THE USE OF GEOPHONE GROUPS TO IMPROVE THE SIGNAL‐TO‐NOISE RATIO OF THE FIRST ARRIVAL IN REFRACTION SHOOTING*

For a group of n geophones at a single station, a signal‐to‐noise improvement of √n can only be expected when the background noise, between the individual detectors in the group, is completely uncorrected. This paper gives results, from four different sites, for the variation of correlation coefficient with distance between the detectors. It is found that at these locations the geophone spacings required to reduce the correlation coefficient to zero varied from 30 to 300 feet.

These results are in agreement with the predictions of the theoretical model of a two‐dimensional isotropic noise field described by Hills and Faran (1952). This model provides estimates of the spacing required between geophones to reduce the correlation coefficient to zero, provided that the velocity of the noise is known and that its spectrum is of a simple form.

Often it is impractical to reduce the correlation coefficient to zero because of the large spacing required between the geophones in the group. For this case a simple method is described in which the signal‐to‐noise improvement can be calculated given that the correlation coefficients between the geophones in the group are known. This procedure can easily be used in the field because the correlation coefficients can rapidly be computed from paper records by using Tomoda's method (Tomoda, 1956) which gives values of sufficient accuracy to be used for on‐the‐spot calculation.