The research employed various techniques, including data collection, calculations, analysis, and verification. Hydrogeological conditions, isotope ratio computations, and geoelectric and magnetotelluric interpretations were used to validate spring catchment regions. The study commenced with a literature review, the gathering of secondary data, and a field study to locate springs and other water sources. Indonesia’s Ministry of Energy and Mineral Resources published groundwater basin, hydrogeological, and geological maps, which were used in the study.
2.1 Study Sites
Ababi Village is located in Abang District, Karangasem Regency, Bali Province, and boasts a spring at 8° 24' 8.14" S and 115° 35' 12.85" E (Fig. 1). Lake Batur extends further northwest, while Mount Agung is located northwest of the Ababi Spring. Ababi Village, spanning 10.86 km² and with an average elevation of 573 m above sea level (ASL), is a distinct and lovely place. Generally speaking, the village area is situated at the base of Mount Agung, with a Southeast-facing land slope. The tallest region is in the north, while modest hills, primarily rice fields, may be found in the east and center. There are plains to the west and south. The range of daytime temperatures was 29 to 35°C.
2.2 Hydrogeological environment
The Amlapura groundwater basin includes Ababi Village (Mudiana & Setiadi, 2008). The Amlapura groundwater basin occupies an area of roughly 213.6 km2 and is situated in the Karangasem Regency in the eastern portion of Bali Province. With an average annual rainfall of 1,000 to 3,500 mm, it has a shallow groundwater potential of around 60 million m3/year and a profound groundwater potential of about 2 million m3/year. The basin's radial river flow pattern originates at the volcanic cone and expands outward in all directions, creating plains, rolling hills, and volcanic cones as morphological units. The basin's topographic elevation ranges from 0 to 3,500 m above sea level (Dinas Pekerjaan Umum Provinsi Bali, 2014).
The main lithology of this basin, as shown in Fig. 3, is alluvium deposits found in rivers and on beaches, which serve as an aquifer, which are primarily made up of medium-gradient sand and gravel; Agung volcanic rocks (Qhva), which are primarily made up of medium-gradient agglomerates, tuffs, lavas, and lahar deposits; and Seraya volcano rocks (Qvps), which are made up of low-gradient volcanic breccia and lava. (Purbo-Hadiwidjojo et al., 1998). A spring with a 5 l/s discharge rate is present (Sudadi et al., 1986).
2.3 Stable Isotopes
Sun-evaporated seawater initiates hydrological cycles by becoming airborne through evaporation, with the wind subsequently carrying it to various locations such as the land, aquifers, rivers, lakes, and back to the sea. As the altitude decreases, the air pressure decreases, with mountains having a lower air pressure than the coast. Water-laden air rises or falls under atmospheric pressure, which affects the movement of water in the hydrological cycle. Height decreases the water-laden air temperature (lapse rate). Hydrogeologists use water-forming isotopes such as hydrogen (1H, 2H, and 3H) and oxygen (16O, 17O, and 18O) (Mazor, 2004; Goldscheider & Drew, 2007). The evaporated water had the following hydrogen and oxygen isotopes: 1H 99.985%, 2H 0.015%, and 3H less than 0.001%; 16O 99.63%, 17O 0.0375%, and 18O 0.1995%. 16O and 2H precipitation decrease by 0.15–0.5 and 1–4 per 100 m in elevation, respectively (Clark, 2015).
The D/1H and 18O/16O isotope ratios in water measured very low relative abundances (R) of 18O and 2H (or deuterium = D). RStd, SMOW, or seawater's D/1H or 18O/16O isotope ratios were compared. The ocean has the most significant evaporation process in the hydrological cycle; therefore, seawater has been used internationally as a reference. Most water samples (excluding seawater) had negative D and 18O isotopes. Figure 4 shows the hydrological cycle D and 18O isotope fractionation (Pang et al., 2017; Kresic & Stevanovic, 2010; Mazor, 2004). In units of ‰ (per mil), the notations (δD)SMOW and (δ18O)SMOW are denoted as (δD) and (δ18O), respectively. δsmpl was used to express the sample isotopes' relative abundance (Nuha et al., 2020):
\({\text{δ}}_{\text{sample}}\text{= }\frac{{\text{R}}_{\text{sample}}\text{-}{\text{R}}_{\text{std}}\text{ }}{{\text{R}}_{\text{std}}}\text{ x 100\%}\) | (1) |
The relative abundances of the samples were the 18O/16O and 2H/1H isotope ratios, where RStd is the SMOW standard ratio. The 18O and 2H concentrations of the hydrological cycle vary due to these water isotopes' freezing points and vapor pressures. Evaporation, condensation, freezing, thawing, chemical reactions, and biological processes can cause isotope fractionation (Pang et al. 2017). Unless altered by magma, mixing, or evaporation, groundwater's 18O and 2H concentrations lie along the local meteoric line (rainwater). The straight line for rainwater differs from the 18O and 2H graphs.
