SNMR measurements in Thailand to investigate low‐porosity aquifers

Abstract

ABSTRACT Integrated geophysical surveys have been carried out at two different locations in Thailand in the context of the Thai‐German Research Project “Investigation of Waste Disposal Sites ‐ WADIS”. Surface nuclear magnetic resonance (SNMR, MRS) was used in addition to conventional geophysics. Although a high total intensity of the earth’s magnetic field (TMI) offers advantageous physical conditions for SNMR, only very low signal amplitudes could be measured. Due to the long acquisition times caused by high stacking rates, diurnal variations in TMI may have had a considerable influence on the SNMR signal. The two investigated sites are characterized by different geological and physical ground conditions. An unusual feature is an extremely high local magnetic anomaly caused by magnetic landfill material at the Nong Harn dump site in northern Thailand. Because of strong field inhomogeneities, SNMR measurements were impractical in the sphere of influence of this anomaly. The Dan Khun Thot site in central Thailand is characterized by very high ground conductivities, obviously due to saline pore fluids. Despite the evidence of water occurring in several narrow sand layers observed in shallow holes, the water content deduced from the SNMR measurements turns out to be very low. An explanation for the apparent discrepancy is the irregular distribution of the water in lens‐shaped confined aquifers. Forward modelling of an elementary 3D situation and its comparison with 1D inversion illustrate that this hydrogeological situation cannot be treated correctly using the usual 1D data collection and 1D interpretation algorithms.