Sub-surface configuration in the northern part of Lembang groundwater basin recharge area

Lembang groundwater basin has an area of 209 km2. Increased tourism activities in the Lembang Groundwater Basin require sufficient groundwater resources to support the sustainability of these tourism activities. The purpose of this study was to analyze the hydrostratigraphy of the groundwater recharge zone of the Lembang groundwater basin in the Cikole-Lembang. Electrical Resistivity Tomography (ERT) with dipole–dipole electrode array and seismic refraction was used to analyze the hydrostratigraphy. The results of ERT and seismic refraction inversion show that Tangkubanparahu Pyroclastic Fall 2 can be characterized as aquifers. This layer consists of low (123–292 Ω\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega $$\end{document}m) and intermediate (293–700 Ω\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega $$\end{document} m) resistivity value and also has low (300–1350 m/s) to medium (2700–1350 m/s) velocity. Tangkubanparahu lava (Tl) layer has a high resistivity value (701–3875 Ω\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega $$\end{document} m) and high (1350–2999 m/s) velocity. The Tangkubanparahu Pyroclastic Fall 1 has a resistivity of 300–700 ohms. Tangkubanparahu (Tl) lava deposits can be characterized as fracture aquifer and impermeable layers in the Tangkubanparahu volcanic hydrogeological system depend on the historical structural geology event.


Introduction
Water is a valuable component of the environment and is essential for the survival of life living on the earth (Hossain et al. 2021). Water resources play a significant role by providing an adequate food supply and sustainable environment for all living organisms globally (Foster and Loucks 2006). The availability of worldwide freshwater is only 2.5%, about 0.3% can be extracted economically, and over two-thirds of this is frozen in polar ice caps and glaciers (Smith 2003). Groundwater is an important freshwater resources. Groundwater is an essential life-sustaining resource for billions of people worldwide, providing a buffer against precipitation variability and water shortages during drought (Giordano 2009). Groundwater is the most essential and valuable resource for agriculture, domestic and industrial usages and playing a vital role in supporting the country's economy, atmosphere, and livelihood (Böhlke 2002;Lee and Song 2007). The rising water demand in combination with urbanization, climate, and land use change poses an increasing stress on the worldwide groundwater resources (Firman et al. 2011;Taylor et al. 2013). The stress due to overpopulation, intensification of agriculture, urbanization, industrialization, and the impacts of climate change. Groundwater storage is shown to be declined in all populated continents (Konikow and Kendy 2005). Besides, many of the world's aquifers are rapidly being depleted (Cassardo and Jones 2011;Oki and Kanae 2006;Shiklomanov, 1998). When the continuous over-pumping of groundwater occurs, the recharge of groundwater cannot replenish the drawdown of the aquifers (Awaleh et al. 2017;de Vries and Simmers 2002;Mauser and Ludwig 2016). Not only the demand of groundwater is increasing, but rainfall and recharge to groundwater are also expected to decline in many semi-arid regions where irrigation mainly depends on groundwater (Tian et al. 2015). Groundwater is used as the water supply for various purposes in major Indonesian cities. Its use has been continuously increasing in recent decades (Tirtomihardjo 2016). In several large cities in Java, such as Bandung, deterioration in both the quantity and quality of groundwater has been observed, while degradation of the environment such as land subsidence due to groundwater overexploitation is readily evident (Schmidt et al. 2011). Lembang City is located within the Lembang Groundwater Basin. Lembang has an area of 95.56 km 2 . Lembang City can be reached around 15 km from Bandung. Lembang groundwater basin has an area of 209 km 2 . In the southern part of the lembang, groundwater basin is bordered by the Bandung-Soreang groundwater basin. In the northern region, it is bordered by the Ciater groundwater basin, in the eastern part is bordered by the Sumedang groundwater basin, and in the west, it is bordered by non-groundwater basins. The population in lembang city is approximately 196,690 people. Lembang has 16 villages, one of which is the village of Cikole. The tourism sector is the growing sector in this city. There are around 36 tourist destinations. With the increasing activity of tourism, the need for water resources, especially groundwater, to support these activities will increase. Some of the Lembang's destination tourism object are located in the groundwater recharge zone, especially around Mount Tangkubanparahu. This situation has threaten the sustainability of groundwater in this basin. However, the detail sub-surface hydrogeological information in this area are limited. The aim of this study was to analyze the subsurface configuration of the groundwater recharge area especially, in the northern part of the Lembang, groundwater basin to obtain the detail information.
