Experimental Studies Of Seismoelectric Measurements In A Borehole

Abstract

Experimental and theoretical studies show that there are two kinds of electromagnetic (EM) fields generated by seismic waves in a fluid-saturated porous medium. First, at an interface where the formation properties are different, the generated seismoelectric wave is a propagating electromagnetic wave that can be received anywhere. The second kind of field occurs inside a homogeneous formation where the seismic wave generates an electromagnetic field which exists only in the area disturbed by the seismic wave and whose apparent velocity is that of the seismic wave. An electrode, used as a receiver located on the ground surface, can only receive the propagating EM wave. However, when an electrode is in a borehole and close to the porous formation, it can receive both of the above EM waves. In this study, electrokinetic measurements are performed with borehole models made of natural rocks or artificial materials. The results of the experiment show that the Stoneley wave and other acoustic modes excited by a monopole source in the borehole models generate seismoelectric waves in fluid-saturated formations. The electrical components of the seismoelectric waves can be received by an electrode in the borehole or on the borehole wall. The amplitude and frequency of the seismoelectric wave are related not only to the seismic wave, but also to the formation properties, such as permeability, conductivity, etc. Therefore, seismoelectric logging may explore different properties of the formation than those investigated by standard acoustic logging. Electroseismic measurements are also performed with these borehole models. The electric pulse introduced through the electrode in the borehole or on the borehole wall induces a Stoneley wave in the fluid-saturated model which can be received by a monopole transducer in the same borehole. These measurement methods, seismoelectric logging or electroseismic logging, can be applied to field borehole logging to investigate formation properties relating to pore fluid flow.