Subsurface Identification using Resistivity Method for Infrastructure Planning in Benowo Area, Surabaya City

Subsurface rock identification in infrastructure construction is important to get the information in planning and monitoring the rock carrying capacity and groundwater systems for infrastructure construction. To get the subsurface condition information that are relatively shallow with detailed results, Resistivity method with Wenner configuration is used. Research is in Benowo area, where dominated by pond area. This research used 5 Resistivity lines, which are divided into 2 locations separated by distance of about 2400 meters as Z1 and Z2 areas. For the subsurface resistivity section at Z1 area there are 2 lines, while at Z2 area there are 3 lines. It is known at Z1 area, top layer is topsoil (2.26-33.00 Ωm) and the bottom layer was identified as clay (0.30-2.26 Ωm). At Z2, top layer being topsoil (0.306-0.708 Ωm), followed by clay layer (0.100-0.306 Ωm), and the sand layer (0.231-0.535 Ωm) in the bottom of resistivity model. Groundwater system shows topsoil acts as a vadose zone, clay layer acts as an aquiclude and the sand layer acts as an unconfined aquifer. Saturated water influences the low resistivity value measurement related to the environmental condition dominated by pond area, beside measurements in rainy season are also suspected to be the cause of low resistivity measured. To be applied infrastructure development in the Benowo area need further treatment activity like soil and/or rock pavement to support infrastructure safety. This research result is related to geotechnical aspects like rock coring, pavement, and optimum architectural design can be an alternative for further research.


Introduction
Infrastructure is a type of building close to and has an important role in daily human activities [1].Roads, bridges, and other public facilities are types of infrastructure that are always related to human activities today [1], [2].Infrastructure development and maintenance activities can't be separated from proper development planning processes and monitoring [3], [4].A kind of attention to subsurface conditions of the infrastructure location, where the subsurface conditions will determine how infrastructure is followed up in the next step [5], [6].
One of the geophysical methods that can be applied to map subsurface conditions is the geoelectrical resistivity method [7], [8].Resistivity method is used to determine the subsurface rock's resistivity values distribution [9], [10].This method can map subsurface conditions in detail with a relatively shallow depth [11], [12].From the resistivity values distribution, it can be identified the 1250 ( 2023

Result
Measurement results from research location 1 (Z1) consist of 2 geoelectrical resistivity lines, including Line 1A and Line 1B are depicted in Figure 5 and Figure 6, respectively.In Line 1A, 2 lithological layers were identified, where the top layer is topsoil with a resistivity value between 2.26 -17.00 Ωm, while the bottom layer was identified as clay with a resistivity value between 0.30 -2.26 Ωm.Line 1B also shows 2 layers presence, where like line 1 A, the top layer is topsoil with a resistivity value between 2.26 -33.00 Ωm, and the bottom layer was identified as clay with a resistivity value between 0.30 -2.26 Ωm.Research location 2 (Z2) consists of 3 geoelectric resistivity lines, including Line 2A, Line 2B, and Line 2C.Line 2A as shown in Figure 7, shows the identification results in the 3 lithological layers form, with the top layer being topsoil (0.306 -0.708 Ωm), followed by clay layer (0.100 -0.306 Ωm), and the sand layer (0.231 -0.535 Ωm) in the bottom of resistivity model.In Line 2B as shown in Figure 8, 3 layers were identified, with the top layer being topsoil (0.306 -0.708 Ωm), followed by clay layer (0.100 -0.306 Ωm), and the sand layer.(0.231 -0.535 Ωm) in the bottom of resistivity model.For Line 2C in Figure 9, 3 layers were identified, where the top layer was topsoil (0.306 -0.708 Ωm), the second layer was clay layer (0.100 -0.306 Ωm), and the sand layer.(0.231 -0.535 Ωm) in the bottom of resistivity model.Table 1 displays information related to the range of resistivity values in each research area.

