Borehole Dielectric Probe for the Detection of Mylonite Zones and Other Structures

The measurement of the high-frequency electrical properties of the materials surrounding a borehole can provide a means of logging changes in seam material and structure with distance along a hole and can possibly also give some positional information on roof and floor.

The electrical properties of materials can vary considerably with the measurement frequency used and various geophysical methods and instruments reflect this. For example, conductivity as measured by a 4 terminal method is usually a DC or low frequency AC method in which the conduction current completely dominates the displacement (or capacitive current). As frequency is increased however, displacement current increases and finally becomes the dominant component. The transition or crossover point, where conduction current equals the displacement current, can be calculated from

s = we_re_{0
where   s= conductivity,  er is relative permittivity, e0  is permittivity of free space.
For example, for  s = 1 mS/m and   er = 5, the crossover point is 3.6 MHz}

In the conduction region well below the crossover point, conductivity logging can be carried out by contact electrodes or by induced current methods. In the conduction region also, RIM is used as an imaging tool and relies on the rather diffuse propagation of energy from a transmitter to a receiver which are located in drill holes on either side of the region being investigated. RIM works well in coal seams which tend to offer a waveguide or channelling effect and a tomographic form of imaging can be carried out using two well separated drill holes in a seam, one hole containing the transmitter probe and the other the receiving probe.

Dielectric constant (+ loss angle) measurements are made in the displacement (or capacitive) current region. Logging can give information on the materials around the borehole and in particular on moisture content variation since water has a high dielectric constant, 81, compared to dry soil, coal etc which are in the region 4 to 9. If the bulk properties of the material surrounding the borehole can be measured rather than the near surface properties, then changes in structure and materials around the hole can be more readily observed. Dielectric logging does not require electrode contact with the walls of the hole.

Radar also operates in the displacement current region and is a non-contact method as well. However, in general, performance is poor unless  s <<we_re_o . Coal is a fairly good radar medium in most cases, although structure such as clay banding may present problems. Radar is also much more complex and difficult to interpret than changes in dielectric constant.

This report details advances made in the development of a dielectric borehole probe.