Dynamic Streaming Currents From Seismic Point Sources In Homogeneous Poroelastic Media

In a porous medium saturated with a fluid electrolyte, mechanical and electromagnetic disturbances are coupled. The coupling is electrokinetic in nature since it is due to an excess of electrolyte ions that exist in an electric double layer near the grain surfaces within the material. Mechanically-induced streaming currents generated by point sources in homogeneous, isotropic porous media are presented. The electrically-induced streaming current is shown to be second-order in the electrokinetic coupling coefficient and can be neglected. This decouples the mechanical behavior from the electromagnetic behavior with respect to the induced fluxes and simplifies the analysis of the relative fluid flow and dynamic streaming current. We used Biot theory to calculate the amount of induced relative flow by the solution to Green's function. The transport coefficients-conductivity, dynamic permeability, and the electrokinetic coupling coefficient-and their sensitivity with respect to porosity, dc permeability, and frequency changes are evaluated. Conductivity decreases with increasing dc permeability. It has a k[subscript 0][superscript -1/2] dependence when grain surface conductances are more important than the bulk fluid phase conductivity. Stationary phase relative flow and streaming current solutions are calculated for an explosive and vertical point source acting on the bulk and a volume injection source acting on the fluid. The streaming currents are induced both by P and S waves. The streaming current decreases with increasing fluid conductivity. This is consistent with the decrease of the diffuse double layer thickness and ζ-potential. The porosity effect on the streaming current induced by S waves is different from the currents induced by the P waves. The porosity affects the bulk moduli of the solid. Its effect, combined with the frame bulk modulus and compressibility of the saturating fluid, determines the streaming current amplitude induced by a P wave versus porosity. The increase in streaming current amplitude induced by S waves with increasing porosity is due to the decrease of the shear frame modulus with increasing porosity. The streaming current behavior with respect to dc permeability is found to differ for sources applied to the elastic frame and volume injection sources.United States. Dept. of Energy (Grant DE-FG0293ER14322)Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Modeling Of Seismoelectric Effects In A Borehole

We present a method to simulate the propagation of seismic and converted electromagnetic waves generated by a mechanical borehole source embedded in a layered poroelastic medium. The electroseismic conversions occur at both the borehole wall and the layer boundaries. Most studies in electroseismic effects have been modelled and tested with seismic sources and detectors (geophones and antennas) at the surface. In this paper, we investigate the case of a seismic source in a borehole and receivers either at the surface or embedded in the medium. The method is formulated as a boundary element technique (where the poroelastic displacement and relative flow Green's functions are calculated by the discrete wavenumber method. The singular properties of the Green's functions are determined analytically using static Green's functions to regularize the integrals. This is necessary to calculate the element's self interaction. The borehole is cylindrical and its axis rulls normal to the interfaces. The coupled electroseismic effects in the layered media are included by using the global matrix technique. The developed method is an extension of the model of Biot-Rosenbaum, who applied the wavenumber integration technique to investigate the effect of formation permeability on Stoneley waves, using Biot's theory to model the wave propagation effects of a homogeneous permeable formation surrounding a borehole. We extend the Biot-Rosenbaum model by including the effect of a heterogeneous permeable formation surrounding the borehole. The effect of formation permeable zones (or fractured zones) on Stoneley waves can now be investigated. The other modification is the inclusion of conversions of mechanical into electromagnetic waves at mechanical and/or electrical contrasts in the poroelastic formation. The converted electromagnetic fields are sensitive to large permeability contrasts and fluid chemistry contrasts inside a reservoir. Using the electroseismic method downhole will provide more information about permeability/permeability contrasts in the formation, as well as additional lithological information (salinity of the fluids)

