`
`
`Ex. PGS 1038
`EX. PGS 1038
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`(EXCERPTED)
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`GEOPHYSICAL MONOGRAPH SERIES
`David V. Fitterman, Series Editor
`William H. Dragoset Jr., Volume Editor
`
`NUMBER7
`
`A HANDBOOK FOR SEISMIC DATA
`ACQUISITION IN EXPLORATION
`By Brian J. Evans
`
`SOCIETY OF EXPLORATION GEOPHYSICISTS
`
`Ex. PGS 1038
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`

`

`I
`
`····: .
`
`. •
`
`:
`
`.
`
`.
`
`.
`
`.··
`.
`.
`-
`included several technical i.JVtovations that furthered the development of
`seismic data. acquisition equipment,and the interpretation of seil)mic 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, seismk 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(cid:173)
`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 1980s, leading to more
`
`Crew Count
`700
`
`600
`
`500
`
`400
`
`300
`
`200
`
`100
`
`0
`1930
`
`1940
`
`1950
`
`1960
`
`1970
`
`1980
`
`1990
`
`Fig. 2. U.S. seismic crew count (Goodfellow, 1991).
`
`Ex. PGS 1038
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`

`

`1. Seismic Exploration
`
`'·.'·
`
`H90.
`
`Fig. 3~ U.S. price per barrel(courtesyU.S. Bureau of Mines, API} . .
`. '
`...... ....
`.
`
`• ,
`
`' '··· ' ..
`
`'
`
`'.'
`
`accurate and realistic imaging of earth. This was partly responsible for the
`increased use of.seismic data by the production arm of the oil industry.
`
`1.2.2 Modern Data Acquisition
`Because subsurface geologic structures containing hydrocarbons are
`found beneath either land or sea, there is a land data-acquisition method and
`a marine data-acquisition method. The two methods have a common goal(cid:173)
`imaging the earth. But ~ecause the environments differ so, each requires
`unique technology and terminology.
`In this section, simple examples of both methods are described in a presen(cid:173)
`tation of the basic concepts of seismic data acquisition. Also, a hybrid of the
`two methods, called transition-zone recording, is described briefly.
`Consider the simple land acquisition diagram shown in Figure 4. A seis(cid:173)
`mic wave is generated by exploding an energy source near the surface to
`cause a shock wave to pass downward toward the underlying rock strata.
`Some of the shock wave's energy is reflected from the rocks back to the sur(cid:173)
`face. The geophones vibrate as the reflected seismic wave arrives, and each
`generates an electrical signal. This signal is passed along cables to a recording
`truck, where it is digitized and recorded on magnetic tape or disk. The
`recorded information is taken to a computer center for processing. The seis(cid:173)
`mic recording technique often is referred to as seismic surveying, so the
`words "recording" and "surveying" are interchangeable.
`The positions at which the energy sources are detonated are called shot(cid:173)
`points. The energy-receiving geophones-"phones" for short-are placed
`
`Ex. PGS 1038
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`1. Seismic Exploration
`
`9
`
`cally monitored by radio navigation so that shots (or "pops") can be fired at
`the desired locations.
`Just as with land records, marine shot records also are recorded and dis(cid:173)
`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(cid:173)
`dowed on the right-hand record. A comparison of the land shot record (Fig(cid:173)
`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(cid:173)
`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 place . while the shot position is moved several
`times. To record an extensive number of lines on land is clearly time consum(cid:173)
`ing because of the need to reposition the geophones manually. In marine
`
`Seismic ship
`
`Sea surface
`
`Sea bed
`
`Fig. 6. Marine recording technique.
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`

