Experimental and Theoretical Study of the Electromagnetic Propagation Tool in Layered and Homogeneous Media

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Experimental and Theoretical Study of the Electromagnetic Propagation Tool in Layered Propagation Tool in Layered and Homogeneous Media Summary. We report a detailed experimental and theoretical study of the electromagnetic propagation tool (EPT) in homogeneous and layered media. In this study, we measured the tool response in a number of homogeneous media, the standoff response in three formations, and the field maps for the magnetic field components parallel to the tool pad. We compared the experimental results to four theoretical models of different complexities. The best agreement is obtained with the model that takes into account the dominant geometric effects of EPT structure and the effect of the mutual coupling between the different slots. This study of EPT response is limited to relatively fresh fluids. Introduction Early work that established the feasibility of dielectric measurements to determine water saturation in borehole formations led to the development of the EPT. This tool measures the dielectric constant and the conductivity of the invaded zone of the formation by measuring the speed of propagation and the rate of attenuation of a 1.1 x 109 -cycle/sec [ 1.1 -GHz] electromagnetic wave. The EPT has a wall-contacting pad with four identical cavity-backed slot antennas. The pad is shown in Fig. with the coordinate system used in this paper. The antennas are spaced by 3.1, 1.6, and 3.1 in. [8, 4, and 8 cm). The development of the EPT has stimulated research in measuring the dielectric properties of water-saturated rocks, understanding the mechanisms contributing to the observed polarization effects, and developing interpretation methods to determine the water-filled porosity of rocks from EPT logs. The success of any interpretation method for the tool, however, depends critically on the physical model used to relate the desired geophysical properties-e.g., water saturation-to electrical properties (permittivity and conductivity). Of equal importance properties (permittivity and conductivity). Of equal importance is the electromagnetic model that describes how the permittivity and conductivity of the formation are related to permittivity and conductivity of the formation are related to the measured attenuation and phase shift. This paper takes a closer look at the latter issue by focusing on the electromagnetics of the EPT sensors, thereby examining the relationship between the dielectric properties of the formation and EPT response. In most properties of the formation and EPT response. In most of the work previously published, EPT behavior has been investigated through pure modeling efforts. The simplest model used to relate permittivity and conductivity of the formation to measured attenuation and phase shifts is the plane wave model. Although there is no firm scientific plane wave model. Although there is no firm scientific basis for such a model, it has been used to describe EPT response with a reasonable degree of success. A computer experiment by Freedman and Vogiatzis showed that the plane-wave interpretation model was semiquantitatively plane-wave interpretation model was semiquantitatively correct, with relative errors less than 5 %. Their conclusion was based on a comparison made with a vertical dipole model, where they modeled the EPT as electric dipoles oriented perpendicular to an infinite flat conducting plane. They used both single-point dipoles and distributed dipoles to account for the finite size of the antenna. A later study by Chew and Gianzero modeled the EPT as an infinite magnetic-line source (parallel to EPT antenna slots) by use of asymptotic approximations, wherever possible, to derive approximate expressions for the field at EPT receivers. In contrast to previous works, our approach in this paper has been to measure the response in media experimentally with properties that are known independently of EPT measurement and to compare our measured data directly with a number of theoretical models. The experimental investigation of the tool was divided into three parts. In the first part, we mapped out the magnetic-field components parallel to the tool pad in waters of different salinities with well-understood properties. We also studied the field maps in ethylene glycol because its dielectric properties resemble those of a "typical" rock. In the properties resemble those of a "typical" rock. In the second part, the homogeneous-medium response of the tool was measured in different saline waters and in ethylene glycol. SPEFE P. 289