Driving point impedance and physical property logging

Geophysicists have a need for quantitative characterization of properties in the subsurface. A new logging-tool paradigm, based on driving point impedance measurements, promises to provide such quantitative measurements of in situ rock properties. Research to date has focused on using a down-hole orbital vibrator (OV) of the type found to be effective in cross-well seismic imaging methods. Previous experiments qualitatively show that the shape and amplitude of the vibrator's orbit is altered by the medium surrounding the OV. This paper begins the process of establishing a physics-based link between OV motion and physical (or mechanical) properties of the surrounding medium. It is well known by the borehole seismic community that a point force in a fluid-filled well has the same far-field behavior as a point force in an infinite elastic medium. However, in this paper we present measurements indicating that the driving point impedance of an OV in a fluid filled well is inertial, which is fundamentally different than the driving point impedance of a point force in an infinite elastic medium. This insight may provide a new way of determining subsurface properties.