Elevation and precipitation frequencies affected the D/18O isotope ratios. (D) and (18O) increase with decreasing frequency and precipitation. Rain and precipitation frequency affect (D) and (18O) at each sampling site; hence, equations 2 and 3 must be used to calculate the average value (Kresic & Stevanovic, 2010; Clark, 2015):
\({\text{δ}}^{\text{18}}\text{O= }\frac{\sum _{\text{i=1}}^{\text{n}}{\text{P}}_{\text{i}}{\text{δ}}_{\text{i}}\text{18}\text{O}}{\sum _{\text{i=1}}^{\text{n}}{\text{P}}_{\text{i}}}\) | (2) |
\({\text{δ}}^{\text{2}}\text{H= }\frac{\sum _{\text{i=1}}^{\text{n}}{\text{P}}_{\text{i}}{\text{δ}}_{\text{i}}\text{2}\text{H}}{\sum _{\text{i=1}}^{\text{n}}{\text{P}}_{\text{i}}}\) | (3) |
where the following are included in the data set: Pi is the rainfall quantity between samples (i-1) and (i) (mm per month), δi18O is the isotopic ratio of 18O to SMOW in rainfall to (i) (‰), and δi2H is the isotopic ratio of 2H close to SMOW in rainwater to (i) (‰).
2.4 Electricity Sounding Vertically (VES)
The VES measures the electrical resistance. Surveys measuring electrical resistivity provide direct current between the electrodes while evaluating the potential between them. The current penetration matched the electrode distance. Resistance stratification is visible in the electrode spacing (Hamzah et al., 2007). The Naniura NRD-300 HF was used for the VES survey, along with a laptop, electrodes, a machine, a cable, a connector, and a clamp (Fig. 5). Four straight-lined electrodes were spaced differently. Four electrodes require 200 m of wire, while 2 m are required. The two outer electrodes are currents, and the two inside electrodes have potential in a Schlumberger array (Koefoed). The spacing of the current electrode was 2–200 m (AB/2 = 1–100 m). 0.5–10 m separated potential electrodes (MN). Changes in the electrode spacing between measurements were collected from field data (Hamzah et al., 2007). According to Suryadi et al. (2018), the resistivity value can be differentiated or partitioned using the master curve developed by Schlumberger. Plotting the apparent resistivity on a graph log was the first step, after which the shape of the plotted apparent resistivity was compared or approximated by looking for a curve. The master curve can be used to calculate the resistivity and thickness partition.
2.5 Audio Magnetotelluric (AMT)
An audio magnetotelluric (AMT) passive geophysical investigation utilizes high-frequency electromagnetic pulses ranging from 000 to 5 Hz to explore shallow to intense subsurface electrical conductivity. The AMT method determines subsurface electrical conductivity by measuring electromagnetic waves produced by the earth (Chave & Jones, 2012; Simpson & Bahr, 2005). The subsurface electrical conductivity of Earth is estimated via the electromagnetic passive-source inductive audio magnetotelluric model. Subsurface conductivity was measured using transient surface electric and magnetic fields. Two external electromagnetic (EM) signals were used in the AMT and magnetotelluric (MT) investigations. The magnetosphere and atmosphere release EM signals. Low-frequency MT signals are emitted by the solar wind, auroras, and Earth's magnetosphere. Around the world, high-frequency audio range (AMT) emissions (> 1 Hz) are produced by lightning and thunderstorms. In the crust of the Earth, EM signal exchanges produce horizontal telluric currents (Simpson and Bahr 2005). MT surveys measured magnetic signals and telluric currents.
Using orthogonal sensors, the AMT examines the electric and magnetic changes caused by telluric currents. Time-series data was processed using the frequency domain method. In the measured field frequency domain, an estimate of the transfer function (complex impedance matrix) is made. Using forward modeling and inversion techniques, resistivity distribution models (1-D, 2-D, and 3-D) were created from the MT data. The isotope analysis results were confirmed by field testing using VES and AMT. The recharge and discharge locations provided the VES and AMT data. Six AMT and VES trajectory points were used. The data on the 18O and 2H isotope ratios from rainfall samples gathered in the recharge area—which is close to Mount Agung—was also used for verification. AIDU 2D software was used to assess the 2D ADMT Method survey from the Anbit ADMT-300HT2 Smart AMT instrument (Fig. 6).