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The Seismic reflection method

What is a seismic section?

The seismic reflection method works by bouncing sound waves off boundaries between

different types of rock (Figure 1). The reflections recorded are plotted as dark lines on a

seismic section. A seismic section resembles a geological cross-section, but it still needs to be

interpreted.

One major difference between a geological cross-section and a seismic section is that the

vertical axis is in time, rather than depth. In the earth's crust, seismic waves travel typically at

about 6000 m/s so that 1 second of two-way travel time corresponds to about 3 km of depth. All

the seismic sections presented in this atlas are plotted at 1:1 (no vertical exaggeration)

assuming an average crustal velocity of 6000 m/s.

Another difference is that the reflections are plotted halfway between the source and the

receiver. These are referred to as unmigrated data. The process that moves the reflections intheir correct spatial position is referred to as migration, and the resulting seismic section is

referred to as a migrated section. Interpreters like to use both, and both unmigrated and

migrated data are presented in this atlas.

Why the seismic reflection method?

The science of LITHOPROBE is spearheaded by the seismic reflection method because it is

the geophysical technique which produces the best images of the subsurface. These data

resolve mappable features such as faults, folds and lithologic boundaries measured in the 10'sof meters, and image them laterally for 100's of kilometers and to depths of 50 km or more

(Varsek, 1992).

Seismic reflection profiling is the principal method by which the petroleum industry explores for

hydrocarbon-trapping structures in sedimentary basins. Its extension to deep crustal studies

began in the 1960s, and since the late 1970s reflection technology has become the principal

procedure for detailed studies of the deep crust.

Seismic data acquisition

The method works by bouncing sound waves off boundaries between different types of rock

(Figure 1). As opposed to earthquake seismology, where the location and time of the source is

an unknown that needs to be solved for, seismic reflection profiling uses a controlled source to

generate seismic waves. On land, LITHOPROBE has been using large truck-mounted vibrators

as a source (the "Vibroseis" method), and occasionally dynamite is used. At sea, large arrays

of airguns, which rapidly eject compressed air, are deployed. The reflected signals are

recorded by geophones, or hydrophones at sea, which resemble ordinary microphones.

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Figure 1. Seismic data acquisition.

During a seismic survey, a cable with receivers attached to it at regular intervals is laid out

along a road or towed behind a ship. The source moves along the seismic line and generates

seismic waves at regular intervals such that points in the subsurface, such as point P in Figure

1, are sampled more than once by rays impinging on that point at different angles. As a shot

goes off, signals are recorded from each geophone along the cable for a certain amount oftime, producing a series of seismic traces. The seismic traces for each shot (called a shot

gather) are saved on magnetic tape in the recording truck.

Seismic data processing

Digital data processing is applied to raw seismic data to produce a seismic section (Figure 2).

The following is an example of typical processing sequence.

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Figure 2. Seismic data processing.

The data are read from tape and the shotrecords (i.e. all traces recorded for a given shot) are

displayed (1). Bad seismic traces, due to noise or a short circuit in the recording equipment,

are edited out (2). The traces are then reordered (3) so that each gather of traces belongs to a

common reflection point, such as point P in Figure 1. Non-reflected arrivals, such as surfacewaves and direct arrivals, are removed by digital filtering and/or muting (zeroing of the data) (4).

A correction is made for the time the reflected ray spends travelling laterally, so that the

reflected arrivals now line up (5). These traces are then added to produce a single output trace

(6). This process, referred to as stacking, cancels out random noise and reinforces the reflected

signals. The waveform is then shrunk by frequency filtering or deconvolution to improve the

resolution (7). Steps (4) to (7) are repeated for each common reflection point, and the resulting

seismic traces are displayed as a seismic section (8) which is then interpreted (9).