A novel display of common-offset profiles is found to be a powerful analytical tool for seismic data. The offset panel displays six sets of common-offset, single-channel data taken from a conventional marine seismic survey. The single-channel profiles are displayed below one another and arranged vertically by offset and horizontally by common depth point. This arrangement causes effects due to near-surface geologic changes to generate geometric patterns that are different from patterns due to changes in seismic source or receiver. Both reflection and refraction data are used in the display. Reflection data of the near trace or offset are at the top of the display and indicate the presence of acoustic boundaries. Refractions observed on distant traces are sensitive to chan es in the near surface. They allow properties of acoustic layers to be determined through their alteration of transit times and amplitudes.

This display takes advantage of the exchangeability of seismic source and receiver to relate variations in refraction arrival time and amplitude at different offsets. Data commonly muted by the processor allow the detection of near-surface velocity anomalies from the simple geometric patterns they generate. The geometric patterns are symmetrical around the anomaly. A variation in refraction arrival due to change in source or receiver lacks symmetry and may be distinguished from a velocity anomaly.

Refraction arrivals penetrate to a depth of perhaps one-fifth the source to receiver distance, dependent on velocity distribution. A velocity anomaly anywhere in the path can be identified if one can compare refraction arrivals with and without the anomaly in the path. The offset panel arranges the data so that a comparison can be made.

The simple symmetry of the data in such a display renders it useful in many areas of analysis. For the data processor, the most important functions of the offset panel are to (1) verify field recording geometry, (2) monitor the seismic source, and (3) determine processing parameters. For the interpreter, the display serves to: (1) detect anomalous velocity zones in the near surface and allow for their correlation with deep structure, (2) locate and identify shallow, high-amplitude reflections, and (3) indicate shallow geologic changes.

Geotechnical and drilling engineers can use the display to: (1) locate zones of unstable sediments on the seafloor, and (2) locate and evaluate shallow drilling hazards.

Refraction data recorded by the conventional seismic spread are found to be very important in resolving problems associated with the near surface. They are most useful in the direct detection of shallow drilling hazards.

End_of_Article - Last_Page 1689------------