Report 14 Improved Autocorrelogram Imaging Condition for Obtaining IVSP’s While Drilling

I present the equations for migrating IVSPWD (inverse VSP while drilling) autocorrelograms. These equations extend the Katz IVSPWD method to 2-D and 3-D media, and also provide a formal mathematical justification for inverting the reflectivity distribution from IVSPWD autocorrelograms. In addition, I describe a new imaging condition which images the primary-reflection energy compared to the Katz IVSPWD method which images the layer reflections that emanate from a freesurface ghost reflection. Raypaths associated with ghost reflections are typically 2-3 times longer than primary reflections, and so attenuate much more from geometrical spreading and anelastic losses. Consequently, the primary-reflection imaging condition promises to provide an order-of-magnitude improvement in reflection strength over that of the ghost-imaging condition. It is recommended that this new imaging condition be used for the standard autocorrelation IVSPWD method. INTRODUCTION TOMEX is a method for obtaining inverse vertical seismic profiles (IVSP) from seismic signals generated by a rotating drill bit and recorded by surface-located geophones. A geophone is placed at the drilling rig to record the source wavelet associated with the rotating bit, and this wavelet is used to deconvolve the reflected signals recorded by a seismic array on the surface. Although the TOMEX method has provided useful results to the oil industry, DOE recently funded an effort to look at an alternative IVSP method based on the patent of Lew Katz.