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`GEOPHYSICAL MONOGRAPH SERIES
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`David V. Fitterman, Series Editor
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`William H. Dragoset ]r., 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]. Evans
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`‘g
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`SOCIETY OF EXPLORATION GEOPHYSICISTS
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`SEISMIC DATA ACQUISITION
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`- included several teclmical innovations that furthered the development of
`_seismic data acquisition equipment and the interpretation of seismic data.
`Beginning in the early 1930s seismic exploration activity in the United
`States surged for 20 years as related technology was being developed and
`refined (Figure 2). For the next 20 years, seismic activity, as measured by the
`U.S. crew count, declined. During this period, however, the so—called digital
`revolution ushered in what some historians now are calling the Information
`Age. This had a tremendous impact on the seismic exploration industry. The
`ability to record digitized seismic data on magnetic tape, then process that
`data in a computer, not only greatly improved the productivity of seismic
`crews but also greatly improved the fidelity with which the processed data
`imaged earth structure. Modern 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 (Goodfellow, 1991). More than 96% of this
`(US$2.110 billion) was spent on petroleum exploration.
`Despite the recent decline 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 a vertical cross-section of earth but an entire
`volume of earth. The technology improved during the 19805, leading to more
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`Crew Count
`700
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`TOTAL LAND AND MARINE CREWS
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`600
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`500
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`400
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`300
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`200
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`100
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` MARINE ONLY '
`0
`1930
`1940
`1950
`1960
`1970
`1930
`1990
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`Fig. 2. U.S. seismic crew count (Goodfellow, 1991).
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`1. Seismic Exploration
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`9
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`cally monitored by radio navigation so that shots (or ”pops”) can be fired at
`the desired locations.
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`Iust 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 surface—generated noise.
`The marine record——being relatively noise free—is 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 roll-along
`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 placevwhile 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 u 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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`grams 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 Planning
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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 correctly. 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, large line spacing
`(50 km+) may give a regional picture, and in—fi1l lines with small spacing
`(500 m+) may be added later. If an interpreter cannot follow the geologic hori-
`zons 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 min 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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`STRIKE
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`°'P
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`\\
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`Fig. 31. Dip and strike directions.
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`3._ Seismic Energy Sources
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`_'
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`-_
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`__
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`Z
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`149 _
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`ENERBY lN FCDT FCUHD6 AT 30 FT DEPTH
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`100
`10
`l
`.I
`.01
`JJCO1
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`EQUi‘1'h'._ENT PCIJND6 OF DYH AMITE AT 30 FT DEPTH
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`tooo
`100
`I0
`1
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`,:
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`EQUIHFALENT PCUND6 OF DYNAMITE
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`Fig. 114. The Rayleigh-Willis diagram relating pulse-bubble period to
`potential energy.
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`3.5 Source and Receiver Depth (Ghost Effect)
`On land, the burial depth of a dynamite charge can affect the exploding
`wavefront’s amplitude and shape. Tests have been conducted with charges
`loaded in clay, sand, water-filled holes and cemented holes over the years
`(eg, McCready, 1940). The frequency spectrum may increase with depth but
`can be distorted by the surface ghost. Shallow charges often have poor ampli-
`tude and frequency content because of detonation within a porous weather-
`ing layer. Ideally, the charge should be placed beneath the weathering for
`improved statics corrections and superior signal-to—noise ratio, plus less sur-
`face noise.
`a
`interference. As
`The charge depth governs a phenomenon called ghost
`shown in Figure 115, a ghost is created by the downward reflection of the pri-
`mary pressure pulse from the surface, the weathering layer, or both. A ghost
`has a polarity opposite to that of the primary.
`If the ghost arrival time corresponds with a true reflection, the shot depth
`must be adjusted immediately. This tends to be more of a problem with land,
`where hole depth may be greater than 30 In (100 ft), than marine, where air-
`gun depth is generally 6-7.5 m (20-25 ft). Another problem, however, is not of
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`SEISMIC DATA ACQUISITION
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`Primarg Pulse
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`/ / Ghost
`’f\r1f\‘
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`Shot
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`7
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`Shot
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`LVL
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`Fig. 115. Ghost generation.
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`7 shows field examples of the signa-
`' gun array as the array changes depth. PTP refers to the peak—to—
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`Fig. 116. Surface ghosting and sinusoidal wave cancellation.
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`changes the streamer depth from 5 to 15 m, while Figure 119 repeats the exer-
`cise but with the source array at 10 in. These examples show how streamer
`depth can affect the location of notches in the spectra and how important it is
`to maintain a constant source and receiver depth.
`The ghost notches are not of infinite depth because of noise in the recorded
`signatures and their finite length. In particular, the signature truncation pro-
`duces a finite DC component (at 0 Hz). This has the effect of making the ghost
`notch that actually occurred atf = 0 appear instead at aboutf = 3 Hz.
`Ideally, a streamer should be towed at a depth designed to minimize the
`impact of the receiver ghosts on the spectrum of the seismic data. At depths of
`less than about 6 m, the ghost notch atf = 0 begins to seriously attenuate the
`low end of the seismic spectrum. At depths of 15 m or more, the first nonzero
`ghost notch affects the higher end of the spectrum. A 10-m streamer depth is a
`reasonable compromise that has become something of a de facto standard for
`streamer surveys.
`For land work, the ghost becomes a real problem if good—quality recording
`requires the shot to be placed beneath a thick w_eathering layer. Ghost notch-
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`SEISMIC DATA ACQUISITION
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`1
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`Traces
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`4
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`3
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`S
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`5
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`»l
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`7
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`‘r
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`at’ ¢
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`‘l\
`fl
`‘T
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`- »*’event
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`Aliased
`\ \A1iased
`event
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`~
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`Fig. 159. Stacked section 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 level is acceptable. During marine surveys, cable
`jerk, air-gun 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—120 m); 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 f-k) repeat the aliasing in f-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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`SEISMIC DATA ACQUISITION
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`During the early days’ of recording marine 3-D surveys, data were I
`recorded using a single vessel, a single streamer, and several air-gim 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
`(1970s) survey had parallel lines about 10 km 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 time 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 ”package” 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 back-deck 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 CIVIP line 1 at streamer 1,
`CMP line 2 at streamer 2, and CMP line 3 at streamer 3. When 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 CMP bin size, current industry practice is
`to aim always for average positional errors of 5 m or less.
`In some areas, such as the North Sea, changing and unpredictable winds
`and currents cause the initial CMP 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. Ideally, 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-
`array 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
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