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MRS

A new geophysical technique for groundwater work

IGP

View larger version (62K): [in this window] [in a new window]   Figure 1. MRS setup. (bottom) Typical MRS field layout using a square loop. (top) NUMISPLUS system of IRIS Instruments: (1) DC/DC converter, (2) main unit, (3) wire loop, (4) tuning box, (5) rechargeable battery, and (6) control and data acquisition PC.

View larger version (9K): [in this window] [in a new window]   Figure 2. Two popular loop configurations for MRS: A = square and B = square-8. The latter configuration allows attenuation of noise interference at the cost of shallower depth of investigation.

View larger version (28K): [in this window] [in a new window]   Figure 3. MRS data set (at Waalwijk-1 in the Netherlands) and inversion. (left) MRS data summary of field and model response of inversion. (middle) MRS inversion results: water content and decay time constant versus depth, (right) Lithological log inferred from three nearest boreholes; in this model, the WVP1A unit has a higher permeability than the SDL1B and Kd31k units. The symbols for the formations correspond to the standard used in the Netherlands. (Log from TNO, 1998 data)

View larger version (31K): [in this window] [in a new window]   Figure 4. MRS investigation on a paleo-channel near Maun, Botswana. (a) Summary of data set. (b) MRS inversion results with multidecay time analysis. (c) TDEM resistivity. (d) Lithology of site bh 8351 (after Mangisi, 2004; Roy and Lubczynski, 2005).

View larger version (17K): [in this window] [in a new window]   Figure 5. MRS forward models (Bs = 48000 nT, dip = 65°, 150-m square loop). (left) Aquifer depth versus Q relationship with 20% porosity, 10–m thick saturated aquifer at mean depth 10–165 m. (right) MRS equivalence under indicated conditions when thickness < 10 m. With porosity*thickness of 2 m of water at a mean depth of 45 m, the water content and thickness can no longer be discriminated and only their product determined.

View larger version (63K): [in this window] [in a new window]   Figure 6. MRS water content MRS and GW storage parameters; in addition to the bound water/free water boundary, the unsaturated zone, including the dewatered cone of pump GW extraction, has an extra boundary, specific retention versus specific yield. Except for well-sorted coarse-grain aquifers, effective porosity (saturated part) is larger than specific yield (dewatered cone). Also, except for carbonates, MRS detects essentially only free water; this is a characteristic of current instrumentation. Capillary water on this figure is from the hydrogeology context: contrary to NMR petrophysics, it is not limited to micropore-size (after Lubczynski and Roy, 2005).

View larger version (21K): [in this window] [in a new window]   Figure 7. Calibration of MRS results in terms of hydraulic parameters. (left) T from pump tests versus MRS T-estimator: MRST1*2Z showing lithology dependence (broadly classified as granites, sands, and chalk). (right) MRS-T calibration after integration of lithology-dependent factor (Vouillamoz, 2003).