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`GEOPHYSICAL MONOGRAPH SERIES
`
`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
`I
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`SOCIETY OF EXPLORATION GEOPHYSICISTS
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`included several technica_l'in_novati_ons that "furthered the development of .
`._ seismic data acquisition "equipment and the interpretation of seismic-data. '
`'
`"_'Beginning in the early 1930s seismic'explora_tion activity in the United
`"States surged for 20 years as- related .technology was -being developed ‘and
`refined (Figurez). For thenext 20 years, seisr_r1_i_c'a_ctivi_ty,'as measured by. the
`U.S. crew count, declined. During this period, however, the so~called-digital
`revolution ushered inwhat 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
`
`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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`0
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`MARINE ONLY
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`1930
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`1940
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`
`1960
`1970
`1980
`I990
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`1950
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`Fig. 2. U.S. seismic crew count (Goodfellow, 1991).
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`1.SeisinicE_xplorz_ztio_n .
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`U._S._pric_:e per barrel (courtesy T._l.S.'Bur_eau_ of l_VIin_e_s,_AI’_I)_. __
`_-
`accurate and realistic imaging of earth. This was partly responsible for the
`increased use ofseismic data by the production arm of the oil industry.
`
`1.2.2 Modern Data Acquisition
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`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—
`imaging the earth. But because the environments differ so, each requires
`unique technology and terminoiogy.
`In this section, simple examples of both methods are described in a presen-
`tation of the basic concepts of seismic data acquisition. Also, a hybrid of the
`two methods, called transitiomzone recording, is described briefly.
`Consider the simple land acquisition diagram shown in Figure 4. A seis-
`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-
`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-
`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-
`points. The energy—receiving geophones—"phones" for short~—are placed
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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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`Equation (17) describes a hyperbolic shape geophysicists .call the normal
`rnoveout hyperbola, 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 ix 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 tmveltime.
`During data processing NMO is removed from the data by shifting each
`trace sample upward by an amount 6, = 1.‘, —t1. The quantity :5, is called the
`_NMO correction. Further discussion of NMO appears in Section 1.4.
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`1.3.1.16 Events on a Shot Record
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`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 traveltirne 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 inidpoint, 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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`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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`1
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`2
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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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`_
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`‘r
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`‘ \
`at’ ¢
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`‘l\
`fl
`‘T
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`\
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`- »*’event
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`Aliased
`\ \A1iased
`event
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`~
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`\
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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.
`
`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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`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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`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.
`Vlfhether 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 sel~
`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.
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`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
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`Surveg lines
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`—>
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`Current
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`direction
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`Streamer
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`drift
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`Fig. 169. Streamer drift can cause midpoints to be located off-line.
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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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