Discussion
There is a similarity between the resistivity values and lithological interpretations in Line 1A and Line 1B, which shows the subsurface resistivity section in Z1 area is consist of a topsoil layer in the range of 0-2 meters depth from the ground surface, as well as a clay layer starting at a depth of 2 meters and continue to the resistivity section bottom.Like the Z1 area, the subsurface resistivity section in Z2 area is also interrelated between layers in each line.Line 2A, Line 2B, and Line 2C show the subsurface resistivity results in the Z2 area which consists of a topsoil layer at 0 -2 meters depth from the ground surface, a clay layer at a 2 -8 meters depth, and a sand layer starting at 8 meters depth to the resistivity section bottom.This result shows that every intersecting path in this research can be correlated with each other in the subsurface rock's resistivity model interpretation.
If it is associated with groundwater systems, each part of the lithology that is interpreted has its own role.Topsoil acts as a vadose zone that drains water from the soil surface to the saturated zone.Clay layer is considered to act as an aquiclude that is capable to storing groundwater, but it is difficult to drain large amounts of water.Sand layer acts as an unconfined aquifer, which can store and drain groundwater, but the aquifer system in this layer is predicted to have a groundwater level below the ground surface level because it does not have sufficient groundwater pressure to naturally emerge to the surface.
If we look in more detail, we will find different resistivity values in the Z1 and Z2 areas, although similar lithological types are identified.This result can happen because each lithological characteristic in an area with other areas can have differences, beside variations in measurement time and land use factors.Lithological control through geological data is important in the subsurface resistivity model interpretation.In addition to lithological control from geological data, the measurement location, which is a former pond area makes the soil conditions around the research location relatively saturated with water.
Layers of clay and sand identified in this study are interpreted as sedimentary deposits.This can be seen from the relatively low rock resistivity values (ranging from 0.3 -33 Ωm on Z1 and 0.1 -0.7 Ωm on Z2).Fluid influence in the form of water is also thought to cause the low resistivity value measurement.It can be seen from the environmental condition around the research location dominated by pond area, likewise with most land uses in the Benowo area.Measurements in the rainy season are also suspected to be the cause of the low resistivity measured because rainwater effect at the time.
From subsurface resistivity data interpretation, it is known to be able to be used as an infrastructure development in the Benowo area need further treatment activity, such as soil and/or rock pavement to support the infrastructure foundations and bodies can be built in this location.Further research related to subsurface conditions can be carried out by applying drilling to obtain rock samples core, which will validate the subsurface conditions directly.Further research on soil and/or rock pavement activity at the research site can be an alternative for geotechnical studies, as a follow-up to this research.As for alternative suggestions for further research regarding the optimum foundation type and infrastructure design in accordance with the relatively "soft" subsurface conditions at the research location, it will be very helpful to assist researchers, consultants, and contractors in determining the best architectural work plan and appropriate infrastructure materials.

Conclusion
This study obtains the subsurface conditions overview from Z1 and Z2 areas as a research study in Benowo area, western part of Surabaya City.In the Z1 area, interpretation results with 2 layers are obtained, the top layer being topsoil layer (2.26-33.00Ωm), while the layer below is clay layer (0.30-2.26 Ωm).For the Z2 area, it is interpreted that there are 3 layers, with the topsoil layer (0.306-0.708Ωm) being at the top, clay layer (0.100-0.306Ωm) underneath, and below the clay layer is sand layer (0.231-0.535Ωm).If it is associated with groundwater systems, topsoil layer is expected as a vadose zone, the clay layer is expected as an aquiclude, and the sand layer is expected as an unconfined aquifer.Identified from the relatively low resistivity value, it is estimated that the identified lithology is sedimentary with relatively "soft" material, where the research location is a pond area that is saturated with water.There are differences in the range of resistivity values with the same lithology type in the Z1 and Z2 areas, where this is influenced by variations in rock characteristics, differences in measurement time, and land use factors.Geotechnical studies related to soil pavement technique types of foundations and infrastructure design can be an alternative for further research from this research.

Figure 5 .
Figure 5. Subsurface resistivity model in line 1A, with west-east line oriented.

Figure 6 .
Figure 6.Subsurface resistivity model in line 1B, with north-south line oriented.

Figure 7 .
Figure 7. Subsurface resistivity model in line 2A, with northwest-southeast line oriented.