ERTH3021 Note: Terminology of Seismic Records

This note is intended to assist in understanding of terminology used in practical exerciseson 2D and 3D seismic acquisition geometries.A fundamental distinction to be clear about is between shot records and stacked images. We start with the 2D concept. This can be easily extended to 3D.

2D SHOT RECORD (aka field record)

This shows the recorded ground motion at each geophone (or geophone group). 2D land records generally are made with geophones on both sides of the source. This is termed a split-spread. Fig1 and 2 are some examples of 2D land shot records.

Figure 1: 2D Land shot record showing refractions, reflections and surface waves. The top record is raw. The bottom shows the same record after amplitude equalisation, applied to make deeper events clearer.

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Figure 2: 2D Land shot record (Galilee Basin). The record also has had amplitude equalisation applied to make deeper events clearer.

A shot record can be considered as a whole, and this allows identification of the different wave arrivals (or events) such as refractions, reflections, surface waves. If you look more closely at a shot record yousee it is made up of individual traces. For example, the record in Figure 2 has 240 traces. Each trace is the recording at one geophone group. Hence the record in Figure 2 was obtained using 240 geophone groups, 120 on each side of the source.

Figures 3 and 4 show examples of 2D marine shot records. Because the source and hydrophone streamer are towed behind the boat, all the detectors are on the same side of the source. These records are referred to as off-end.

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Figure 3: 2D shallow-marine shot record, showing refraction, reflection and surface-wave events. A 48 channel recorder has been used. Each trace represents the recording at a single hydrophone. Here the boat would be moving towards the right.

Figure 4: 2D marine shot record, showing refraction, reflection and surface-wave events. Here the boat would be moving towards the left.

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2D STACKED IMAGE (stacked section)

The acquisition of a 2D seismic line usually results in hundreds or thousands of shot records. Theseare then processed to produce a stacked image (also called a stacked section or a stack). The stack is meant to indicate geological structure. Figure 5 shows a 2D stacked image presented in standard wiggle-trace format. This clearly shows that the image is made up of a number of individual traces from left to right. What do these traces represent?

A fundamental step in seismic processing is CMP stacking. This involves the summation of a number of traces from field records to produce a single stacked trace at each bounce point, or CMP. The image in Figure 5 shows the stacked traces obtained at 200 such CMP locations (numbered from CMP 350-550). Each of these traces has been obtained by stacking together a number (typically 20-100) traces from different shot records.

Figure 5: 2D stacked image, presented in standard wiggle-trace format. Each trace on this image is the output of the stacking process at a single CMP location.

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There are often a very large number of traces on a stacked image. For this reason, the individual traces are not always shown on the image. Figures 6 shows an alternative mode of presentation (called colour variable-density) in which individual traces are not shown. However, in reality the image is constructed as a large number of individual data traces from left to right. Each trace corresponds to the output of the stack at a single CMP location.

Figure 5: 2D stacked image, presented in colour variable-density format. Although individual traces are not shown, the image is constructed as a large number of data traces from left to right. Each traceon the image is the output of the stacking process at a single CMP location.

Figures 6 and 7 show more examples of colour variable-density 2D stacks, which have been interpreted geologically.

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Figure 6: 2D stacked image, presented in colour variable-density format. A geological interpretation has been overlaid.

Figure 7: 2D stacked image, presented in colour variable-density format. A geological interpretation has been overlaid.

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3D SHOT RECORD

The concepts discussed above can be easily extended to 3D. Figure 8 shows a 3D land shot record. This is essentially a series of 2D shot records displayed side by side. Each 2D subset represents the recordings made at a sequence of geophones on a single receiver line.

Figure 8: 3D land shot record. In this example, the 3D recording patch consists of 10 receiver lines, each having 36 geophones. The source point is situated between Receiver Lines 6 and 7, counting from the left. Note that refractions appear linear for receiver lines close to the source, and become curved for receiver lines further away from the source.

Similarly, a 3D marine shot record is obtained by towing a number of hydrophone streamers in parallelbehind the boat, and resembles a number of 2D shot records plotted side by side.

3D STACKED IMAGE (aka 3D Volume)In a 3D survey a single stacked trace is produced for each bin location. The result can be thought of as a series of intersecting 2D stacked sections, referred to as a 3D Volume (see Figure 9).

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Figure 9: (top) 3D Stacked volume. Looking down in plan view, we visualise a large number of bins on a rectangular grid. A single stacked trace is produced at each bin location. Th3 3D volume can be thought of as comprising a large number of intersecting 2D sections (bottom). These are referred to as Inline sections and Crossline sections.

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