Measurements of attenuation from vertical seismic profiles

Abstract

Values of attenuation have been extracted from five detailed velocity surveys. Significant amounts of attenuation are observed in all five wells. Measured values of attenuation vary by a factor of 10, ranging from less than 0.1 to 0.9 dB/wavelength. Correlation of attenuation with lithology is good, considering the resolution limits of the surveys. One of the surveys was performed in West Texas, and the other four were carried out in the Gulf Coast area. The total number of recording depths in each survey varied from 31 to 208, with downhole recordings taken every 10 or 20 ft over key stratigraphic zones. In all five surveys, a monitor geophone was placed near the well to check for source consistency. Our procedure for measuring attenuation involved constructing the spectral ratio between each downhole pulse and one particular reference pulse. A value of cumulative attenuation (CA) was then extracted from each spectral ratio and plotted as a function of geophone depth. This method allowed us to use many measurements in determining the attenuation over each depth interval.