Electroseismic Waves From Point Sources In Layered Media

The macroscopic governing equations controlling the coupled electromagnetics and acoustics of porous media are numerically solved for the case of a layered poro-elastic medium.It is shown that these coupled equations decouple into two equation sets describing two uncoupled wavefield pictures. That is, the PSVTM picture where the compressional and vertical polarized mechanical waves drive currents in the PSV particle motion plane that couples to the electromagnetic wavefield components of the TM mode. And the SHTE picture where the horizontal polarized rotational mechanical waves drive currents in the SH particle motion plane that couples to the electromagnetic wavefield components of the T E mode. The global matrix method is employed in computing electroseismograms in layered poro-elastic media in the PSVTM picture. The principal features of the converted electromagnetic signals are the following: (1) contacts all antennas at approximately the same time; (2) arrives at the antennas at half of the seismic traveltime at normal incidence reflected P waves; and (3) changes sign on opposite sides of the shot. The seismic pulse is shown to induce electric fields that travel with the compressional wavespeed and magnetic fields that travel with the rotational wavefield. The frequency content of the converted electromagnetic field has the same frequency content of the driving incident seismic pulse, as long as the propagation distances are much less than the electromagnetic skin depth. Snapshots in time and converted electromagnetic amplitudes versus seismic point source-antenna offset-are calculated for contrasts in mechanical and/or electrical medium property. Conversion happens there where the seismic wavefront passes a contrast in medium properties due to generated imbalances in current across the contrast. The TM component amplitude radiation pattern away from the interface shows similarities with an effective electric dipole radiation pattern, or its dual, an effective magnetic current loop radiation pattern centered right beneath the source at the contrast's depth. The TM mode amplitudes decay rapidly with traveled distance and suggest the importance of a Vertical Electroseismic Profiling geometry to enhance recording of the converted electromagnetic signal by positioning the antennas closer to the target (contrast) of interest.United States. Dept. of Energy. Office of Energy Research (Grant DE-FG02-93ERI4322

Experimental Studies Of Electroseismic Conversion In A Fluid-Saturated Porous Medium

The coupling between seismic and electromagnetic waves in a fluid-saturated porous medium is essentially controlled by the electrokinetic effect. The inverse effect of the seismoelectric conversion, the electroseismic conversion, is investigated experimentally in the laboratory. The electric field induces movement of the ions in a pore fluid relative to the solid matrix. The interaction between the pore fluid and the solid matrix generates acoustic waves known as electroseismic waves. Our studies confirm experimentally that the electroseismic conversion at ultrasonic frequencies is the electrokinetic effect in nature. In measurements with a homogeneous rock cylinder, P- and S-wave transducers receive, respectively, the P- and S- components of the extensional and flexural waves generated by an electric pulse with ring or parallel electrodes when the electrodes are on the surface or inside a porous medium. The electroseismic waves are measured in layered models, made of sandstone or artificial materials, to determine the area where the electroseismic waves are generated. Further experiments with the layered model investigate the relationship between electroseismic conversion and the conductivity of the fluid-saturated medium or the pore fluid. When fluid conductivity increases, the amplitude of the electroseismic wave increases. Experimental results show that electroseismic conversion is different from the piezoelectric effect of quartz grains in sandstone and is closely related to the relative motion between the fluid and the solid. The results also eliminate the possibility that the electroseismic wave is generated by a spark of a high-voltage pulse. Electroseismic waves can be generated at low voltage and increased continually with the voltage, without a big voltage jump similar to the spark in an isolated material. Our results confirm the existence and measurability of electroseismic conversion in porous formation at ultrasonic frequency ranges. Therefore, electroseismic measurements may be an effective means to investigate the pore fluid flow and rock properties.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-93ER14322

Experimental Studies Of Seismoelectric Measurements In A Borehole

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.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-93ER14322

Electroseismic Investigation Of The Shallow Subsurface: Field Measurements And Numerical Modeling

Electroseismic phenomena in porous media, first observed almost 60 years ago (Ivanov, 1939), were recently "rediscovered" due to their potential to detect zones of high fluid mobility and fluid chemistry contrasts in the subsurface (Thompson and Gist, 1993; Haartsen et al., 1995). However, a limited number of field studies of these phenomena reported in the literature were not able to support the results with an explicit comparison to theoretical predictions. In this paper, we demonstrate that electroseismic phenomena in porous media can be observed in the field, explained, and modeled numerically, yielding a good agreement between the field and the synthetic data. We first outline the design of our field experiment and describe the procedure used to reduce noise in the electroseismic data. Then, we present and interpret the field data, demonstrating how and where different electroseismic signals originated in the subsurface. Finally, we model our field experiment numerically and demonstrate that the numerical results correctly simulate arrival times, polarity, and the amplitude-versus-offset behavior of the electroseismic signals measured in the field.United States. Dept. of Energy (Grant DE-FG02-93ER14322)Massachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Coupled electromagnetic and acoustic wavefield modeling in poro-elastic media and its application in geophysical exploration

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995.Includes bibliographical references (leaves 389-300).by Matthijs W. Haartsen.Ph.D