Ground Penetrating Radar (GPR) is a geophysical method used to identification subsurface condition at a certain depth, using a source of electromagnetic waves in the form of radar. Geoscanners Akula A9000C and Antenna Gecko 60 with 60 MHz antenna frequency were used for data acquisition for this study. Survey data was conducted as many four lines, located on the south and east sides of the lake, with the distance range from 0 to 145 m. This paper reports basic signal processing GPR data using matGPR for the analysis and modelling. The results show that there is anomaly which is suspected as water seepage appear on depth of 2 m. On the lengthwise line, anomalies show on a distance of 20 m and 35 m (line A) and 125 m and 135 m (line B), while on the crosswise line anomalies show on a distance of 35 m and 50 m (line C), 25 m and 95 m (line D). Water seepage can be found in this area as it is formed by alluvial depositional environment, remembering that the characteristics and rocks lithology in this area can stimulate water infiltration process. Water seepage will cause erosion of the lake and can collapse the lake’s structure.

The objective of this study was to identify the lithological layer around the lake body to determine the ability of the reservoir body structure to withstand surrounding seepage water. This research was conducted using the MAE X820S seismic instrument, carried out on a 46-meter-long track and 24 geophones. Seismic refraction processing is done by picking at the first break in each seismic trace. From the picking results, distance and time parameters will be obtained so that a seismic velocity model can be generated to be analyzed for lithological types based on the P-wave velocity. The results of the processing show that this research area consists of two rock layers. The range of P wave velocity owned by the first layer at a depth of 0-19 m is 648.11 m/s, interpreted as sand-gravel. The range of P wave velocity owned by the first layer at a depth of 19-30 m is 1553.21 m/s, interpreted as a layer of clay. Rock layers that have a sand-gravel lithology have the ability to seep or pass water, while layers that have a clay lithology cannot pass water. Therefore, the first layer is thought to have a less strong ability to withstand surrounding seepage water.