Worldwide assortment of shot records

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Seismic Data Analysis
Seismic-data-analysis.jpg
Series Investigations in Geophysics
Author Öz Yilmaz
DOI http://dx.doi.org/10.1190/1.9781560801580
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store


Forty shot records, both land and marine, from North and South America, Europe, the Middle East, North Africa, and the Far East, are presented in Figures 1.3-1 through 1.3-40. Source types are vibroseis, Geoflex, dynamite, air gun, Maxipulse, Aquapulse, and Aquaseis. The recording parameters, including the number of traces, number of samples per trace, sampling interval, trace interval, and inner offset, are indicated in Table 1-13. Study the field records to learn how to recognize different types of waves. For display purposes, an instantaneous type of gain (AGC) (gain applications) was applied to all 40 records. These records will be referred to by their record numbers in the following discussions.

The main goal in processing reflection seismic data is to enhance genuine reflection signal by suppressing unwanted energy in the form of coherent and random ambient noise. In the following paragraphs, shot gathers are examined to point out the different types of seismic energy.

Record 1 is a correlated vibroseis data set. (For vibroseis correlation, refer to the 1-D Fourier transform.) A number of reflections is present in this record with reasonably good signal-to-noise ratio. A genuine reflection is recognized on common-shot gathers by its hyperbolic nature. Reflections behave the same way on CMP gathers. A flat horizon with no dip yields a symmetric hyperbola on both common-shot and common-midpoint gathers recorded using split-spread geometry. (In split-spread geometry, the source is located somewhere in the middle of the receiver cable, usually at the center.) A dipping horizon yields a skewed hyperbola on a common-shot gather, while still yielding a symmetric hyperbola on a common-midpoint gather. Reciprocity of sources and receivers provides this symmetry. From the reflection hyperbolas in Record 1, note that the subsurface is made up of nearly horizontally flat layers. Any irregularity in the shape of the moveout hyperbola can be attributed to near-surface effects and/or lateral variations in velocity.

Record 2 is an asymmetric shot gather. Note the reflection energy between 1 and 2 s, with rather irregular moveout.

Record 3, which was obtained by using dynamite, contains a series of reflections with nearly perfect hyperbolic moveout, especially between 1 and 3 s. This record is from the times of analog recording. It is not uncommon to digitize old analog data and process it with modern techniques.

Record 4 contains events with complex moveout between 2.5 and 3.5 s. Events A, B, and C have skewed hyperbolic moveouts, which, in this case, suggests that they are dipping up toward the left. Also note the traveltime distortions along the moveouts caused, most likely, by near-surface irregularities.

Record 5 has some ground-roll energy, which is characterized by its low-frequency, high-amplitude appearance, particularly on short-offset traces. This kind of energy typically is suppressed in the field by using a proper receiver array.

Record 6 contains weak and strong, nearly flat reflectors (A and B). The break in the reflection hyperbola (C1-C2) suggests the presence of a fault C1 on the upthrown side and C2 on the downthrown side). Again, note the ground-roll energy with its dispersed low-frequency character on inside traces (event D).

Record 7 contains three interesting events. Event A is a skewed hyperbola, which suggests that it is dipping up toward the left, while event B is nearly symmetric, which suggests a flat dip. Event C shows a discontinuity F along its moveout curve, indicating the presence of a fault.

Record 8 shows a record with excellent signal quality. This dynamite record has a number of reflections and associated interbed reverberations. Note the progressive decrease in the signal-to-noise ratio at late times. This is true for almost all seismic data. Event A has large moveout because it is shallow, while event B has small moveout because it is deep. (Linear energy C is referred to in Exercise 1-3.)

Record 9, which is a correlated vibroseis data set, has a series of reflections and ground roll. Unlike data from impulsive sources such as dynamite, first breaks in vibroseis data may not be distinguishable (compare, for example, Records 8 and 9). This is because the correlated vibroseis record contains some of the side lobes of the sweep signal autocorrelation. Note the increase in random noise in the later part of the record below 3 s.

Record 10 contains two strong shallow reflectors, A and B, in addition to ground-roll energy C. Also, a bundle of energy with extremely large moveout is noted between 2.5 and 5 s (D1-D2). This coherent noise may be attributed to side-scattered energy, which is caused by inhomogeneities in the subsurface (particularly at the water bottom) that behave as point sources.

Record 11 contains four prominent reflections. This record is from Alaska, where the thickness of the permafrost layer can be irregular. Such near-surface irregularities can have lateral dimensions that range from less than a group interval to wavelengths that are several times a cable length. As seen on the right flank of the hyperbolas (events A, B, C, and D), these irregularities cause substantial time shifts in reflection arrivals. Such distortions in moveout could be dynamic (time-dependent) or static (time-independent). They should be corrected before stacking. Except for these distortions, all events seem to have symmetric, hyperbolic moveouts that indicate nearly horizontally layered substrata.

