Development of field techniques for improving MRS quality in shallow investigations

Abstract

ABSTRACTThe application of magnetic resonance sounding (MRS) to near‐surface studies, especially in the vadose zone, requires a significant improvement in the signal‐to‐noise ratio. Small dimension loops with several turns must be used in order to achieve a high vertical and lateral resolution. This increases the loop impedance, limiting the current intensity and increasing the criticalness in the requirement to produce excitation pulses with a good shape.Due to the high current intensity required to create the excitation field, the tuning system used in MRS equipment is achieved selecting the capacitance values amongst several fixed configurations in discrete steps that depend on the loop characteristics and the emission frequency. When the equipment works with high inductances, this procedure implies a wide range in the tuning frequency that corresponds to a given value of capacitance. For this reason, it is necessary to change other system characteristics in order to achieve an optimal resonance fitting to the Larmor frequency.Moreover, a way to improve the excitation field in the very shallow zone of the subsurface and to achieve the dipolar approximation of the electromagnetic field of the loop in this zone, would be to increase the distance from the plane of the antenna to the surface of the ground.In this paper, several processes that have been developed as an aid to optimize the recording parameters for shallow MRS investigations are presented: 1) a methodology to improve the tuning through an optimized configuration of the transmitter antenna is proposed, 2) a quantitative index has been derived to evaluate the quality of the excitation pulse, as an aid to decide whether the design of the antenna should be modified and 3) a portable and low‐cost device to lift the antenna above the ground has been designed and constructed in order to increase the excitation of shallower water.These developments have been carried out using a NumisLITE Iris Instrument. Field experiments have shown the validity of the proposed theoretical and field techniques.