VELOCITY AND Q ESTIMATION FROM THE SEPARETED UPGOING AND DOWNGOING WAVEFIELDS IN VSP DATA

Quality factor (Q-factor) evaluates the attenuation of seismic wave propagation, playing a fundamental role of reservoir characterization, which can be obtained accurately from Vertical Seismic Profile (VSP). The common methods usually use the downgoing wavefields in VSP data. However, the downgoing wavefields consist of more than 90% energy of the spectrum of the VSP data due to the energy fraction of the upgoing and downgoing wavefields, which makes difficult to estimate the viscoacoustic parameters accurately. Thus, a joint viscoacoustic waveform inversion of velocity and Q-factor is proposed to measure the difference between the separated upgoing and downgoing wavefields in VSP data based on the multi-objective functions. A simple separating step is accomplished by the reflectivity method to obtain the pure individual wavefields in VSP data, and then a joint inversion step is carried out to make full use of the information of the individual wavefields and improve the convergence of viscoacoustic waveform inversion. The sensitivity analysis about the velocity and Q-factor shows that the upgoing and downgoing wavefields contribute differently to the viscoacoustic parameters. Numerical examples and a field test indicate the accuracy and efficiency of the proposed method.