Table 1-13. Parameter index of a worldwide assortment of common-shot gathers.
Record Number Area Number of Samples per Trace Number of Traces Sampling Interval, ms Trace Interval, ft or m Inner Offset, ft or m Source
1 South Texas 1,275 48 4 330 ft 990 ft V
2 West Texas 1,025 120 4 100 ft 400 ft V
3* Louisiana 1,500 24 4 340 ft 340 ft D
4 Turkey 1,275 48 4 100 m 250 m V
5 South America 3,000 48 2 100 m 200 m D
6 Far East 1,250 48 4 100 m 150 m D
7 South America 2,600 48 2 100 m 300 m V
8 Central America 1,300 96 4 50 m 100 m D
9 Alaska 1,000 96 4 220 ft 990 ft V
10 North Africa 1,325 120 4 25 m 300 m V
11 Alaska 1,000 96 4 220 ft 990 ft V
12 Mississippi 1,275 48 4 330 ft 990 ft V
13 Offshore Texas 2,025 48 4 220 ft 875 ft A
14 Offshore Texas 1,525 48 4 220 ft 690 ft P
15 Offshore Canada 2,500 48 2 25 m 360 m A
16 South America 1,275 48 4 25 m 233 m A
17 South America 2,000 48 4 50 m 250 m A
18 Offshore Louisiana 1,500 120 4 82 ft 716 ft A
19 Turkey 1,250 216 4 10 m 50 m D
20 South Aleutians 2,025 120 4 82 ft 921 ft A
21 Denver Basin 1,550 48 2 220 ft 220 ft V
22 Williston Basin 1,550 48 2 110 ft 110 ft V
23 San Juaquin Basin 1,550 48 2 220 ft 220 ft V
24 Arctic 3,000 48 2 220 ft 220 ft S
25 Alberta 2,000 96 2 50 m 50 m D
26 Alberta 1,500 48 2 67 m 67 m D
27 Canada 1,791 92 4 50 m 200 m A
28 Canada 2,500 48 2 25 m 300 m A
29 Offshore Spain 2,000 48 4 50 m 250 m M
30 Offshore Crete 2,125 96 4 25 m 230 m A
31 North Sea 1,550 96 4 25 m 228 m A
32 North Sea 1,550 96 4 25 m 178 m A
33 North Sea 1,625 96 4 25 m 200 m A
34 Celtic Sea 1,500 60 4 50 m 253 m A
35 Denmark 2,500 52 2 100 m 100 m D
36 Middle East 1,024 48 4 50 m 250 m V
37 Turkey 1,000 48 4 75 m 187 m V
38 North Africa 2,500 60 2 100 m 100 m V
39 Middle East 2,500 60 2 50 m 100 m G
40 West Africa 2,600 96 2 30 m 120 m D
* Analog recording. V: vibroseis, D: dynamite, A: Air gun, P: Aquapulse, M: Maxipulse, G: Geoflex, S: Aquaseis. All vibroseis records have been correlated. Aquapulse and Maxipulse are registered trademarks of Western Geophysical Company of America. Aquaseis and Geoflex are registered trademarks of Imperial Chemical Industries.

Record 12 is a field record with a low signal-to-noise ratio. A complex subsurface structure is implied between 2 and 3.5 s.

From Record 13, note the high-frequency hyperbolic energy S that is associated with a side scatterer, possibly at the water bottom.

Record 14 has three identifiable reflections: A, B, and C. Reverberations and multiples also make up a significant portion of the data.

Record 15 is a marine record. The hard water bottom causes refraction arrival A. This shot gather primarily contains guided waves, which are manifested as linear trends such as B, C, and D. The genuine reflection E has little moveout.

Guided waves are trapped within a water layer and travel in the horizontal direction. They are dispersive — each frequency component travels at a different speed, which is called horizontal phase velocity. Their behavior is variable, primarily dictated by water bottom conditions and the thickness of the water layer. They are an important source of coherent noise and are confined mainly to the supercritical region of propagation, where no transmission occurs into the substratum.

Wave packet A in Record 16 is made up entirely of guided waves. Direct arrivals B carry the highest frequency components, while lower frequencies C arrive earlier. Moderate frequencies D make up the later portion of the dispersive wave packet. This record has a reflection E and long-period multiples M1 through M4. The reflection and its multiples also have an accompanying reverberating wavetrain that is nearly 300 ms long.

Record 17 is longer than the common length (4 to 6 s) used in seismic data acquisition. There is no apparent signal after 4 s; nevertheless, very weak signals can sometimes be uncovered by stacking.

Record 18 has some events worth mentioning. Dispersive waves, which include the head wave and direct arrivals A make up the early portion of the record. Some reflections, B, C, and D, are followed by short-period reverberations. In the deeper part of the record, note the events with extremely large moveout E, which is unusual for deep data. These events represent the side-scattered coherent noise.

