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Acquisition of Seismic Data

Geophone Hydrophone

Parameters of data acquisition

Some parameters of a seismic acquisition program are:

· maximum offset: distance from the source to most remote receiver;

· minimum offset: distance from the source to nearest receiver;

· group interval: distance between geophone arrays. Constant for a survey;

· shot intervals: distance between holes;

· fold coverage: number of times a subsurface point is surveyed by different sources and detectors;

· sample interval: the time interval between digital samples of the signal, which varies from less than 1ms to 4ms. This sample rate is chosen not to limit the vertical resolution and to record the desired maximum frequencies;

· choice of source and geophone arrays;

· number of recording channels.

Acquisition of parameters and noise

Fig. 1 below is a schematic seismic section showing the major signal and noise features. It has been corrected for normal movement (NMO) which is the time correction applied to each trace to account for its offset. The first arrivals at the top of the record are labeled P-waves; these are typically refracted waves from near-surface formations. These are followed by two coherent noise patterns. The first one, characterized by low frequency and a velocity that varies from 3500 to 5500ft\sec., is the surface wave (ground roll). The second one is the air wave, which is energy traveling from the source to the detectors through the air having a high-frequency component and low velocity of 1100ft\sec. The third type of noise is the multiple, which is a repeat reflection from the same interface. It may be either a simple multiple from one of the shallow reflectors or an intrabed multiple bounced between two reflectors and back to the surface or one of several other types in which the reflected ray reverses direction at some point.

Elements of a data acquisition system are:

1. sources and arrays;

2. detectors and arrays;

3. instrumentation;

4. field geometry or survey configuration;

5. surveying, positioning and navigation.

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