One part of the geoelectric method is DC (Direct Current) resistivity measurement. Physical parameters measured by the system are the current and potential difference. From these two parameters, the resistance value is obtained based on Ohm’s law. Based on the current-potential electrodes configuration in the DC resistivity measurement, there is the possibility of contact between the potential-current electrodes and also the position of the potential electrodes that can be exchanged. Thus, an auto-range Voltmeter is needed which can measure the potential difference with the order of mV up to hundreds of volts and can measure negative potential difference value (reversed polarity). This research aims to build a Voltmeter prototype using ADS1115 (namely ADC with 16-bit resolution) and Arduino Uno, which can be applied in DC resistivity measurement. In this system, there is internal resistance and switch. The internal resistance is around 2.4 MΩ, arranged into a voltage divider circuit to be able to measure the maximum potential input around 1,600 V. The switch function is to set the measurement mode (as auto-range), which is set by Arduino Uno. Furthermore, ADS1115 uses a differential measurement method to be able to measure a negative potential difference. Based on the results of tests on several resistor components with an adjustable voltage source, the Voltmeter is good at measuring the potential difference in resistor values below 10 KΩ with a confidence level of around 97%.

Infrastructure construction made andesite’s demand has increased, particularly in Lampung Province. In this research, its distribution in West Sungkai of North Lampung is mapped based on Electrical Resistivity Tomography (ERT) data from 6 lines, each of them was 186 m in length. Due to its excellent vertical resolution, Wenner configuration is performed. The research area is part of Quarter Holocene Volcanic (Qhv) formation. Lajur Barisan members consist of volcanic breccia, lava, and andesite-basalt tuff; thus, resistivity modeling is built within this aisle. Subsurface resistivity model has been created using the non-linear inversion method with promising low error at the third to fifth iterations, which marks an acceptable value. Using 2D and 3D ERT modeling, it is estimated that there are three mains of rocks based on their resistivity value: sandy tuff with 65 – 212 Ω m; tuff with 212 – 655 Ω m; and andesite with resistivity more than 655 Ω m. Andesite within this area is likely lava andesite which spread from the middle to the West and north. It is located at 5 – 35 m in depths with the reserve estimation of andesite is about 1.65 million tons.

Desa Gayau yang terletak di Kecamatan Padang Cermin, Kabupaten Pesawaran sering mengalami kekeringan air pada saat musim kemarau sehingga perlu dilakukannya pengukuran geolistrik untuk mengidentifikasi keberadaan lapisan akuifer air tanah. Pada penelitian ini konfigurasi yang digunakan adalah Schlumberger dengan panjang bentangan MN/2 sebesar 1, 5, 10, dan 20 meter. Sedangkan panjang bentangan AB/2 sebesar 6 hingga 300 meter. Pada tahapan inversi data VES, kami menggunakan algoritma Particle Swarm Optimization (PSO) untuk memperoleh nilai parameter resistivitas dan ketebalan lapisan. Algoritma ini dipilih karena cepat menuju konvergen dan relatif stabil. Hasil dari inversi ini diperoleh bahwa lapisan akuifer pertama berada pada kedalaman 21.4 - 52.1 meter dengan litologi batupasir berbutir halus dengan sisipan tufa Lapisan akuifer kedua berada pada kedalaman 52.1-70 meter dengan litologi breksi. Pada kedalaman 70 meter diinterpretasikan sebagai akuifer air tanah, namun memiliki debit yang kecil. Hal ini disebabkan karena litologi lapisan berupa breksi, dasit dan lava basal dari Formasi Hulusimpang. Sistem aliran air tanah pada lapisan ini merupakan sistem media pori yang berakibat pada debit rendah dan waktu pengisian kembali lapisan air tanah relatif lama.

An initial subsurface survey to predict the aquifer is important to avoid the unprospect drill location and getting groundwater with the right discharge. In this study, prediction of the aquifer was carried out using the seismic refraction method in Jatimulyo Village, Jati Agung District, South Lampung Regency. The data was collected on October 6, 2019 using the SUMMIT X One seismic tool with 24 Geophones. The first break data from p-wave travel time is processed and interpreted using the Hagiwara method. Interpretation results show that there are three layers of subsurface lithology in the study area. The weathering layer was found at 2 m to 3.5 m depth with average velocity of 360 m/s. The second layer is dry sand with a velocity of 890 m/s at a depth of 2 m to 8 m. The third layer with a 2300 m/s velocity is a mixture of clay rock and saturated sand at depths of more than 8 m. As a comparison there is resistivity distribution data on the same measurement line in the study area. The overall interpretation shows that at a depth of 12 m with saturated clay sedimentary rock as a prospect of aquifer in this study area.

Kegiatan pengabdian kepada masyarakat (PkM) telah dilakukan di desa Gaya, kecamatan Padang Cermin, kabupaten Pesawaran, provinsi Lampung. Permasalahan yang dialami desa tersebut adalah sarana penampungan air yang sudah dibuat tidak optimal digunakan karena sumur bor yang dimiliki, kondisi airnya sering kering. Sarana tersebut diperuntukkan bagi warga desa ketika musim kemarau tiba. Tim PkM mengajukan suatu pendekatan dari segi kerekayasaan geofisika untuk mengatasi permasalahan tersebut, berupa peninjauan kembali kedalaman akuifer sumur bor desa. Pendekatan tersebut menggunakan metode resistivitas 1D. Metode ini bertujuan untuk mengetahui lapisan batuan bawah permukaan bumi dengan dilihat dari distribusi nilai resistivitas terhadap kedalaman. Ada 3 titik pengukuran di sekitar sumur desa tersebut dengan posisi sumur berada di antara ketiga titik tersebut. 1 titik memiliki bentangan 400 m dan 2 titik yang lain sepanjang 600 m. Hasil analisis dari metode resistivitas 1D menjelaskan bahwa estimasi jenis batuannya berupa endapan alluvium, lempung tufaan, batu pasir tufaan, kerakal/ kerikil, dan lava andesit. Akuifer berada pada batu pasir tufaan atau kerakal/kerikil. Hasil analisis juga memberikan informasi bahwa posisi dan kedalaman sumur tidak berada pada akuifer. Dengan demikian, memberikan rekomendasi perlu adanya pengeboran ulang. Kemudian tim juga memberikan gambaran bahwa ada 2 posisi yang cocok untuk dibor beserta dengan kedalamannya.