Interval Attenuation Estimation

Abstract

The goal of this paper is to study the accuracy of estimating the Quality Factor (Q) from ultrasonic pre-stack P-wave reflection data. Q is estimated by applying the spectral ratio method to the top and bottom reflections of the target. The data used to estimate Q are shot gathers acquired over different target materials submerged in a water tank. Using this setup, Q is estimated for Lucite, rubber and Berea sandstone. The behavior of Q estimates with offset is also investigated. Theoretical error analysis shows that non-zero estimates are contaminated by the overburden (water) attenuation. It is predicted that this error is directly proportional to the ratio of target depth to its thickness, target Q value, overburden attenuation and the difference between the top and bottom reflection ray trajectories in the overburden. The estimation error for Lucite is around 20%. For the Sandstone the error is about 10%. The estimated rubber Q is between 17 and 20. The estimates of all the targets show increasing estimated Q with increasing offset. This behavior is correctly predicted in the error analysis and is due to the difference between the top and bottom reflection ray path lengths within the overburden and the target Q value.