The Response of the Electromagnetic Propagation Tool to Bed Boundaries

Abstract

Summary Because of its high vertical resolution, the electromagnetic propagation tool (EPTSM) has proved to be valuable in thin-bed analysis, in addition to its primary use in determining water-filled porosity. Using computer models corroborated by experimental data, we analyze the response of the EPT to bed boundaries, showing that thin beds just under 2 in. [5.08 cm] can be resolved, and the response to even thinner beds may be predicted. Introduction The EPT measures the phase shift and attenuation of a 1.1 × 109 cycle/sec [1.1-GHz] wave as it propagates past two receivers. These two measurements are scaled as propagation time (in nanoseconds per meter) and attenuation (in decibels per meter) and are displayed as two curves on EPT logs. The major factor influencing electromagnetic propagation at large (GHz) frequencies is dielectric permittivity, and because the dielectric constant of water is relatively high compared with that of hydrocarbons and most minerals, the propagation measurement can be used to determine water-filled porosity independent of water salinity. EPT measurements are made in a borehole-compensated (BHC) mode with a linear array consisting of two transmitter and two receiver microwave slot antennas (Fig. 1). Each transmitter is turned on separately, while phase and amplitude measurements are made at each receiver. In this way, the phase shift and attenuation between the two receivers is recorded for signals traveling in opposite directions. Averaging the two readings serves to eliminate imbalances in the receivers, helps to correct for imperfections in the borehole wall, and symmetrizes tool response to thin beds. The large cavity-backed slot antennas of the early EPT-D2 have been replaced by two interchangeable arrays of small slot antennas that act as point-magnetic dipoles. These two versions of the new tool, known as the EPT-G, 3 differ in the orientations of the dipoles on the tool pad. In one version, the antennas are mounted so that the dipole moments point end-on to each other. This is called the axial dipole array (ADA). In the other version, the antennas are mounted so that the dipole moments are side-by-side. This is referred to as the transverse dipole array (TDA). (For the field tool, ADA and TDA are referred to as the endfire-magnetic-dipole and the broadside-magnetic-dipole arrays, respectively.) The two arrays are shown in Fig. 1. The dipole moments are indicated by the arrows along the antenna slots. The response of these pure dipole arrays is much easier to interpret and to model than that of the large slot antennas of the EPT-D. The point-dipole behavior of the new small slot antennas has been verified with modeling and with laboratory experiments at the 1% level; i.e., contributions to the fields measured by the receivers from higher-order multipoles (in the range of media the tool is likely to be exposed to) is at least 40 dB less than the dipole field. This was also verified for both layered (axial and radial) and homogeneous media. Therefore, it is valid to assume that the antennas are point dipoles in all the following discussions regarding tool characterization.