Record 19 is a walk-away noise test. It is actually a composite of six shot records. The receiver cable was held constant while the shots were moved away without overlap. The receiver group interval is 10 m. The receivers in each group were bundled together without array forming. This allowed all signal and noise wavelengths to be recorded. Horizontal wavelength is determined by measuring the dominant frequency (the reciprocal of the time between successive peaks or troughs) and the horizontal phase velocity (the reciprocal of stepout Δtx) of the unwanted ground roll. The horizontal wavelength then is used to design the receiver array length that is needed to suppress this energy (see Exercise 1-4). Wave packet A1-A2 (between 1.7 and 4.6 s on the far-left trace) is an excellent example of ground roll. The linear coherent energy with opposite dip A1-A2 is the backscattered component. Reflection C is being disrupted by ground-roll energy.

Ground roll is different from guided waves, although both are dispersive. Ground roll is one type of Rayleigh wave that arises because of the coupling of compressional waves (P) and the vertical component of shear waves (SV) that propagate along the free surface [1]. On the other hand, guided waves are one kind of compressional wave that travels within a layer just as sound waves travel in an organ pipe.

Record 20 (marine) shows a variety of wave types. Direct arrivals A are significantly suppressed by receiver arrays in the field. We can see the water bottom reflection B on short-offset traces. Note a shallow reflector C and associated refraction arrival D. At 1 s, another reflector E is seen. Much of the energy between 1 and 3 s most likely is multiples associated with B, C, and E. Linear noise (possibly cable noise) F and the lower-frequency propeller noise G appear in the deeper portion of the record after 4 s.

Record 21 (vibroseis) has a weak A and a strong B shallow reflector. Below 2 s, ambient noise dominates the record.

Record 22 is another vibroseis record. Note reflection arrivals A, B, and C. Although reasonably hyperbolic, there are some fluctuations in traveltimes that may be attributed to near-surface complexity (as inferred by the first breaks). Record 23 shows similar characteristics.

Record 24 is a marine shot gather that was acquired with an Aquaseis source. Direct arrivals A, water-bottom reflection B, and first-order multiples M1, M2, are recognized easily. A primary reflection P and its peg-leg multiple PL also are distinguished.

Record 25 (land) has a very good signal quality. In addition to the several primary reflections, note ground-roll energy A. It is predominantly low frequency and travels with low group velocity (the speed with which the energy in a wave packet travels). Also note the near-surface effects that cause traveltime distortions along the right flanks of reflections B, C, D, and E.

Record 26 (dynamite) does not have a well-developed ground-roll energy; however, it still is recognizable from its low-frequency character A. Traveltime paths that correspond to reflections (for example, B, C, and D) have been disrupted by ground roll and possibly distorted by irregularities in the near surface.

Record 27 (marine) is interesting. Note the change in the cable geometry A (Exercise 1-6). There is a well-developed dispersive wave packet B that spans between 1.9 and 2.9 s at the far-offset trace. This includes the head wave and direct arrivals. Also note the distinct moveout difference between events C and D. Event C, with a larger moveout, belongs to the short-period multiple wavetrain that is associated with the water-bottom reflection. Event D, with a small moveout, is a primary reflection with its own peg-leg series F.

The air-gun data in Record 28 contain highvelocity reflections with little moveout. Note the predominant guided wave packet C that spans between 0.7 and 1.9 s at the far-offset trace. It results from the strong water-bottom refractor D.

Record 29 is puzzling (see Exercise 1-7). The skewness of the reflection hyperbolas (B, C, D, and E) increases in depth.

Record 30 is a deep-water shot record. The direct arrival A, water-bottom reflection B, and shallow reflection C, can be identified easily. First-order water-bottom multiples M and peg-leg multiples PL, which are associated with the shallow reflector C, also are prominent in this record.

Record 31 is a shot record primarily made up of guided waves. The following wave types are identified: A is the refraction arrival, B is its multiples, C is the direct arrival, D is the dispersive medium-frequency components of guided waves between 1.8 and 3 s at the far-offset trace, and E is the backscattered energy associated with side scatterers at the water bottom.

Record 32 is another marine record that contains strong guided wave energy. A refractor A, direct arrival B, and dispersed wave packet C span between 1.2 and 4 s at the far-offset trace. Events D and E represent subcritical reflection energy, most of which is reverberation.

The events on Records 33 and 34 are referred to in Exercises 1-9 and 1-10, respectively.

The four far left traces in Record 35 are associated with the channels that are used to record auxiliary information.

Record 36 seems to have no events with hyperbolic moveout. Record 37 has a few reflections, AD, BE, CF; however, they are buried in strong ambient noise. Record 38 contains virtually no reflections. A strong dispersive wave (ground roll) makes up the early part of the record, while the remaining part contains primarily random noise.

Record 39 is a Geoflex record containing strong ground-roll energy A. Additionally, the record from top to bottom contains a strong high-frequency reverberating wavetrain and short-period multiples that are associated with the water bottom and, perhaps, a few shallow reflectors.

Record 40 has a small usable segment — the longer offset traces on the left side between 1 and 4 s. The remaining part of the record contains strong random noise and transient noise, A, B, C, D, and E, which are attributed to electronic instrument noise (possibly resulting from weather conditions).

References

  1. Grant and West, 1965, Grant, F. S. and West, G. F., 1965, Interpretation theory in applied geophysics: McGraw-Hill Book Co.

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Worldwide assortment of shot records
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