Numerical modelling of passive electroseismic surveying

Abstract

SUMMARY This work reports numerical modelling of electroseismic conversions when the electric field source originates in the atmosphere. Layered structures of conductivity anomalies yield rotated electric fields at reservoir depths as large as source fields at the surface. Active-source electroseismic field tests imaged reservoirs 1800 m deep. However, the required high-power, dipole sources mediate against these methods finding practical application in hydrocarbon exploration. We extend previous research by considering the potential for using environmental electric fields to create useful electroseismic conversions. World-wide lightning strikes induce time-dependent electric fields in the atmosphere. In the frequency band appropriate for seismic surveying, 1–100 Hz, electromagnetic field pulses occur at a rate of 10–100 pulses per second. These pulses create horizontal electric fields in the earth’s surface that induce electric currents in the subsoil. Those currents preferentially channel through high-conductivity layers. Charge accumulates at the termini of conducting layers. That charge accumulation induces galvanic currents. Vertical galvanic currents propagate to depth where they generate propagating seismic waves at gradients in electrical properties, such as conductivity gradients at reservoirs. We use 2-D numerical simulations on three different, layered-earth models to estimate the seismic amplitudes generated by passive fields. The modelling shows that the transverse magnetic fields can induce potentially useful vertical electric fields at depth. The generated seismic amplitudes at the top of the reservoir are sensitive to the oil content of the reservoir, the frequency of the primary electric field, the geometry of the conducting layers and various material properties. Finally, a hypothetical laterally complex reservoir configuration was tested which confirms the mentioned results and additionally shows the ability of the proposed method to delineate water–oil contacts.