3. A seismic survey crew have a 120-
channel geophone system, if a 12-fold
coverage is required in every one-
kilometer survey, find out the source
interval and receiver distance.
Answers
Explanation:
Seismic Reflection Methods
The physical process of reflection is illustrated in Figure 1, where the raypaths through successive layers are shown. There are commonly several layers beneath the earth's surface that contribute reflections to a single seismogram. The unique advantage of seismic reflection data is that it permits mapping of many horizon or layers with each shot.. At later times in the record, more noise is present in the record making the reflections difficult to extract from the unprocessed data.
Figure 2 indicates the paths of arrivals that would be recorded on a multichannel seismograph. Note that the subsurface coverage is exactly one-half of the surface distance across the geophone spread. The subsurface sampling interval is one-half of the distance between geophones on the surface. Another important feature of modern reflection-data acquisition is illustrated by figure 3. If multiple shots, S1 and S2, are recorded by multiple receivers, R1 and R2, and the geometry is as shown in the figure, the reflection point for both raypaths is the same. However, the ray paths are not the same length, thus the reflection will occur at different times on the two traces. This time delay, whose magnitude is indicative of the subsurface velocities, is called normal-moveout. With an appropriate time shift, called the normal-moveout correction, the two traces (S1 to R2 and S2 to R1) can be summed, greatly enhancing the reflected energy and canceling spurious noise.
This method is called the common reflection point, common midpoint, or common depth point (CDP) method. If all receiver locations are used as shot points, the multiplicity of data on one subsurface point (called CDP fold) is equal to one-half of the number of recording channels. Thus, a 24-channel seismograph will record 12-fold data if a shot corresponding to every receiver position is shot into a full spread. Thus, for 12-fold data, every subsurface point will have 12 separate traces added, after appropriate time shifting, to represent that point.