Analysis of Geological Structure based on 3D Virtual Outcrop Model and Physical Properties of Rocks in Wringinanom District, Gresik Regency

The Covid-19 pandemic has caused geological mapping activities to be hampered. This study aims to create a 3D virtual rock outcrop model which is compiled from 2D rock outcrop images with Measured Stratigraphy (MS), strike & dip and the value of rock physical parameters to analyze geological structures in Wringinanom District, Gresik Regency. Analysis of geological structures will be contained in the form of folding reconstructions with rock physical parameter as a validation and descriptions of fault orientation. This research modeled 21 rock outcrops and will be tested 6 core sedimentary rock samples. Physical rock parameter tests in the form of density tests through the comparison of mass weighing results with dimensional measurements by calipers and P-wave velocity tests using the Ultrasonic Pulse Velocity (UPV) method. Petrological descriptions of core rock samples are based on direct observations. The results of 3D virtual rock outcrop integrated with measurements in the field can be used in analyzing geological structures. The study area consists of anticlines with the direction of layering the south (southwest) and north (northeast) wings and 50-degree reverse faults with southeast dipping orientation (N153°E). The folding reconstruction consists of sandstone (top), sandstone-clay interlude, and claystone (top) in the Pucangan Formation and sandstone (bottom) and claystone (bottom) in the Lidah Formation where the presence of fossil layers of bivalve mollusks is a firm correlation. The physical parameters of rocks cannot validate the reconstruction of folds due to the influence of differences in physical values of the sample supported by petrological descriptions.


Introduction
The Covid-19 pandemic has greatly impacted all general public including the world of education [1] which resulting on unimplemented field data measurements.These problems demand a change in learning methods, one of them is to convert the information needed in the field into virtual such as making 3D virtual rock outcrop models.The 3D virtual rock outcrop model is expected to be supporting information before carrying out direct measurements in the field and as a learning platform that can be easily accessed by all people who are constrained to go to the field.A 3D virtual rock outcrop model integrated with geological mapping in the field can be an innovation in the earth field both during the Covid-19 pandemic and beyond [2].Developments in the era of the industrial revolution 4.0, especially technological aspects, also make the 3D virtual rock outcrop model very potential in developments in the earth field through the creation of 3D virtual reality [3].In addition, 1307 (2024) 012024 IOP Publishing doi:10.1088/1755-1315/1307/1/012024 2 this model indirectly plays a role in maintaining evidence of geological heritage which over time has changed due to the role of nature (weathering) and humans (soil mining), especially at the research site.
Therefore, this research aims to analyze geological structure based on 3D virtual outcrop model in Wringinanom District of Gresik Regency by the application of petrophysics, density and compression wave (Vp) of rocks.A 3D virtual rock outcrop model created by combining 2D rock outcrop photos and adding information such as measured stratigraphy (MS), sample description, and strike & dip from field measurements.Site selection is based on the geological structure in the form of normal faults and anticline folds that have been identified through geological mapping in the field [4].From the 3D virtual rock outcrop model, geological structure analysis will be carried out.The analysis would be validated by the rock physical parameters result to corelate that value for both north and south anticline's wing.It was assumed that for the same lithology has the same rock physical parameters result.In addition, physical parameters of rocks in the form of density values and compression wave speed (Vp) were tested according to the type of test in the form of dimensional measurements using calipers and rock masses using digital balances and laboratory-scale Ultrasonic Pulse Velocity (UPV Test) tests.The results of testing the physical parameters of these rocks will be additional validation in analyzing geological structures.