Groundwater recharge is the amount of adequate precipitation that infiltrates into the unsaturated zone and then percolates as a water yield to the water table after exceeding the soil field capacity. Infiltration refers to water movement downward from the surface into the subsurface (Seiler and GAT, 2012).
This study was conducted in the Cikole-Lembang. The area located on the southern flank of Mount Tangkubanparahu, administratively included in West Bandung Regency. This area lies in the northern part of the Lembang groundwater basin. Tangkubanparahu is an active volcano as the youngest phase of the continuation of the Sunda volcanism system with a complex geological history (Yuniardi et al. 2019). Volcanic area has a great groundwater potential. However, this area has a complex aquifer system that varied in lithology. Resistivity and seismic methods are a robust and reliable combination for mapping subsurface, especially aquifer configurations in the volcanic area (Donohue et al. 2015;McClymont et al. 2011;Onugba et al. 1989;Saribudak and Hauwert 2017). This study, uses the ERT and Seismic refraction methods to analyze the sub-surface configuration. The information from this study will be the valuable output due to the lack of detail information of the subsurface geological information in this study area. This is would become the novelty of this study.
Seismic methods have been widely used in the study of hydrogeology because of the reliability of this method in analyzing subsurface (Grelle and Guadagno 2009;Parker et al. 2010;McClymont et al. 2011;Donohue et al. 2015;Hellman et al. 2017;Saribudak and Hauwert 2017;Adelinet et al. 2018). Seismic refraction can be used to determine volcanic deposits (Adelinet et al. 2018;Clarke et al. 2009). This study has shown that ERT and Seismic methods are suitable to characterize aquifer in the volcanic area to obtained detail information about sub-surface hydrogeological information. (Fig. 1)

Regional geology
The oldest rock unit in this area is a tertiary sedimentary rock, which consists of claystone interfinger with sandstone (Yuniardi et al. 2019). This rock unit is covered by the Pre-Sunda Volcanic Rocks Unit, which consists of lava and pyroclastic, Lower-Upper Pleistocene Sunda Andesite Volcanic unit, Upper Pleistocene Pyroclastic Sunda, early Holocene andesitic Tangkubanparahu and Late Holocene Pyroclastic. Tangkubanparahu is deposited above Pre-Sunda Volcanic Unit. (Fig. 2) There are several rocks units in the study area (Yuniardi et al. 2019  Older Volcanic Product (Qob) consists of Breccia, lahar, and sandy tuff. Tjitalang Formation (Pt) (500-600 m) consists of Tuffaceous marl intercalated by tuffaceous sandstone, breccia, and conglomerate. Kaliwangu formation (Pk) consists of Tuffaceous sandstone, conglomerate, and claystone. rarely calcareous sandstone, limestone, and thin layers of peat and lignite. Sandstone and conglomerate contain abundant mollusks. Tjilanang Formation (Mtjl) consists of Tuffaceous marl and limestone. Well-bedded. The exposure is found on the bank of Tjimanuk near the village of Kapek.
Tangkubanparahu is a volcano formed in the youngest phase of the continuation of the Sunda-Tangkubanparahu volcanic system, which has a fairly long and complex geological history. Tangkubanparahu volcanic rock unit consists of 7 (seven) groups of volcanic units, from the oldest to youngest, i.e., Tertiary Volcano, Pre-Sunda Volcano, Sunda Volcano, Kandangsapi Volcano, Dano Volcano, Bukitunggul-Manglayang Volcanic Complex, and Tangkubanparahu volcano. In addition, there are fluviatile and lake deposits rocks as non-volcanic rock deposits. Volcanic rocks that are thought to be Tertiary are exposed in the southwestern lower slopes and northeastern, central slopes to form the morphology of hill ridges so that they are not covered by younger volcanic deposits (Yuniardi et al. 2019).
