Characterization of Geotomographic Studies with the EMRE System

Abstract

Posiva Oy carries out research and development on spent nuclear fuel disposal in Finland. The repository will be constructed deep in the crystalline bedrock of Olkiluoto island in Eurajoki. Posiva Oy and ANDRA (France) have cooperated actively in examining methods for revealing properties of granitic bedrock. One of the considered methods was an electromagnetic cross-borehole survey, and RIM measurements were conducted in 2009. Olkiluoto migmatitic bedrock has undergone polyphasic ductile-brittle deformation and resistivity in the bedrock varies strongly in range of tens to tens of thousands of Ωm. Field work was successfully performed in one borehole pair. The results are presented in this paper. The tomographic reconstruction of the borehole section is based on the far-field approximation of the electric field. The results prove that the method can be used between boreholes to delineate and follow sulphide-bearing horizons. The detected low and high resistivity zones and their apparent shapes and orientations are in fair agreement with geological and other geophysical results. The obtained information can be used, for example, in assessing the integrity of the rock mass, as the increased electrical conductivity is often associated with rock mass deformation (clay and water bearing fractures, sulphide and graphite bearing zones).

Borehole Antenna Considerations in the EMRE System: Frequency Band 312.5-2500 kHz

“…Lowpass filters are used to isolate the antenna from the winch cable, the technique used in the multiband trap dipoles [17], or to cut off frequencies above the reference frequency of 156.25 kHz. The receiver and transmitter surface units are connected using a ground-level reference line where two low-pass filters are connected [1,14].…”

“…At the surface the signal is amplified and fed to the surface receiver using a groundlevel two-wire line through low-pass filters. The reference signal has an important role in the proper detection of amplitude and phase difference [14]. …”

“…One important functional feature and at the same time a limiting factor in the present receiver is that its dynamic power is <40 dB, and the user thus has to select the appropriate antennas carefully in order to avoid the saturation of the receiver. Saturation can be eliminated by changing the antennas, rejecting the problematic frequency or using longer distances between the transmitter and receiver [14].…”

“…The behaviour of the Q-factor is closely related to the behaviour of the field in a medium where the field exists [7][8]25]. It is thoroughly treated in a paper by Korpisalo [14]. Fig.…”

“…The major factor that limits the use of radiofrequencies in geophysics is the strong absorption of energy in most earth materials, or the penetration may be highly frequency dependent. Thus, the expected attenuation rates of electromagnetic waves in a frequency band should be taken into account when estimating whether the use of the RIM method is justified in the research area [10][11][12][13][14].…”