`

`__ v2"
`
`.
`
`I
`
`-
`
`Equation (17) describes a hyperbolic shape geophysicists .call the normal
`moveout hyperbole, or simply NMO. It describes the relationship of the
`' arrival time of a reflection event to the reflector’s depth (via t1), the source—to-
`phone offset, and the average speed of sound in the earth layers through
`' which the wavefront travels. Because tx represents the total traveltime for a
`reflection—that is, the sum of time the wave travels downward and the time
`it travels upward—tx is called the two-way traveltime.
`During data processing NMO is removed from the data by shifting each
`trace sample upward by an amount 6, = tx —t1. The quantity :5, is called the
`NMO correction. Further discussion of NMO appears in Section 1.4.
`
`1.3.1.16 Events on a Shot Record
`
`The various types of seismic events that are observed on a shot record are
`summarized in Figure 23. Note that the only useful event—the primary reflec—
`tion—must compete with all of the other wave types so far discussed. These
`other wave types commonly are referred to as coherent noise. Other forms of
`noise also are prevalent in day—to-day seismic recording; there are various
`ways to try to attenuate such noise, as listed in Figure 23.
`Direct arrivals and ground-roll travel from the shot-point horizontally.
`Thus, their arrival times across a receiver spread represent one—way time rather
`than the two-way traveltime associated with reflection events.
`
`1.4 The Common Midpoint Method
`
`The common midpoint method of seismic surveying is universally accepted
`as the optimum approach to obtaining an image of earth layers. When a shot
`is fired, the emanating wave has many rays that travel downward. When the
`incident wave is reflected from a horizontal boundary, the point of reflection
`is midway between the source position (shotpoint) and the receiving—phone
`position. This point is called the midpoint. As shown in Figure 24, a reflection
`point can be the midpoint for a whole family of source-receiver offsets. The
`traces in that family have one thing in common—the midpoint lies equidis-
`tant between their source and receiver positions. Hence, the group of traces
`has a common midpoint, or CMP. If the CMP traces are corrected for NMO and
`then summed, the resulting stack trace has an improved signal—to-noise ratio
`(compared to that of the individual recorded traces). This happens because
`each trace in the stack contains the same signal (i.e., the reflection event) that
`sums coherently, but the random noise doesn’t. A collection of traces having a
`
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`38
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`i
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`SEISMIC DATA ACQUISITION
`
`grams are generally only used in special circumstances (such as in transition
`zone or erratic coverage areas).
`
`1.5 Survey Design and Planning
`
`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—fill 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-
`
`STRIKE
`
`D'P
`
`\\
`
`Fig. 31. Dip and strike directions.
`
`Ex. PGS 1038
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`