According to Tephrachronology, volcanism of Sunda volcano-Tangkubanparahu complex divided into four volcanism phases, from the oldest to the youngest Pre-Sunda, Sunda, Tangkubanparahu dan Tangkubanparahu. Two caldera successions resulted in deposits of pyroclastic flow or the Cisarua ignimbrite and the Manglayang ignimbrite, which were spread almost on the entire slope of Tangkubanparahu. Ignimbrite Manglayang contains a number of accretionary lapilli as an indication that the eruption of paroxysms associated with the formation of the Sunda caldera is preceded by eruptions associated with the phreatomagmatic system. The Sunda volcanism phase around 210-105 The relationship between springs and volcanic facies in Tangkuban Perahu consists of three facies: first, the Volcanic Core Facies are located at elevations 3050-3172 m. dpl, consisting of andesites. This facies is impermeable, has no springs. Both Volcanic Proximal Faces are distributed at elevations (500-2076) masl, and consist of 2a) Proximal 1 at elevations (1250-2076) masl is composed of impermeable pyroclastic elevations and fragments andesite, tuff matrix. 2b) Proximal 2 at elevation (650-1250) m.dpl composed of andesitic lava, which generally contains fractures. In this facies there is zone spring 1 consisting of 142 springs with a total discharge of 178 l/sec. The three Distal Faces (Volcanic Distal Facies) are located at elevation (100-650) m.asl, consists of permeable lava, andesite fragments embedded in the tuff matrix or volcanic sand. Rocks show fractures with dimensions and geometry not identified. In this facies, the springs zone 2 consists of 53 springs with a total discharge of 700 l/s.
The volcanism evolution of Tangkuban Perahu produced a two-phase caldera system around 200 thousand and 90 thousand years ago. Therefore, the life of the population on the Tangkubanparahu slope is always facing the threat of disaster due to activities, especially on the north or northeast slopes that experience the direct impact of the eruption. The Southern slope of mount Tangkubanparahu consists of Extensive & moderately productive aquifers. Groundwater aquifers in volcanic deposits are suitable and have good permeability (Yuniardi et al. 2019). The recharge zone around Mount Tangkubanparahu is spread at an elevation of 700-1000 masl and an elevation of > 1300 masl.

Regional hydrogeology
According to (Delinom 2009), younger volcanic material Tangkubanparahu is an aquifer. These deposits consist of coarse tuff fractions. Grain size varies from fine sand to gravel, but the dominant fraction is medium to fine sand. The thickness of these water-bearing deposits is very different from less than 1 to 30 m or more. Effusive materials such as basalt and andesite flow are poor aquifers because they are usually compacted. However, lava flows closer to Tangkuban Parahu appear to be more porous and cracked and they can be considered aquifers. It can be considered as aquifers (Delinom 2011). In the area near the center of the eruption, perched aquifers have been developed in the youngest tuff and ash, underlain by lava. This aquifer is used by dug-well; where morphological conditions are favorable, springs come out (Delinom 2009).
