Environmental and hydrogeological geophysics with applications in Thailand, Laos, and Sweden

Abstract: This thesis presents applications of geophysical methods in the fields of environment and hydrogeology. In relation to environmental problems, two different geophysical surveys have been carried out; one to study an arsenic contaminated area in the southern Thailand (paper I) and a second to locate shallow faults in Quaternary sediments in an area around the Ongkharak Nuclear Research Centre, central Thailand (paper II). For hydro-geological problems, surveys were conducted in southern Sweden (paper III) and in the Vientiane basin, Laos (paper IV). In the arsenic contaminated area, tin and associated minerals, i.e. arsenopyrite and pyrite, have been extracted from granites and natural processes and the mining activities led to arsenic contamination in the environment. Electrical resistivity and self potential (SP) have been used to define the distribution of arsenic contamination in the groundwater. Resistivities of 25 - 100 ohm-m and a positive SP anomaly of 66.0 mV were observed in an area where the arsenic content in auger water at 3.5 to 5.0 m depths was high, 0.5 - 5.0 mg/l. Integrated interpretation of resistivity, seismic refraction, GPR, and gravity data gave a clear image of subsurface structures at a depth to 30 m. There was a good correlation between the resistivity and the gravity data. A subsurface rise was found, which possibly acts as a naturally buried dam, separating a high contaminated area from a low contaminated area. This study has demonstrated that the combination of geophysical methods is successful in delineating contaminated areas and contributes to the understanding of a possible mechanism for the distribution of arsenic. In the Ongkharak Nuclear Research Centre area combined GPR and resistivity pseudosections (dipole-dipole and pole-pole arrays) have given a good image of shallow faults in Quaternary sediments, faults that were originally indicated from regional remote sensing interpretations. Horizontal discontinuities of reflected signals obtained by GPR and images of lateral resistivity variation have been correlated to faults or subsurface movement identified by geological mapping in trenches. This signature of the faults is caused by contrasts in dielectric permittivity and/or in resistivity, which originates from vertical displacement at sedimentary layers and from sediments filling the faults. The positions of sub-faults as identified by GPR and their strike directions obtained from the trenching data (N60°W - N70°W, N65°W - N70°W, and N30°W) agree with the general NW-SE trend of the major faults, the Mae Ping Fault Zone, the Nakhon Nayok-Prachinburi and the Ongkharak faults in central Thailand. Thermoluminescence (TL) dating showed that sub-faults in the area have been active at about 7,500-2,400, 4,800- 1,750, and 9,700-2,300 years ago. Thus, these sub-faults have been classified to be of the same generation and they are defined as "capable faults" with reference to the criteria of U.S. Nuclear Regulatory Commission. Magnetic Resonance Sounding (MRS) has been successfully tested for detecting groundwater and in combination with Vertical Electrical Sounding to characterizing aquifer in southern Sweden and in the Vientiane basin, Laos. The combination of MRS and VES in the southern areas of Sweden shows that low resistivity layers interpreted as clay are sometimes identified close to the surface. The results here have shown that the MRS signals penetrate through the clay and that deeper aquifer can still be detected. The MRS data suggest aquifers that are not only hosted in soft sediment materials (moraine, sand, and mixed materials), but also hosted in basement rocks. Based on the MRS and borehole pumping test data, the hydraulic conductivity of aquifers has been estimated and the results agree with yield, average water content and subsurface geological data. The results from the measurement in the Vientiane basin have shown that there is usually two - three water bearing layers, and the "best aquifer" is found at depths between 15 and 25 m, with regard to high water content, permeability and resistivities indicating fresh water. MRS has also shown to be an important tool in constraining layer thickness and distinguishing low resistive layers of impermeable rock from what could have been interpreted as water in the VES interpretation. MRS data also suggest a clay layer at depths between 30 and 50 m, which is overlying halite deposits. This clay layer may act as a naturally sealing layer to protect the fresh water in above aquifer from salt contamination. On the basis of this drilling for fresh water is recommended not to penetrate deeper than 20 to 40 m, depending of the local depth to the clay layer.