`1. Seismic Exploration
`
`45
`
`Communications: The vessel is supplied with all the necessary communi-
`cation equipment installed. It may be necessary to review communications in
`difficult areas of operation.
`Streamer cables: Most seismic ships can tow more than one streamer at a
`time. For 3-D surveys, costs, data quality, and target illumination are all
`affected by the number of cables towed. Designers of 3-D surveys must be
`aware of the tradeoffs involved in selecting a streamer pattern. For 2-D and 3—
`D lines, the length and group interval of the streamer(s) are important. Also,
`special consideration must be given to the equipment needed to determine
`streamer positions during a survey.
`Energy source: The energy pulse signature of the source should be known
`before a survey begins. If possible, perform a pulse test prior to survey startup
`to obtain the source signature for later processing. Alternatives are to use a
`previous test result or to model the source signature numerically.
`
`Exercise 1.1
`
`During and after drilling a well, a geophysical tool known as a sonde or ‘
`sonic tool may be run down the well to obtain sonic velocity information
`about the rock strata. The tool has a transmitter that sends a short sonic pulse
`through the rock to its receiver. Knowing the transmitter/ receiver separation
`allows a computation of the rock layer or interval velocity. In a similar well—
`logging run with a density detecn'on tool, relative rock densities are deter—
`mined. Knowledge of interval velocity and density allow a crude form of seis-
`mic modeling.
`Figure 32 represents a well drilled through four dipping layers. R is the
`reflection coefficient at each boundary, and p and V represent sonic density
`and velocity, respectively. Four interval velocities have been determined for
`each of the four beds; a density log is also available. Values are provided in
`Figure 33. Draw the seismic model representing the well, in the form of a
`”stickogram,” with the reflection coefficient K, being represented by a stick
`and having maximum deflection amplitude values of :1. Ignore traveltimes.
`The stickogram would have positive or negative reflections at levels R1, R2,
`and R3, as indicated below.
`
`
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`72
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`1)
`
`SEISMIC DATA ACQUISITION
`
`Depth controller noise—Water flow turbulence along the streamer and
`over the birds may be reduced if birds are placed away from live
`hydrophones. Such turbulence also can generate extreme noise when
`birds are diving hard.
`Poor—ballast noise—If a streamer is poorly ballasted, the birds will tilt
`their wings to greater angles trying to pull the streamer up or down to
`the desired depth. As a result, local turbulence is generated which pro-
`duces bird noise on the streamer.
`
`Rough sea state noise—Sea swell causes up-welling and down—drafting
`of volumes of sea water. This turbulence often generates a short-wave-
`length vertical pressure wavefield causing individual live sections to
`raise up or drop down depending on the direction of the surface swell.
`When individual live sections are moved relative to adjacent live sec-
`tions, high-amplitude noise bursts are observed. In rough seas, as a
`rule of thumb, a maximum depth of turbulence which causes unac-
`ceptable streamer noise bursts occurs at half a wavelength of the swell
`beneath the swell trough (Figure 55). Such noise bursts can be over-
`come by trough shooting (Figure 56). During trough shooting (a tech-
`nique often used in the North Sea where the weather window is
`limited), the troughs drop the whole cable down, and it is lifted up
`
`Mean Sea Level
`
`_ _ _
`
`— — — —Max1mumdepth
`ofturbuience
`
`(MSL)
`
`
`Fig. 56. Trough shooting.
`
`Ex. PGS 1038
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`3 seismic Energy Sources
`
`149
`
`
`
`I
`
`103
`
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`
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`103
`
`ENERDY m r001 Pounce AT so FT DEPTH
`
`1o0
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`100
`10
`I
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`.01
`.0001
`_f——L_—_.l—.l_—.L—l
`
`EQUIVALENT FCUNDB 0F DYNAMITE AT 30 FT DEPTH
`
`.|
`I
`
`_
`
`I
`I
`
`10
`I
`
`1000
`100
`| —I—
`
`EDUW PLENT POJNDS OF DYNAMITE
`
`Fig. 114. The Rayleigh-Willis diagram relating pulse-bubble period to
`potential energy.
`
`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
`(e.g., 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.
`
`The charge depth governs a phenomenon called ghost interference. As
`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 m (100 ft), than marine, where air-
`gun depth is generally 6—7.5 m (20—25 ft). Another problem, however, is not of
`
`Ex. PGS 1038
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`

`

`T
`
`150
`
`SEISMIC DATA ACQUISITION
`
`Primarg Pulse
`
`/ / Ghost
`4H-
`
`LVL
`
`Shot
`
`7
`
`/
`
`Shot
`
`Fig. 115. Ghost generation.
`
`the ghost interfering with a reflected event but the ghost actually causing
`no tching of the frequency spectrum.
`in Figure 116, the solid line represents a direct downgoing wave and the
`dashed line represents the wave reflected from the surface. The physics of
`sinusoidal wave propagation states that when two waves have the same
`wavelength, destructive interference (cancellation) occurs when they arrive
`exactly 180" out of phase. Because the reflection coefficient at the surface is
`negative, the downgoing reflected wave experiences a 180 ° phase shift rela-
`tive to the direct wave. However, the reflected wave experiences a further
`phase shift because of the additional distance, 261, that it travels relative to the
`direct wave. If destructive interference is to occur, that distance must be an
`integral number of wavelengths. That is, destructive interference occurs when
`2d :2 n)», where n = O,1,2,.... Since 9» = V/f, the notch frequencies Where destruc-
`tive interference is experienced are given by
`
`f=Z—g, n=0,1,2,....
`
`(35)
`
`For example, if V = 1500 m/ s and d = 6m, then source ghost-notch fre-
`quencies are at 0, 125, 250,... Hz. Figure 117 shows field examples of the signa-
`ture of an air-gun array as the array changes depth. FTP refers to the peak-to;
`peak strength, and the PBR refers to the primary-to-bubble ratio.
`Figures 118 and 119 display normalized versions of the normalized ampli-
`tude spectra for source and receiver arrays as a function of source and
`receiver depths. Figure 118 maintains the source array constant at 5 m and
`
`Ex. PGS 1038
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`

