การสำรวจธรณีฟิสิกส์แบบบูรณาการเพื่อตรวจสอบการซึมของอ่างเก็บน้ำหนองปลาสะวาย จังหวัดลำพูน และอ่างเก็บน้ำห้วยม่วงตอนล่าง จังหวัดเชียงใหม่

Abstract

Geophysical surveys including electrical resistivity tomography (ERT), ground penetrating radar (GPR), and multichannel analysis of surface wave (MASW) were acquired at Nong Pla Sawai reservoir area, Lamphun province and Lower Huai Muang reservoir area, Chiang Mai Province, northern Thailand. The surveys aim to detect seepage locations and pathways of both reservoirs because it is suspected that the reservoirs may have had seepage problems since they cannot retain adequate volume of water even in the rainy season.

ERT, GPR and MASW surveys were performed along the flat surface of the dam ridge of the reservoirs. Additional ERT surveys were acquired on the reservoirfloor area, and across main channel of the watershed of each reservoir. Geological field surveys at the areas surrounding the two reservoirs were also conducted to investigate the geologic setting and to support geophysical data interpretation.

At Nong Pla Sawai reservoir, a possible reason of the seepage is that the basement of the reservoir is unconsolidated sediments. Moreover, the basement beneath the dam ridge also has an inhomogeneous characteristic. This basement interpretation is based on corresponding anomaly locations in both ERT and MASW sections at the dam ridge and is also supported by the geological field data. Additional information about the unconsolidated sediments is from the ERT sections at the reservoir floor area and across the main channel of the watershed. The ERT results show that the unconsolidated sediment layer has a possible thickness of greater than 47 m beyond the maximum depth of investigation. The unconsolidated sediments have high permeability; therefore, water can seep through the basement of the reservoir.

At Lower Huai Muang reservoir, all geophysical surveys at the dam ridge do not show any reservoir seepage anomaly in the dam core. The main reason that causes the seepage problem is uncompact material in the basement below the dam ridge. This is indicated by the ERT and MASW surveys that show high resistivity anomalies and low (less than 180 m/s) respectively. The geologic field data show that Lower Huai Muang reservoir is located on unconsolidated sediments, the same as Nong Pla Sawai reservoir.

Moreover, the nearby borehole data show that the thickness of the unconsolidated sediments is more than 72 m. Therefore, this could make the reservoir unable to retain water because of the high permeability of the reservoir basement.

The results about locations and causes of the seepage of both reservoirs from this study can be very helpful for the reservoir remediation