Geophone ground coupling

Abstract

The geophone ground coupling for vertical and horizontal geophones has been measured in the laboratory and in the field. The data can be fit to a calculated geophone response with a single coupling resonant frequency and damping factor. For frequencies much lower than the coupling resonant frequency, the geophone accurately follows the ground motion, but for higher frequencies the coupling can alter both the amplitude and phase of the seismic signal. The normal planting of vertical geophones in the field results in coupling adequate for conventional recording that uses frequencies less than 100 Hz. However, for very loose soils or for high‐frequency seismic recording, I recommend that the geophones be buried to place the geophones in firmer soil. The coupling resonant frequency for vertical geophones is determined by the firmness of the soil, and I have measured resonant frequencies ranging from 100 to 500 Hz at different locations. The coupling resonant frequency is insensitive to changes in the mass or base diameter of the vertical geophones. Because the firmness of the soil increases with depth, the coupling resonant frequency can he increased by burial of the geophones or by the use of longer spikes. Adequate coupling is very important in shear‐wave recording because the rocking of horizontal geophones causes a low‐frequency coupling resonance. It is crucial that horizontal geophones be planted with their bases firmly contacting the soil. Geophones so planted have a resonance around 130 Hz, whereas those 1 cm off the ground can have a resonance of 30 Hz or lower. Soil conditions have little effect on the resonant frequency. Horizontal geophones with 1-inch spikes are as well coupled as those with longer spikes, but the best coupling is achieved by burial of the geophones.