Geological Setting
The physiography of the East Java divided area into several parts including Kendeng Zone or Kendeng Anticlinorium, the study area [5].Basically, the Kendeng Zone is divided into two parts (east and west) based on separation by the Bengawan Solo River.The research area, Wringinanom District, is included in the eastern part of the Kendeng Zone which bordered by the Rembang Zone in the north and in the south is spread the North Coast Alluvial Plain (Figure 1).Part of the Kendeng Zone is also composed of anticline mountain ranges with a maximum width of 30 km which continue to decrease eastward, starting from Mount Pandan to the area around North Mojokerto and Surabaya (research location).The naming of Anticlinorium in the Kendeng Zone is due to the formation of geological structures in the form of folds and faults that take place intensively.
The location of this research is included in the Geological Maps of Mojokerto [6] and Surabaya-Sapulu [7] which act as a reference in determining regional geological map with stratigraphy containing the age of each formation (Figure 2 and 3).The formation order with the arrangement from young to old is listed as follows: 1. Alluvium (Qa) as the youngest formation with Holocene age which consists of gravel, sand, silt, clay and there are fossils in the form of shell fragments that do not spread.2. Kabuh Formation (Qpk) includes sandstones-tuff inserted with claystones, light gray, coarsegrained, aquatic and cross-structured structures, and there are tuffs and conglomerates, poorly disaggregated, have open packaging, and are arranged in layers.3. Pucangan Formation (QTp) divided into the lower part which is composed of well-layered sandstones, with conglomerate and claystone inserts, and there are mollusk and plankton fossils scattered and at the top is dominated by layered tufan sandstones with both aquatic and crosscutting structures.4. Lidah Formation (Tpl) composed of blue, blackish, solid, fossil-poor, and hard when dry with a thin lens of sandstone.5. Sonde Formation (Tps) identified as the oldest formation of the study site consists of marl-tuff, yellowish-white in color, contains fossil diatoms or algae, and there is unshattered limestone.

Research Methods
There are 3 main stages on this study which are literature reviews, input, and process.Related topics that has been review consist of regional and local, operation of drones as Unmanned Aerial Vehicles (UAVs) and determination or interpretation of geological structures from 3D virtual rock outcrop models.Input includes geological mapping which includes of MS which contain compositional information from clastic sediment deposition either through rock outcrops or subsurface drilling [8], Strike & dip, photogrammetry measurement (UAV Data) and sampling of representative rocks types.
The UAV Data is related to the size of the desired outcrop area through the correlation of the binding points of each photo [9].Process includes the petrologic observation from rock sampling data, density and Vp of core rocks laboratory measurement, reconstruction of 3D virtual outcrop model that has been created from UAV Data for geological structure analysis.Petrologic observation related to minerals, texture, and rock structure.Density calculation has been done by dividing weight of grain (g) over bulk volume (cm 3 ) [10] and Vp measurement was calculated from rock sample length (m) and travel time (s) division [11].From the 3D virtual rock outcrop model is divided based on the sides or wings of the geological structure in the field in the form of anticlines that extend with a west-east orientation so that the division into a combination of south and north wing models.The combined anticline south wing model consists of outcrops S-20, S-17, S-18, S-12, S-13, S-21, S-7, S-6, S-11, S-10, S-9, and S-8.Meanwhile, the northern anticline wing contains outcrops S-16, S-15, S-14, S-19, S-2, S-5, S-4, S-1, S-3.The outcrops display lithological information from 3D virtual outcrop models, MS, stereonet and rose diagrams, as well as photos of 2D outcrops (Figure 4).