The characteristic of aquifers in the study area can be classified into two. First, extensive moderately productive aquifer i.e., the aquifer of largely varies transmissivity. The groundwater table is generally deep. Spring discharge largely varying, generally great in the vesicular lava flow. Reaching more than 100 l/sec wells yields generally less than 5 l/sec. Second, Moderately productive aquifers. Groundwater flow is limited to fissure or fractures of vesicular lavas. The water table varies extremely. Springs discharge is generally high, more than 100 l/sec. (Fig. 3)

Study area
The study area is located in Cikole-Lembang, West Bandung Regency, West Java at coordinates 6° 46′53.7 ″S 107° 38′10.6″ E. This study used a single electrical resistivity tomography (ERT) traverse and single seismic refraction traverse which will be discussed further in the next section. Electrical resistivity tomography (ERT) measurements using a dipole-dipole configuration with 10 m electrode spacing

Electrical resistivity tomography (ERT)
Data from ERT measurements in the study area were analyzed as part of this study. Field measurements were conducted from a single ERT line. The line is placed near the seismic refraction line to get a representative correlation of aquifer resistivity. ER measurements are collected using Advanced Geosciences Inc. (AGI) SuperSting R8, an 8-channel ER meter. Direct current is introduced to the subsurface through the current electrodes, and the resulting voltage is measured through the potential electrodes. An array of up to 56 electrodes spaced 10 m apart, was pushed about 0.25 m into the ground, and connected via electrode cables and switch boxes to the central ER meter. The dipole-dipole collection configuration is used for better geological variability of images related to volcanic sediment aquifers. The dipole-dipole array has better horizontal resolution and better depth coverage at the ends of the lines (Neyamadpour et al. 2010).The measurement time is set at 1.2 s, and measurements are stacked (average) twice. Except if the standard deviation is more than 2%. In that case, a third measurement is taken and is included in the average. (Fig. 4) AGI EarthImager 2D software version 2.4.0 were used to analyzed the data. The software is used to create a 2D resistivity profile. The inversion method combines the finite element model and the Smooth Model Inversion, which is recommended for its stability and durability for all data types. The default EarthImager "surface" inversion setting is used on all data sets to ensure comparison can be made across sites (Vogelgesang et al. 2019). A fine model inversion method is used. The inversion mesh is fine for near-surface areas in each transect and is roughened with depth. Resistivity values below one Ohm-m or above 10,000 Ohm-m have been removed because these values usually represent incorrect data. The stop reversal criteria are saved in the default settings. The final model has an average root mean square of 2.92% and an average L2 ratio of 0.9 (Vogelgesang et al. 2019). For all tomograms, negative pseudo resistivity or N50% non-conforming relative data are removed (Vogelgesang et al. 2020). (Fig. 5)

Seismic refraction
Seismic refraction surveys conducted in the field involve one line. Lines are shot in forward and backward order. The source of the wave is the plate which hit the 4 kg sledgehammer. 24 geophones of natural frequency of 10 Hz are regularly scattered on a line and measure the acceleration of waves in a vertical direction. The distance between two geophones is 5 m. No frequency filter applied during data acquisition. Destinations differ by site. The shot sample collects raw data. Tracing the normalized amplitude is used for display purposes. Data were analyzed using the Pickwin software program (ver 5.1.1.2) and Plotrefa (ver 3.0.0.6) of the SeisImager software package developed by Geometrics Inc.

Result and discussion
Result ERT line has a length of 540 m with an electrode spacing of 10 m using a dipole-dipole electrode configuration. This line has an east-west direction. The maximum depth that can Resistivity classified into three zones: the low, medium, and high resistivity zone. The low resistivity zone has a resistivity of 123-292 Ωm. It has colored dark to light blue. The zone is identified at a depth of 0-25 m. The low resistivity zone layer is found in all parts of the cross-section from the west to the east ERT line. The layer with low resistivity value is thickened on the east side of the cross-section. The medium resistivity zone has a resistivity of 293-700 Ωm. It has a color of green to yellow. This layer is spread in the west-east part of the cross-section. This layer is located under the low resistivity layer-this layer situated at a depth of 13-77 m and 77-103 m. The medium resistivity layer is thickened on the east side of the cross-section and thinned on the west side of the cross-section. The high resistivity zone layer has a resistivity of 701-3875 Ωm. The layer located in the middle of the ERT cross-section. This layer lies at a depth of 27-77 m with a layer thickness of 50 m. This layer is thickened in the west section and thinning in the east section. This layer is in the middle resistivity zone. (Fig. 6) The seismic refraction line has a length of 115 m with 5 m of geophone spacing. The seismic line has a relatively east-west direction and is about 25 m to the north of the ERT Line. Based on the results of the seismic refraction inversion using PlotRefa software, the maximum depth of inversion values is 40 m. The inversion results can generally be classified into three-layer zones. The layer zone with low velocity is at a depth of 0-8 m with the value of the velocity of 300-1350 m/s colored pink to yellow, spread from the west to the east seismic cross-section. The layer zone with medium velocity is at a depth of 8-23 m with the value of velocity 2700-1350 m/s colored green to dark green. The layer zone with high velocity is at a depth of 23-40 m with the value of velocity 1350-2999 m/s colored blue. (Fig. 7)

Discussion
There are several rocks unit in the study area (Barkah and Daud 2021). From the oldest to the youngest Alluvium (Qa), Colluvium (Qc), Sandy Tuff (Qyd), Breccia and Agglomerate (Qyb), Pumiceous Tuff (Qyt), Undifferentiated Young Volcanic Products (Qyu), Undifferentiated Old Volcanic Products (Qvu), Older Volcanic Products (Qob), Tjitalang Formation (Pt) and Kaliwangu Formation (Pk). Alluvium (Qa) consist of Caly, silt, sand and gravel mainly deposits of Holocene streams. Colluvium (Qc) mainly debris from old volcanic products. This result have not yet specified in detail the geometry of each rock unit/formation and are still qualitative in the results of geological mapping so that the configuration of each formation is still somewhat ambiguous.
