Study of the surface noise correlation distance in seismic arrays

Abstract

The behavior of noncoherent surface noise within geophone arrays of variable geophone spacing was investigated by means of recording a dense geophone patch of single geophones. Arrays with variable geophone spacing were subsequently simulated from the geophone patch. The noncoherent noise was evaluated by comparing the RMS amplitude of the simulated geophone arrays to the predicted RMS amplitude (square root law). Also, correlation coefficients at the zero-lag for all possible combinations of two geophone traces of the geophone patch were computed for all geophone spacing. The results confirm the existence of surface noise correlation distance of the noncoherent noise. The array's efficiency to attenuate noncoherent noise decreases from the predicted square root law when the geophone spacing is smaller than the surface noise correlation distance. The effect is visible over the full seismic bandwidth, but is frequency dependent and more pronounced with decreasing frequencies of the noncoherent noise. At lower frequencies the spatial correlation of the noncoherent noise will manifest itself at relatively larger geophone spacing as a deviation from the square root law. The resulting relationships between bandwidth, geophone spacing and noncoherent noise attenuation suggest a maximum useful geophone density of geophone arrays. Using these relationships may yield a better signal to noise ratio and a more efficient use of the field equipment, a main contributor to the overall cost of present day 3D seismic surveys.