Application of Spread-Spectrum and Frequency Hopping Techniques to Geophysical Inversion Problems

In this paper we present a theoretical framework for the application of Direct Sequence Spread-Spectrum techniques in conjunction with slow Frequency Hopping to the estimation of acoustic wave propagation times in a geophysical medium. Instead of measuring travel-time via manual (visual) estimation of the direct-path arrival time of a one-shot disturbance sent from the source to the receiver, we opt to measure travel-time by establishing a direct sequence spread-spectrum minimum shift keying link between the source and the receiver. By sending timestamp data between the source and the receiver over this spread-spectrum link, travel-times can be calculated In order to combat the problem of false locking of the spread-spectrum system on strong indirect-path waves, we frequency-hop the center frequency of the spread-spectrum signal and calculate the travel-time via the decision algorithm of "plurality rules " applied to the travel-times computed at each center frequency.