Tangkubanparahu Pyroclastic Fall 2 (Tjp 2), consists of scoria, pumice, andesite and basalt lithic fragments of lapilli to pebble size. This deposit is well stratified, loose to consolidated, normal grading, weathered brownish yellow color, the thickness varies from thin lamination to 70 cm. it is distributed in the summit area of Tangkubanparahu reach as far as the rim of Sunda caldera. Below Tangkubanparahu Pyroclastic Fall 2 there are Tangkubanparahu Lava(Tl). Basalt lava flows consist of plagioclase, pyroxene with few olivine set up in fine grained minerals groundmass. Blocky structure on the upper part and commonly massive to sheety joints in the middle and lower part of the flows. These lava flows are the eruption products of the main crater of Tangkubanparahu. Well exposed around the summit area, its flank and river valleys such as at Cisarua, Sagalaherang, Cibeureum, east Cimahi, Maribaya reach as far as Dago in northern Bandung. These lava flows are exposed alternatedly with pyroclastic fall 1 (Tjp 1) around crater walls of Ratu, Upas and Baru craters. Below Tangkubanparahu Lava(Tl) deposited Tangkubanparahu Pyroclastic Fall 1 (Tjp 1), consists dominantly of pumice 1-5 cm in diameter, scoria, basalt, and andesite lithic fragments, occasionally volcanic bombs are found. This unit is characterized by thin layers to 1 m thick. Each layer is often separated by soil horizon. The colour ranges from yellowish brown, blackish rey to yellowish white. The thickness between one and other periods varies from 5 to 25 m. Widely spread around the summit area and on the ridges of its flanks. Below Tangkubanparahu Pyroclastic Fall 1 (Tjp 1), deposited Sunda Pyroclastic flow  (Sap). Consist of about 20% juvenile material such as pumice, scoria, and obsidian and the rests (80%) consists of lithic fragments from the country rocks. The juvenile clasts are black vesicular scoria, grey pumice and few obsidian. The composition ranges between basaltic andesite to andesite (53-63% SiO2). The lithic material of 5 cm in diameter at the same distance. Well exposed in valley walls and quaries. Generally the lithic fragments are valley walls and quaries. Generally the lithic fragments are angular to sub angular, loose to moderately compacted, contain wood charcoal and the age resulted is 38.300 years. The deposits cover an area of about 200 km2, the thickness varies from 8 to 180 m on valley walls. There are Sunda Lava (Sl) below Sunda Pyroclastic flow deposits (Sap). It is consists of andesite lava, dark grey, porphyritic texture with plagioclase and pyroxene phenocrysts set up in volcanic glass and fine grained mineral groundmass. Generally, they show massive structure, blocky surface but columnar joint is also found. Most of them are weathered. The stratigraphy of the Tangkubanparahu complex is explained in detail in Nasution et al. (2004). Pre-caldera, Syn-caldera & post-caldera episodes. The post-caldera is divided into two major units namely Old Tangkubanparahu Volcanics and Young Tangkuban Parahu Volcanics. Old Tangkubanparahu Volcanics from its eruption in the form of Magmatic-PhreatoMagmatic with the oldest stratigraphy sequence is TP Andesite and the youngest is TP Pyroclastics. For major Tangkubanparahu Volcanic major units, the eruption type is Phreatic in the order of stratigraphy, TP Tefra, Domas, Ratu, Siluman, Ciater and Baru. Kartadinata et al. (2002) can classify in young Tangkubanparahu tephra or phreatic deposits structures. The authors do not specify exactly how deep each rock unit is in the study area.
