Quantitative characterisation of groundroll (Rayleigh Waves) in the Western Niger Delta, Nigeria

Abstract

A study of the character and dispersion patterns of groundroll was undertaken in Western Niger Delta in order to provide much needed information and solution for its suppression during seismic surveys. The quantitative analysis was carried out on raw monitor seismic records. The results revealed the occurrence of groundroll with predominant phase velocities of 500-1020 ms-1 (mean of 780 ms-1), frequency content of 4.0-9.0Hz (mean of 6.1Hz), wavelength of 61-240m (mean of 122m) and group mean velocity of 787 ms-1, with average standard deviation of 158 ms-1. The uphole data acquired was analyzed through direct or analytical determination of weathering velocities and thicknesses. The results shows that weathering and sub weathering velocities vary between 510 - 1012ms-1 with average value of 770ms-1 and 1368- 2474ms-1 with an average value of1734ms1respectively. The determined thicknesses of the weathering layer ranges from 3.8 to 52.8m with an average value of 19.4m. The result showed that the weathering and sub weathering velocities as well as thicknesses of the weathering layers in the study area vary erratically both vertically and laterally. The result further showed that western Niger Delta is not a homogeneous half space but exhibits non homogeneous character with unequal phase and group velocities as well as variations in the thicknesses of the layers. This behavior showed that western Niger Delta is dispersive in nature which occurs as a result of velocity layering. The attenuation strategy designed has shown that elaborate geophone array of 28m and source pattern of 122.5m can provide excellent suppression of the bands of the wavelengths of noise in this area of study.