Instruments and field work to measure a Magnetic Resonance Sounding

Abstract

The various instruments performing Magnetic Resonance Soundings are briefly introduced. The NUMIS equipment includes two to five modules depending on the investigation depth to reach. It is controlled by a PC which carries out soundings through an automatic procedure leaving to the operator the final choice of the frequency and of the number of stacks to apply. The double pulse technique is used for the determination of the T1 time constant. PC software gives an automatic 1D estimation of the porosity and of the permeability of the water layers with depth at the end of the acquisition process. The GeoMRI equipment features four transmit/receive surface coils, one or more being reference sensors. An adaptive signal processing is used for noise mitigation. An imaging process determines the signal contributions from individual voxels, and an estimation is made for the amplitude, phase, decay time constant T2* of the free induction decay signal from where a water content and a permeability image is derived. The HYDROSCOPE system uses a maximum pulse of 20000 A.ms and a standard sequence of twenty-eight excitation moments. Each field curve e(t) is decomposed in the sum of three exponentials with fixed decay times values of 40, 100 and 200 ms, which in the inversion process are converted in three porosity values. The way the field surveys have to be carried out is presented, mainly through the use of the NUMIS equipment range. The maximum excitation current which determines the maximum investigation depth depends on the maximum output voltage of the equipment, on the size of the loop and on the local Earth magnetic field. The practical penetration is of the order of magnitude of the antenna dimensions. For detecting a MRS signal, it is necessary to have a stable Earth magnetic field, a low magnetic susceptibility, and a low EM noise conditions. Specific filtering and stacking process are required to improve the signal to noise ratio. Various combinations of loops are also available (eight shape, compensated loop …) to help reducing the relative effect of the noise in relation with the investigation depth required. The validation of a MRS signal is made by comparing the signal and the noise curves, by controlling the decaying shape of the signal curve, and by checking the frequency analyzed in the received signal. A 1D software automatically provides water content, depth and estimate of permeability based on T1 time constant for the various water bearing layers identified.