The effect of conductive layers on the 1D inversion of MRS data



MRS method is one of the new geophysical methods that directly detects the Groundwater. By using this method, one can determines some parameters of aquifer (porosity, permeability and transmissivity). The purpose of this research is the application of MRS method in the conductive ground. Combination of MRS and RS method is used for better detection of the aquifer in the region of Abyek located at southeast of Qazvin province.
Processing MRS sounding data, EN/IN and S/N are equal to 2.66 and 1.14 respectedly for 8sq-75 sounding. The external noise in 8sq-75 is 3 times of the internal noise, so the parameters in this sounding are not considerable. As we can see the amount of permeability and transmissivity (8.5 × 10 -6 m2/s ) are very low in this sounding.
For the sounding of sq-150-1 the EN/IN and S/N were 1.29 and 1.4 respectively. The external noise and internal noise is almost equal and this sounding is reliable. The signal and the noise in this sounding showed equal amount, so there is a little amount of water bearing in the aquifer exist upto 150 m depth beneath this sounding. The amount of transmissivity in this sounding is 8.3 × 10-4 m2/s.
The EN/IN for sounding sq-150-2 is 1.29, and S/N is 1.6 which is better than sq-150-1 and 8sq-75 soundings in noise level condition. The EN/IN and S/N for 8sq-37-5 are 1.05 and 1.64 which is the best of all in low level noise situation. The transissivity of the sounding 150sq-1 showed the highest of all among the 4 soundings, so this is the best in water bearing of all in this region.
Inversion of MRS soundings showed, the region is in the low level of water bearing aquifer, because the transmissivity is less than 0.015 m2/s, so the water production is not efficient.
The comparison of the MRS method and RS method determined that the aquifer layers are located at very low resistivity zones. For 8sq-75 sounding the aquifers layer have 4-5 ohm-m resistivity. The aquifers in 150sq-1 sounding have almost 1 ohm-m resistivity. The aquifers in 150sq-2 sounding have almost 0.5 ohm-m, which is the lowest of all in this region. The aquifers in 8sq-37-5 sounding have resistivity from 1.5 ohm-m to 7 ohm-m.
In the present work, influence of the conductivity on MRS phase, has been applied in the three regions, namely Abyek Qazvin with low resistivity (5 ?-m), Hamedan with medium resistivity (40 ?-m) and the southwest of Iran with high resistivity (approximately 300 ?-m) were compared with each others.
If the resistivity of the subsurface decreases, the phase variation will be increased and amplitude of signal will be decreased. In the low resistivity zone (Abyek Qhazvin) has 270 degree phase variation, while in the medium resistivity zone (Hamedan) has 183 degree variation and in high resistivity zone (Lorestan) has 34 degree variation. The signal amplitude for low resistivity zone (Abyek Qhazvin) is 21 nvolt, for medium resistivity (Hamedan) is 34 nvolt and for high resistivity zone is 381 nvolt.