be further delineated by a modified program. One
`quick-look approach in marine 3-D recording is to bin short oifset traces in a
`low fold 3-D volume, which may be rapidly processed and provide an early
`indication of data quality as wet] as profiles and time slices through the 3-D
`volume.
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`Exercise 4.1
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`1}
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`their respective centers) were
`If two seismic lines (which tie at
`recorded by different source, receivers, and instruments, what tests
`would be needed on the fielcbacquisition system to ensure that the
`data phase ties in data processing would be made correctly?
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`5
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`238
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`SEISIVIIC DATA ACQUISITION
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`2
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`Traces
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`Missed
`K ‘ -»“"‘event
`‘for’. R
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`event
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`Fig. 159. Stackedusection trace aliasing. The addition of a trace at station 6
`would define the dip direction.
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`The minimum near~offset distance should be long enough to ensure that
`the shot—generated noise levei is acceptable. During marine surveys, cable
`jerk, ajr—g11n bubbles, water turbulence, and ship-propeller noise can cause
`excessive near-trace noise. With land Work, the shortest offset tends to be one
`station length (about 25 m). In marine operations, it tends to be the distance to
`the farthest gun from the towing vessel (60—12U rn); otherwise, the near
`receiver would be saturated by gun tow and / or bubble noise.
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`6.5.2.3
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`Station Spacing
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`Receiver stations should be close enough together to avoid the possibility
`of spatial aliasing. If spatial aliasing occurs on shot records, some transforms
`(such as fik) repeat the aliasing inf—k space, so they are no help in reducing
`coherent noise levels. Spatial aliasing occurs when sampling is inadequate for
`the frequencies and apparent dips present in the data. For example, spatial
`aliasing can cause misinterpretation of dipping events (Figure 159). Picking
`the correct dipping event is just guesswork because the data are aliased.
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`6
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`250
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`SEISMIC DATA ACQUISITION
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`During the early days of recording marine 3-D surveys, data were 1
`recorded using a single vessel, a single streamer and several air—gun strings
`acting as a single energy source. This meant that each traverse of the survey
`area by the sail line produced one line of subsurface coverage. A typical early
`(19705) survey had parallel lines about 10 ion long, spaced some 50 In apart. If
`the seismic vessel towed the streamer at 5 knots, then each line would take
`just over one hour to shoot. Because the vessel turning time between lines
`was also about an hour, on such surveys the vessel was productive for only
`half the time. Consequently, contractor service companies preferred to bid for
`seismic surveys on a tirne rate or daily rate, rather than on a kilometer ("turn-
`key”) basis. Many early surveys were recorded and processed by the same
`contractor because a convenient ”pacl<age” cost for acquisition plus process-
`ing could reduce the overall cost to the client exploration company:
`Because the cost of 3-D marine acquisition was so high, during the 19805
`new ideas were considered to increase the speed of data acquisition, thereby
`lowering costs. One idea was to record data using two well-coordinated ships
`sailing side—by—side, each towing a streamer and an air-gun array. The sources
`were fired in an alternating sequence, while data were recorded by both
`streamers for every shot. In this fashion, three seismic lines were collected for
`the price of two. That is, each ship recorded a standard line plus a line cover-
`ing CMPS halfway between the two vessels. This acquisition configuration
`also allowed subsurface coverage to be obtained under obstructions such as
`producing platforms (see Section 7.4).
`Economics is the driving force behind the technological advances in 3-D
`marine acquisition. The company with crews that can collect the most quality
`data at the lowest cost will get the most business. If a ship tows two cables
`rather than one, its production rate almost doubles, with a much lower per-
`centage increase in costs. Consequently, during the late 1980s, contractors
`started to tow a number of streamers and sources from a single vessel to
`increase productivity. With two sources in the water, it was possible to fire
`them separately and record data separately on the two streamers. The ship
`power to tow two such streamers would render the conventional seismic ves-
`sel (which was often little more than a modified rig supply tender) as inade-
`quately powered. Furthermore,
`towing two streamers (known as dual:
`streamer operations) and air-gun arrays required Wider bacl<—decl< space and
`greater air compressor power.
`The result was the commissioning of so—called ”super ships” by contrac-
`tors such as Western Geophysical and Geco—Prakla. An example of a ship tow-
`ing three streamers and two gun arrays is shown in Figure 168. If gun array 1
`fires first, then the vessel would record data from CMI’ line 1 at Streamer 1,
`CMP line 2 at streamer 2, and CMP line 3 at streamer 3. l/Vlleri gun array 2
`fires, data of CMP line 2 are recorded at streamer 1, CMP line 3 at streamer 2,
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`SEISMIC DATA ACQUISITION
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`monitored by the ship’s radar. The front section of the streamer and the
`source were located using acoustic triangulation measurements. Some crews
`used tow sensors to measure the angle at which the streamer left the ship. All
`of these data were processed in real time to provide a continuous monitoring
`of subsurface coverage.
`With the advent of ships towing several streamers and sources, the posi-
`tioning systems became more elaborate. Figure 170 shows an example. Typi-
`cally, the near-offset receiver and source positions are determined by a system
`of transponder pingers and receivers: Each such pair provides an acoustic
`range measurement of the distance separating the pair. Many such measure-
`ments can be combined to determine accurate positions, just like in the range-
`range ship-navigation systems described in Chapter 5. Acoustic systems are
`often also deployed at the tail end of the towed streamers and sometimes at a
`middle offset. GPS receivers and laser range finders may be positioned on
`streamer tail buoys and other buoys to provide additional positional data. All
`of the data together make up a so-called positional network. The network data
`are inverted in real time by powerful Workstation-class computers to provide
`accurate positions for all of the sources, receivers, and midpoints. A CMP cov-
`erage map is maintained by the computer so that any coverage shortcomings
`can be seen and subsequently fixed by shooting in-fill lines. Although required
`positional accuracy is dependent on CIVIP bin size, current industry practice is
`to aim always for average positional errors of 5 In or less.
`In some areas, such as the North Sea, changing and unpredictable winds
`and currents cause the initial CM? coverage to have many holes. Sometimes
`as much as 30% of data acquisition time is spent shooting in—fill lines to cor-
`rect coverage deficiencies. Survey budgets should allow for such contingen-
`cies in areas where they are likely to occur.
`'
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`7.3 Three-‘Dimensional Land Surveying Method
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`In 3-D land recording, there are a number of source / receiver configura-
`tions that may be used. ldeally, we wish to produce a gather of data contain-
`ing all azimuths when feasible (because if the raypath azimuths are from all
`directions, then the data are truly three-dimensional). To do this properly, the
`source]receiver lines may be positioned at right angles” to each other, as
`shown in Figure 171. This configuration is commonly known as the crossed-
`azrmy approach, in which the source is fired along the source line toward the
`receiver line as a broadside shot, eventually crossing the receiver line in split-
`spread manner, then continues firing as it moves away from the receiver
`spread. The shot records commence with the reflected waves arriving broad-
`side, becoming progressively hyperbolic until in the split-spread configura-
`tion, when they appear like normal split-spread shot records before becoming
`
`8
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