Density and Vp
Core rock samples that have been obtained in the field amounted to 6 rock samples (S1, S2, S3,..., S6) scattered at the research site.The sample will be described in brief petrology in more detail (Table 1).The purpose of this rock description is to determine the type of lithology as well as a reference in its influence on physical parameter testing.Petrological descriptions related to minerals, texture, and rock structure need to be done because they greatly affect the test results of physical parameters such as density and Vp [13].From the results of density calculations, density values are obtained that vary against different types of rocks.The density value range of the entire sample is between 1,261 and 2,592 g/cm3.This value is in the range of rock density values from [14] and [15].There are differences in the density value of the measurement results in the type of rock in the form of clay and sandstone-carbonate against the reference density value.This is assumed to occur due to the characteristics of clay in dry conditions there will be cracks in the rock.Meanwhile, sandstone-carbonate when observed directly has more porosity than other types of rock.Physical parameters of rocks are macroscopic which are closely related to mineral composition, texture, and rock genesis [16].The density value of rocks can differ very drastically depending on the mineral composition and porosity or empty space in the rock.
Testing of rock physical parameters in the form of P (Vp) wave velocity tests on core rock samples from the study area uses the same assumptions as density tests where the samples to be tested are in dry conditions.This test is carried out by calculating the difference in wave travel time.The step in determining the difference in wave travel time is done manually depending on the interpretation of the operator.The results of wave travel time (ms) from each core rock sample.Vp values ranging from 868 to 2708 m/s.Vp used in the form of Vp of rock layers obtained by multiplying L layer and Vp sample and dividing by L sample.The Vp layer value is in accordance with the reference value.There are differences in Vp values from direct measurements in the field with laboratories.Rock characteristics that have been taken from a rock outcrop will affect the pressure so that it can produce microcracks.Therefore, the Vp value of measurement results in the laboratory cannot be directly compared with the results of measurements in the field and there is an influence of the frequency used on the Vp value to be produced [17].

Geological Structure Analysis
Based on the results of data collection in the field, data were obtained in the form of 3D virtual rock outcrop models, Measured Stratigraphy (MS), strike & dip, and rock samples.The data will be correlated with the aim of analyzing geological structures.The folding structure in question is an anticline with an orientation of straightness tending to point west-east and a reverse fault identified as heading northeast.The geological analysis process is divided into making 3D anticline reconstructions as well as brief descriptions of existing fault structures in the study area.
In Figure 6 with its caption in Figure 7 shows the results of Measured Stratigraphy (MS) correlation of the south and north wings of the anticline.Rock lithology units that have been correlated and arranged from old to young rocks include claystone, sandstone, claystone, sandstone-clay interlude, and sandstone units.The correlation process of each lithological unit is described by firm and dotted lines.The firm line shows a correlation that can be ascertained.Meanwhile, the dotted line as an estimated correlation or it can be said that there are no layer characteristics, lithological contacts, or mineral content in the same rock.The correlation with the firm line is evidenced by the presence of fossil bivalve mollusks found during field observations at stop sites or rock outcrops on the southern (S-17) and northern (S-15) anticline wings (Figure 5).Stereonet and rose diagrams of S-15 and S-17 In Figure 9 shown as a location where there is evidence of geological structure in the form of ascending faults.The location is marked as stopsite S-12.Photo of an outcrop can be seen with the direction of movement of the fault plane.In addition, it can be determined the direction of the fault plane where in this study the orientation of the fault plane is towards the southeast while the straightness is towards the northeast.The orientation of the fault plane is depicted through a stereonet and rose diagram.Then, a 3D virtual rock outcrop model of the fault plane coloured in yellow and claystone in green.2. There are 4 rock samples identified in the Pucangan Formation with code S1, S2,...,S4 and 2 rock samples in the Lidah Formation, namely S5 and S6.The density and Vp values range from 1,261-1,763 g/cm 3 and 868,056-1416,667 m/s.Meanwhile, the values in the Lidah Formation was 1.56 and 2,592 g/cm 3 and 1190,476 and 2708,333 m/s.The validation process of folding reconstruction requires the value of the physical parameters of rocks that tend to be the same from the south and north wings of the anticline.The assumed sampling location is the same, namely in S5 and S6 rock samples taken around S-15 and S-17.The value of the physical parameters of the two rock samples, namely S5 and S6, was different with density and Vp values, including 2,592 and 1,559 g/cm3 as well as 2708 and 1190 m/s.This is due to differences in petrological descriptions related to the process of rock deposition or the depositional environment.In addition, there is a need for additional intake points to validate the reconstruction, especially at stratigraphic cross-section.

