All too often in geotechnical works, slopes fail as a result of high seepage force arising from excessive rainfall or impoundment as well as poor compaction of fill material. One of the challenges facing geotechnical engineers is how to locate the high-seepage zones within slopes early during design and construction with a limited budget and time so that appropriate slope stabilization method can be arrived at. Groundwater Aeration Sound (GAS) survey technique recently developed in Japan is now being used in Thailand for studying seepage in various natural slopes. The technique involves using very high sensitivity accelerometer to pick up vibration or “sounds” of water flowing through unsaturated soil pores with air bubble bursting sounds. The device measures the Groundwater Aeration Sound (GAS) in the manufacturer’s representative index unit known as D value. GAS survey was carried out at two landslide sites in Northern and Southern Thailand and a distinct correlation was found between the locations of high D values (high GAS sounds), landslide scars and geological structures. In addition, tentative correlation between pore-water pressure and D values has been attempted and appeared to be site-specific.