The determination of the seismic quality factor Q from VSP data acquired using distributed acoustic sensing

Abstract

The spectral ratio method is the most commonly used approach to estimate the Quality factor (Q) from vertical seismic profile (VSP) data. An assumption behind this method, indeed all Q-estimation methods, is that the ability of the VSP sensor to record data is not affected by depth. Distributed Acoustic Sensing (DAS) is a relatively new method for acquiring VSP data that utilises a fibre optic cable as the sensor. DAS has the advantage of being a highly efficient method to acquire densely sampled VSP data over a large depth range simultaneously but unfortunately the frequency response of DAS data varies with velocity and thus depth. In this paper we describe a methodology that can be used to correct the frequency spectra and thus successfully estimate unbiased Q values. The methodology is demonstrated using both synthetic and real data.