Plotting the seismic travel time from a point as a function of azimuth and distance yields a surface called a “chronoid”. The chronoid for the standard earth is a surface of revolution, termed the “standard chronoid”. Local anomalies are defined as the set of coordinate transformations required to bring the local chronoid into least-square coincidence with the standard chronoid. In general this is accomplished by means of two mutually orthogonal translations, plus one rotation in an azimuthal plane.

These operations give rise to three distinct components of the local anomaly; a geophysical interpretation of each of the anomaly components is provided.

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