The inversion results using Software 2D EarthImager show the maximum depth that can be modeled is 103 m. While the inversion results using software PlotRefa show that the maximum depth that can be modeled is 40 m. The results of ERT inversion using software EarthImager 2D show that there are three layers of resistivity i.e. low resistivity zones, medium resistivity zones, and high resistivity zones, while the results of seismic inversion using software PlotRefa show that there are three zones of Velocity i.e. low, medium and high-velocity zones. The low resistivity zone has a resistivity of 123-292 Ωm colored by dark blue to light blue. A low resistivity zone is found at a depth of 0-25 m. The medium resistivity zone has a resistivity of 293-700 Ωm colored by green to yellow. This layer is located at a depth of 13-77 m and 77-103 m. The high resistivity zone has a resistivity of 701-3875 located at a depth of 27-77 m. The low-velocity zone has a value of 300-1350 m/s. This zone is located at a depth of 0-8 m. The medium velocity zone has a value of 2700-1350 m/s located at a depth of 8-23 m. The high-velocity zone has a values 1350-2999 m/s at a depth of 23-40 m.
The low resistivity zone and the upper-middle resistivity zone are lithologically resistive cross-sections incorporated in Tangkubanparahu Pyroclastic Fall 2 (Tjp2) consists of scoria, pumice, andesite and basalt lithic fragments of lapilli to pebble size (Fig. 8). This deposit is well stratified, loose to consolidated, normal grading, weathered brownish yellow color, the thickness varies from thin lamination to 70 cm. it is distributed in the summit area of Tangkubanparahu reach as far as the rim of Sunda caldera. Major units are incorporated in Young Tangkubanparahu Volcanics. These deposits originate from phreatic eruptions as in the classification of Kartadinata et al. (2002). At the top, it has a low resistivity value, is likely to be associated with infiltration of rainwater that enters the soil causing the soil / upper rock to be more "saturated" when compared to the middle and bottom of this layer, considering the research area has a high rainfall intensity and ERT measurements are carried out during the rainy season. This layer correlates with low and medium velocity in the seismic refraction cross-section with a velocity of 300-1350 m/s. this layer thickens in the east of the crosssection (resistivity). The thickness of this layer around 36 m in the west and 72 m in the east. The high resistivity value in the cross-section of the inversion results is associated with the Tangkubanparahu (Tl) lava layer. Basalt lava flows consist of plagioclase, pyroxene with These lava flows are the eruption products of the main crater of Tangkubanparahu. Well exposed around the summit area, it is flank and river valleys such as at Cisarua, Sagalaherang, Cibeureum, east Cimahi, Maribaya reach as far as Dago in northern Bandung. These lava flows are exposed alternately with pyroclastic fall 1 (Tjp 1) around crater walls of Ratu, Upas and Baru craters. in the crosssection of the ERT inversion results, there is a resistivity layer with a medium resistivity value. This layer is likely to be associated with the Tangkubanparahu Pyroclastic Fall 1(Tjp1) which consists dominantly of pumice 1-5 cm in diameter, scoria, basalt, and andesite lithic fragments, occasionally volcanic bombs are found. This unit is characterized by thin layers to 1 m thick. Each layer is often separated by soil horizon. The color ranges from yellowish brown, blackish-grey to yellowish-white. This layer has a thickness of 71 m in the west section and thinning in the east section. The existence of this layer is confirmed by the high velocity at seismic refraction (> 2999 m/s). Tangkubanparahu Pyroclastic Fall 1(Tjp1) was successfully identified by resistivity measurements where the resistivity value of this layer is lower than the upper layer (300-700 Ωm). While the existence of this layer is not identified by the seismic cross-section because it lies deeper than the range of the refraction seismic inversion. The existence of the Sunda Pyroclastic flow deposits (Sap) and Sunda lava (S1) are not identified, this is caused by the ERT inversion depth which only reaches 103 m and the depth of the refraction seismic profile only reaches 40 m. While the Sunda pyroclastic flow (Sap) and Sunda Lava (S1) deposits are estimated to be at a depth of > 150.