`

`Y
`
`3. Seismic Energy Sources
`
`151
`
`
`
`Fig. 116. Surface ghosting and sinusoidal wave cancellation.
`
`changes the streamer depth from 5. to 15 m, while Figure 119 repeats the exer-
`cise but with the source array at 10 m. 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 at f = 0 appear instead at about f = 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 at f = 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 weathering layer. Ghost notch-
`
`Ex. PGS 1038
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`

`

`152
`
`SEISMIC DATA ACQUISITION
`
`ing can then occur as a result of reflections from beneath the weathering and
`from the surface.
`
`Gun-depth
`(m)
`
`PTP
`Strength
`
`PER
`
`
`
`20bar—m
`
`Fig. 117. Air-gun array signatures for a varying depth of source
`(after Dragoset, 1990).
`
`80 ms
`l—--—-—-l
`
`
`
`Ex. PGS 1038
`
`

`

`5. Survey Positioning
`
`211
`
`lanes of 180 m on the baseline. Maximum range was 650 km during daylight
`hours and 450 km at night.
`Raydist—This was a phase-comparison system that provided a circular
`network using two base stations, and it used a cesium frequency standard.
`The mobile unit contained a transmitter that sent the synchronizing signal to
`the base stations.
`
`XR-Shoran—The extended-range Shoran system was a range-range sys-
`tem using three base stations, limited to ranges of less than 150 statute miles.
`XR-Shoran operated in the 230 MHz to 450 MHz range with an accuracy and
`repeatability of approximately 25 m under good conditions. The solid state
`version of this became Maxiran.
`
`5.7 Navigation Systems
`
`During marine seismic survey operations, the recording and processing of
`navigation data is handled by a computer. The numerous sensors monitoring
`the vessel’s location and trailing equipment, together with the computer,
`form the navigation system. For example, if GPS is the primary operational
`navigation system, the secondary system may be Syledis supplying data to
`the computer, with peripheral equipment such as streamer compasses, acous—
`tic equipment, the ship’s gyrocompass, attitude sensors, and Doppler sonar
`providing other positioning data. The navigation system integrates all of this
`information to produce real-time estimates of the ship’s position as well as the
`source and receiver positions.
`Having these real-time estimates, the navigation system computer con—
`trols the ship’s speed, its direction, and the shot firing time so that the actual
`seismic positioning data match, as well as possible, the intended, or ”preplot-
`ted,” acquisition program. Computer-generated preplots can contain a vari-
`ety of useful information, either in graphical form or in tabular form. Usually
`preplots show at least the beginning and ending shotpoint coordinates for
`each planned line in an acquisition program. Sometimes every shotpoint posi-
`tion is plotted or listed. If shore-based radio stations are tabulated, the preplot
`often lists the intended ranges between each station and each planned shot
`position (in Table 5.1, for example, ranges are shown for stations A, B, C, and
`D). That information lets the navigation system operator know when a station
`will become unusable because of excessive range from the station to the seis-
`mic vessel.
`
`Before the advent of computers, navigation of a vessel toward the seismic
`line was performed using preplots alone, whlch resulted in inefficiencies and
`lost time steering a vessel into position to begin a line. Today, steering is based
`on information provided by computer monitor screens that visually indicate
`the vessel’s actual position and desired position.
`
`.
`‘
`l
`
`
`
`#—
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`Ex. PGS 1038
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`

