The increasing demand for gypsum as a raw material for construction projects motivates exploration for additional reserves. Electrical resistivity tomography (ERT) and seismic refraction geophysical methods, augmented with borehole and laboratory measurements on core samples, are used here to delineate the top, bottom and lateral boundaries of an important gypsum ore deposit in Thailand, an economically developing region. The gypsum-bearing formation is found throughout the study area to have an irregular upper boundary on account of karstic dissolution processes. The deeper transition from gypsum to anhydrite, however, is not constrained by the measurements. The P-wave velocity measured in the field is consistent with the core specimen measurements. The electrical resistivity of the core specimens, however, is substantially higher than the values measured in the field. The specimen measurements may depend on the presence of micro cracks, whereas electrical resistivity in the field may be affected by the enclosing clay-rich materials.