According to Delinom (2009), younger volcanic material Tangkubanparahu is a water-bearing. These deposits consist of coarse tuff fractions. Grain size varies from fine sand to gravel, but the dominant fraction is medium to fine sand. The thickness of these water-bearing deposits is very different from less than 1-30 m or more. Effusive materials such as basalt and andesite flow are poor aquifers because they are usually compacted. However, lava flows closer to Tangkuban Parahu appear to be more porous and cracked. it can be considered as aquifers (Delinom 2009). In the area near the center of the eruption, perched aquifers have been developed in the youngest tuff and ash, underlain by lava. This aquifer is used by dug-well; where morphological conditions are favorable, springs come out (Delinom 2009).
According to explanation, the formation that has prospects as aquifers are Tangkubanparahu Pyroclastic Fall 2 (Tjp2), although the Tangkubanparahu lava (Tl) layer can also act as aquifer fractures (Fig. 9). Tangkubanparahu Pyroclastic Fall 2 (Tjp2) can act as an unconfined aquifer. This formation remarkably close to the atmosphere and is not covered by an impermeable layer above it. Tangkubanparahu Pyroclastic Fall 1 (Tjp1) can act as a confined aquifer at the study area because this layer is located below the impermeable layer Tangkubanparahu lava (Tl) layer.
This study analyzes the subsurface conditions of the recharge zone in the Lembang groundwater basin. This study also explains what was not revealed in previous studies that focused only on geological mapping. Explanation of subsurface conditions at the research site is very important. This is because in the groundwater basin there is a very massive tourism economic development which requires an adequate supply of groundwater while at the same time maintaining the preservation of groundwater in the area. This study analyzes subsurface conditions only based on ERT data and seismic refraction. It is highly recommended to add subsurface data through geological and groundwater drilling to support the existing data.

Conclusion
The results from ERT and seismic refraction measurements in the study area can be classified into three rock units i.e., Tangkubanparahu Pyroclastic Fall 2 (Tjp2), Tangkubanparahu lava (Tl) layer, and Tangkubanparahu Pyroclastic Fall 1 (Tjp1). Tangkubanparahu Pyroclastic Fall 2 (Tjp2) is associated with the low and medium resistivity zone and also correlated with low and medium velocity values in the Fig. 9 Aquifer in the study area seismic refraction cross-section. Tangkubanparahu lava (Tl) layer is associated with high resistivity values in the crosssection of resistivity inversion. The existence of this layer is confirmed by the high velocity at seismic refraction crosssection (> 2999 m/s). Tangkubanparahu Pyroclastic Fall 1 (Tjp1) is associated with a medium resistivity value in the resistivity inversion cross-section, but unidentified in the refraction seismic cross-section considering the low range of seismic refraction inversion depth (< 40 m). Rock units that have the prospect of being aquifers are the Tangkubanparahu Pyroclastic Fall 2 (Tjp2), although the Tangkubanparahu lava (Tl) layer can also act as aquifer fractures. On the other hand, Tangkubanparahu (Tl) lava deposits can act as impermeable layers, and cover layers for the Tangkubanparahu Pyroclastic Fall 1 (Tjp1) deposited layer beneath.
Acknowledgements It is our honor and pleasure to thank the Director of Research Center for Limnology and Water Resources-BRIN for valuable help and support.
Author contributions All authors contributed to the study of Sub-Surface Configuration in The Northern Part of Lembang Groundwater Basin Recharge Area. HB and RFL contributed to the study conception, design and manuscript review. ERT data acquisition were taken by YS. Seismic refraction data acquisition were taken by JA. Material preparation were performed by ADR. Data analysis were performed by GUN. The first draft of the manuscript was written by GUN, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding The author(s) received no specific funding for this work.
Availability of data and material Data are available upon request.

Conflict of interest
The authors declare that we have no conflict of interest.
Ethical Standard This article does not contain any studies with human participants or animal performed by any of the authors.

Informed consent
In this article, no patient care was involved.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.