`

`238
`
`SEISMIC DATA ACQUISITION
`
`Traces
`
`2
`
`3
`
`4
`
`5
`
`7
`
`Aliesed
`J ,.........-event
`
`'- "-...Aliesed
`event
`
`Fig. 159. Stacked section trace aliasing. The addition of a trace at station 6
`would define the dip direction.
`
`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.
`
`Station Spacing
`6.5.2.3
`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 J-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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`

`

`250
`
`SEISMIC DATA ACQUISITION
`
`During the early days of recording marine 3-D surveys, data were
`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
`(1970s) survey had parallel lines about 10 km long, spaced some 50 m 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 ("tum(cid:173)
`key") basis. Many early surveys were recorded and processed by the same
`contractor because a convenient "package" cost for acquisition plus process(cid:173)
`ing could reduce the overall cost to the client exploration company.
`Because the cost of 3-D marine acquisition was so high, during the 1980s
`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(cid:173)
`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(cid:173)
`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(cid:173)
`sel (which was often little more than a modified rig supply tender) as inade(cid:173)
`quately powered. Furthermore, towing two streamers (known as dual(cid:173)
`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(cid:173)
`tors such as Western Geophysical and Geco-Prakla. An example of a ship tow(cid:173)
`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 CMP 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,
`
`Ex. PGS 1038
`
`

`

`252
`
`SEISMIC DATA ACQUISITION
`
`A recently launched ship has been built to tow as many as 12 cables. The eco-
`nomic incentives to increase productivity probably never will disappear. Con-
`sequently, further technological advances that lower the cost per unit of 3—D
`coverage are likely.
`Whether a seismic ship is towing a single streamer and source or many
`streamers and sources, the positions of the towed systems are affected by
`winds and currents. Figure 169 shows a phenomenon called streamer feather-
`ing, which occurs when there is a current having a component in the cross-
`line direction. Feathering introduces a cross-line component to CMP posi-
`tions. During data processing the location of each trace’s CMP must be
`known so it can be assigned to the correct stack bin. Because of feathering, the
`actual subsurface coverage obtained by one traverse of a survey area is se1~
`dom the same as the planned coverage. Thus, accurate source and receiver
`positioning data must be recorded and processed during data acquisition to
`ensure that the actual subsurface coverage meets the survey coverage specifi—
`cations.
`
`When only a single streamer and source were towed, the positioning
`equipment and processing systems were quite modest. Typically, a streamer
`would contain four to 10 compasses whose data would be integrated to
`reconstruct the streamer shape. The tail-end position of the streamer was
`
`Surveg lines
`
`—>
`
`Current
`
`direction
`
`Streamer
`
`drift
`
`Fig. 169. Streamer drift can cause midpoints to be located off-line.
`
`EX. PGS 1038
`
`Ex. PGS 1038
`
`

`

`254
`
`SEISMIC DATA ACQUISITION
`
`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(cid:173)
`tioning systems became more elaborate. Figure 170 shows an example. Typi(cid:173)
`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(cid:173)
`ments can be combined to determine accurate positions, just like in the range(cid:173)
`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 positiomi.l 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(cid:173)
`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(cid:173)
`rect coverage deficiencies. Survey budgets should allow for such contingen(cid:173)
`cies in areas where they are likely to occur.
`
`7.3 Three-Dimensional Land Surveying Method
`In 3-D land recording, there are a number of source/receiver configura(cid:173)
`tions that may be used. Ideally, we wish to produce a gather of data contain(cid:173)
`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 I receiver lines may be positioned at right angles to each other, as
`shown in Figure 171. This configuration is commonly known as the crossed(cid:173)
`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(cid:173)
`spread manner, then continues firing as it moves away from the receiver
`spread. The shot records commence with the reflected waves arriving broad(cid:173)
`side, becoming progressively hyperbolic until in the split-spread configura(cid:173)
`tion, when they appear like normal split-spread shot records before becoming
`
`Ex. PGS 1038
`
`