Shape Index: A Refraction Attribute to Detect Fractures and Permeable Bodies

Refracted P-waves are currently used to obtain a velocity model both in refraction seismic survey and in acoustic logging. In addition to velocity measurement, the acoustic and seismic data can be processed to measure parameters such as amplitude, attenuation, frequency, distortion of the refracted signal. The distortion of the refracted P-wave can be measured by a qualitative dimensionless attribute called Shape Index (Ic) which is given by the ratio A2+A3 to A1 where A1, A2 and A3 are the amplitudes of the first three picks of the refraction wavelet. Shape index measurement has been introduced in acoustic logging in the 80’s and then extended to near surface refraction data. Through two field examples, we show that it detects the presence of wave interferences due to geological heterogeneities such as fractures. In acoustic logging, the shape index log can be filtered in two components: a short wavelength component and a long wavelength component. The short wavelength component is correlated with the reflectivity function and indicates the changes of acoustic impedance and the presence of fractures. The long wavelength variation seems to be more correlated with petrophysical parameters such as permeability.