An Example of a Time‐Term Survey

The time-term process seeks to minimize the assumptions required for the interpretation of refracted body waves by expressing the travel-time over a distance A in the form t = f(A)+ai+bj, where ai and bj are constants, related to the materials which lie between the refracting boundary and the source and receiver respectively. To a first approximation, the time-terms are independent of A and azimuth. The example referred to was a field operation in Canada, which yielded a set of travel-times between the points of overlapping networks of shot-points and detectors. A preliminary data plot indicated two approximately linear branches of the travel-time curve, with considerable scatter of observations about each branch. A first application of time-term theory yielded ,a criterion whereby points near the intersection of the two branches of the curve were associated with the appropriate refractor. A second iteration led to the assignment of a set of time-terms ranging up to 0.42s, and enabled the standard deviation of a single observation to be reduced to 0.049 s. The time-terms themselves could be fitted on to a simple model of crustal structure. Finally, subtraction of time-terms from appropriate pairs of travel-times yielded a number of independent estimates of local propagation velocity below the refractor, by means of which possible regional variations of velocity could be investigated.