Experimental measurements of seismoelectric signals in borehole models

An experimental system is built for the electrokinetic measurements with a small scaled seismoelectric detector and a high resolution digitizer (1 MS s^-1, 22 bits). The acoustic and seismoelectric experiments are carried out in different borehole models at the high frequency of 90 kHz in the laboratory. All the localized seismoelectric signals that accompany compressional wave, shear wave and Stoneley wave are first clearly observed with a monopole source in sandstone boreholes that are saturated by tap water. The amplitudes of these signals are measured in the range of 1-120 μV, which is useful for designing the seismoelectric logging instruments. Then the amplitude ratio of electric signal to acoustic pressure (REP) for each of the three waves is calculated and compared with the theoretical simulations. Based on the experimental data, we find that seismoelectric logging signals as well as REP become stronger at the more permeable borehole model. We also find that seismoelectric logging signals are more sensitive to permeability and porosity compared with acoustic logging signals. Therefore, this study verifies the feasibility of seismoelectric well logging, and further indicates that the seismoelectric logging technique might be a preferable method to estimate formation parameters in the field measurements.