Gravity survey is one of the passive geophysical techniques commonly used to delineate geological formations, especially in determining basement rock and the overlying deposit. Geologically, the study area is made up of thick quaternary alluvium deposited on top of the older basement rock. The Muda River basin constitutes, approximately, of more than 300 m of thick quaternary alluvium overlying the unknown basement rock type. Previous studies, including drilling and geo-electrical resistivity surveys, were conducted in the area but none of them managed to conclusively determine the basement rock type and depth precisely. Hence, a regional gravity survey was conducted to determine the thickness of the quaternary sediments prior to assessing the sustainability of the Muda River basin. Gravity readings were made at 347 gravity stations spaced at 3–5 km intervals using Scintrex CG-3 covering an area and a perimeter of 9000 km² and 730 km, respectively. The gravity data were then conventionally reduced for drift, free air, latitude, Bouguer, and terrain corrections. These data were then consequently analyzed to generate Bouguer, regional and total horizontal derivative (THD) anomaly maps for qualitative and quantitative interpretations. The Bouguer gravity anomaly map shows low gravity values in the north-eastern part of the study area interpreted as representing the Main Range granitic body, while relatively higher gravity values observed in the south-western part are interpreted as representing sedimentary rocks of Semanggol and Mahang formations. Patterns observed in the THD anomaly and Euler deconvolution maps closely resembled the presence of structural features such as fault lineaments dominantly trending along NW-SE and NE-SW like the trends of topographic lineaments in the study area. Based on power spectral analysis of the gravity data, the average depth of shallow body, representing alluvium, and deep body, representing underlying rock formations, are 0.5 km and 1.2 km, respectively. The thickness of Quaternary sediment and the depth of sedimentary formation can be more precisely estimated by other geophysical techniques such as the seismic reflection survey.

Punca air amat penting bagi kegunaan harian dan kegiatan ekonomi pertanian. Bekalan air bersih di kawasan Pantai Barat Kota Belud, Sabah terhad terutamanya sewaktu musim kemarau. Oleh itu, air bawah tanah boleh dijadikan sebagai punca bekalan air alternatif. Satu kajian telah dijalankan bagi menentukan jenis akuifer di Pantai Barat Kota Belud untuk tujuan ini. Berdasarkan cerapan lapangan pada singkapan yang dijumpai di Kampung Piasau dan analisis log lubang gerudi di Kampung Ratau, kawasan kajian terdiri daripada selang lapis batu pasir dan syal. Gabungan kaedah pemetaan profil luluh hawa dan seismik pantulan menggunakan teknik pemprofilan titik tengah sepunya (CMP) menunjukkan akuifer di kawasan kajian adalah jenis akuifer batu pasir teretak yang boleh mencecah ketebalan sehingga 300 m. Kehadiran kekar pada singkapan batu pasir yang mempunyai gred luluh hawa II hingga IV dan struktur sesar normal dan sesar sungkup di bahagian subpermukaan (melalui pentafsiran data seismik pantulan di Kampung Piasau dan Kampung Kalibungan) telah meningkatkan keliangan dan ketertelapan batu pasir sebagai akuifer.

This study aims to clarify the groundwater recharge and flow processes in a basin located in a tropical humid area with a complex geological setting. We conducted intensive sampling campaigns for river water and groundwater during three different time periods in the relatively dry season (September 2019, January-March 2020, and August-September 2022). The 40 river water samples and 25 groundwater samples were taken from the upper-stream area with an altitude of 527 m to the downstream area with an altitude of 10 m. The oxygen-18 (δ18O) and deuterium stable isotopic compositions and inorganic constituent concentrations were determined on all water samples. We also used the monthly stable isotopic compositions in rainwater observed at a location with an altitude of 26 m, 15 km apart from the Klang River basin, by the Global Network of Isotopes in Precipitation database (GNIP), IAEA. These chemical compositions were used as tracers to investigate the groundwater recharge and flow system. The deep groundwater shows a lower δ18O than the volume-weighted mean of rainwater and higher ion concentrations, whereas the shallow groundwater shows a higher δ18O and lower ion concentrations. This suggests that the deep groundwater with low δ18O seems to be recharged in the mountainous area with an altitude ranging from 70 to 1421 m. Additionally, we conducted a principal component analysis and cluster analysis using inorganic constituent concentrations and stable isotopic compositions, showing that the deep and shallow groundwater samples are classified into two groups. This shows that the deep groundwater in the downstream area is recharged mainly in the mountainous areas with the highest altitude of 1421 m, and the shallow groundwater is recharged partly in the hilly areas with the highest altitude of 250 m. We believe our study serves new findings on the groundwater flow system in mega-cities of tropical climate regions.