Conclusion
The results of 3D virtual rock outcrop modeling integrated with measurements in the field can be used in analyzing geological structures.The geological structure of the study area consists of anticlines with south (southwest) and north (northeast) wing layering directions and reverse faults at the S-12 stop site with a fault plane slope of 50 degrees and a southeast dipping orientation (N153ºE).Reconstruction of folds consisting of sandstone units (top), sandstone-clay interlude, and claystone (top) in the Pucangan Formation and sandstone (bottom) and claystone (bottom) in the Lidah Formation where the presence of mollusk layer 1 (fossil mollusk bivalve) found on the north (S-15) and south (S-17) wings becomes a firm correlation.Moreover, based on the results of rock physical parameter testing cannot validate fold reconstruction due to the influence of differences in rock physical parameter values supported by petrological descriptions in S5 and S6 rock samples.In this study, there are shortcomings or limitations both in its implementation and objectives so that advanced stages can be carried out, among others.The 3D virtual rock outcrop model can be used as a reference in further studies related to the depositional environment.Furthermore, the solution can be done by adding rock sampling points for physical parameter tests in determining the characteristics of rock samples in the south and north wings of the anticline as validation of fold reconstruction.

Figure 1 .
Figure 1.Physiographic map of Java Island and location of the study area in Wringinanom District(squared area), East Java (modified from[5]).

Figure 2 .
Figure 2. Regional Geology and Structure Map of the study area (modified from[6] and[7]).

Figure 4 .
Figure 4. a) 3D Virtual Rock Outcrop Model, b) Measured Stratigraphy (MS), c) Stereonet (top) and Rose Diagram (bottom), d) Lithology contact of Claystone, Sandstone-clay Interlude, and Sandstone with gradation boundary, and e) Photo in the direction of rock layering strike

Figure 5 .
Figure 5. a) Photos of S-15 stop sites containing fossils and b) S-17

Figure 6 .
Figure 6.Correlation of MS south and north anticline wing

Figure 7 .
Figure 7. Description of lithology, symbols, and contact lithology of MS correlation Anticline reconstruction can be done from the correlation of Measured Stratigraphy (MS) of the south and north wings of the anticline with the drawing of a firm line.The construction of anticline reconstructions is also based on strike & dip data from each layer of lithological units.The data can be seen by the creation of stereonets or rose diagrams.Figure 8 shows the reconstruction of the anticline with a firm line or boundary is present in mollusk layer 1 or located on the outcrops of rocks S-15 and S-17.Meanwhile, the dotted line is at the contact between claystone (bottom) and sandstone and claystone (top) and sandstone-clay interlude.The layered orientation of the southern wing of the anticline predominantly points southwest and the north wing points northeast.The identification of each lithological unit can be classified into one formation.The lithological units of claystone (bottom) and sandstone are identified as the Lidah Formation.The Lidah Formation has characteristics where there are dominant claystones and shallow marine depositional environments associated with the discovery of fossil mollusks.Then, the claystone unit (top), sandstone-clay interchange, and sandstone became the constituent unit of the Pucangan Formation.It is necessary to analyze the depositional environment based on a 3D virtual rock outcrop model to assist in the determination of formation.

Figure 8 .
Figure 8. a) Anticline reconstruction at the study site, b) Basemap containing incisions A-A', c)Stereonet and rose diagrams of S-15 and S-17 In Figure9shown as a location where there is evidence of geological structure in the form of ascending faults.The location is marked as stopsite S-12.Photo of an outcrop can be seen with the direction of movement of the fault plane.In addition, it can be determined the direction of the fault plane where in this study the orientation of the fault plane is towards the southeast while the straightness is towards the northeast.The orientation of the fault plane is depicted through a stereonet and rose diagram.Then, a 3D virtual rock outcrop model of the fault plane coloured in yellow and claystone in green.

Table 2 .
Density and Vp of rock values