Advanced seismic processing applications that use inverse Q-filtering for enhanced seismic resolution and true amplitude processing require reliable Q(z) earth models. While interval Q may be accurately estimated by zero offset VSP surveys, quality data are obtained over relatively limited depth intervals, and shallower well sections pose special problems because of hole size, multiple casings, and lack of cementation. To obtain a more complete Q(z) model, we have developed a theoretical method for combining zero offset VSP Q measurements with either effective-Q measurements from walkaway VSP surveys or surface seismic common midpoint (CMP) gathers.

Using a simple crustal model of isotropic and homogenous flat layers, we derive an analytical result for the effective-Q measured from walkaway VSP surveys or from CMP gathers. We have found that effective-Q approaches a weighted harmonic mean 〈Q of the overburden interval Q-values for short source-receiver offsets. This new method allows Q(z) models to be constructed from the surface down to the total depth of the well, or even deeper, by combining offset-dependent effective-Q measurements with depth-dependent interval Q estimates from zero-offset VSP. The technique is also a simple method of compensating surface seismic gathers for anelastic Q absorption in an offset-dependent and time-variant way. We tested our new method using both synthetic data and walkaway and zero-offset VSP field data from the North Sea.

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