Delineation of Coal Identification Using Inversion Microtremor and Borehole at PT X

ABSTRACT


Introduction
Indonesia is one of the countries with the highest level of fuel oil consumption in the world.The increase in fuel oil energy consumption is not matched by depleting petroleum reserves.This has prompted the government to search for new energy sources to ensure energy security in the future.Coal exploration is the right choice because it still holds great potential for mining [1].The potential of coal, which is one of the national and world resources, excessive use causes the amount of reserves to decrease, therefore exploration of new areas to answer industrial challenges is needed and of course the technology used is environmentally friendly [2].
Coal is one type of natural resource that is composed of organic and inorganic materials.The content of this organic material comes from plant remains that undergo various decomposition and changes in physical and chemical properties, both before and after being covered by overlying deposits.Inorganic materials consist of various kinds of minerals, especially clay minerals, carbonates, silicates and various other minerals that are less in number [3].As an energy source, coal is able to produce high enough calories/heat, which is between 5,000 to 9,000 calories per gram [4].
Coal can be utilized in solid form (lump coal or briquettes), liquid form (coal oil and coal water fuel), and gas form (coal gas synthesis).Before the discovery of oil and gas reserves in the 19th century, coal was the world's main primary energy source.Its role in the industrial revolution in the 18th century was huge, especially with the invention of the steam engine [5].
Coal resources can be identified by conducting exploration activities.Preliminary exploration is also often carried out using Geophysical methods.Geophysical method is one of the branches of physics that studies the earth based on physical concepts.In general, geophysical methods are divided into two categories, namely passive and active methods.Geophysical surveys that are often used include magnetic, gravity, geoelectric, electromagnetic and seismic methods.Currently, the passive seismic method is relatively new for hydrocarbon identification when compared to active seismic.Passive seismic is a geophysical exploration method that utilizes microseismic waves to identify the presence of hydrocarbons, one of the passive seismic methods that can be used for preliminary exploration is the microtremor method.
Microtremor method or commonly referred to as ambient noise is a ground vibration that can describe the condition of the area and is generated by natural or artificial events, such as wind, ocean waves or vehicle vibrations [6].Waves generated by human activities such as waves caused by cars, motorcycles, trains have short periods.Waves generated by wind, storms and ocean waves have long periods (2-3 seconds or more) [7].Microtremor analysis is more suitable because it is more environmentally friendly and simple [8].
In recent years, microtremor exploration has achieved results in the field of cave exploration, for example, Ma Guosong (2022) applied the microtremor method for water-rich exploration for the first time.Coal identification research has also been conducted by [9] Nanling in Taiyuan City.The microtremor method only requires about 20-30 minutes of recording time to determine the soil characteristics based on the dominant frequency (f0) and amplification factor (A0) parameters [10] and can determine the shear wave velocity (Vs) of the coal itself because every material has a shear wave velocity (Vs) value that varies with the type of material.This research was conducted at PT X using 30 measurement points.Regionally, the research area belongs to the Lemau Formation and Bintunan Formation which are composed of sandstone, mudstone, siltstone, coal and conglomerate of Middle to Upper Miocene age [11].

Materials
This research uses the microtremor method.The tools and materials used in this research are: a set of Portable Short Period Seismometer Gemini 2 Sn-1405 model to record ground vibrations, geological compass, GPS, camera, laptop and software such as Geopsy Software, Arcgis, Hv-Inv Software, Microsoft Excel, and Coreldraw.

Processing Microtremor data with geopsy
Microtremor data processing is processed first using Geopsy software.Processing is done by inputting microtremor recording data on the import signal.Next, smoothing and windowing are carried out to obtain the results of the H/V curve in the form of the dominant frequency (f0) and the peak value of the amplification factor (A0).The H/V curve must be in accordance with SESAME (2004) validation and then save it in *txt format.The horizontal axis is the dominant frequency value (f0) and the vertical axis the amplification (A0).The dashed line is the deviation of the curve, while the thick black line is the H/V curve.The dominant frequency is the result of the interpretation of the H/V curve.Microtremor data that have obtained frequency and amplification value curves are then inputted into the Hv-Inv software in the form of processed data results.The HV-Inv application is a MATLAB-based computer application developed by García-Jerez et al. that is used to analyze and model subsurface structures using the Monte Carlo (MC) principle [12].Furthermore, it is analyzed using Monte Carlo simulation in order to obtain the most suitable curve.The curve is said to be suitable if the misfit value obtained is small and the H/V graph coincides.In data processing with Hv-Inv, the parameters of thickness, shear wave velocity, compressive wave velocity, and density are obtained [13].

Results and Discussion
This research uses the microtremor passive seismic method at the PT X location.Regionally, the study area belongs to the Lemau formation which is dated around middle Miocene to late Miocene which is a coalbearing formation.
Shear wave velocity is an important parameter in interpreting the subsurface conditions of an area [14].Vs is also a direct indicator of soil stiffness and strength, where the greater the shear wave velocity, the greater the hardness of the soil or rock.According to Posgay (1967), the compressive wave velocity (Vp) of coal is 1600-1900 m/s.Meanwhile, in this study, the shear wave velocity (Vs) is used.Meanwhile, Shear wave speed (Vs) is half the speed of the compressive wave speed (Vp) It can be concluded that the shear wave speed (Vs) of coal is 800-950 m/s.There are several factors that affect the speed of waves in the coal layer, including the modulus of elasticity and the type of coal, anthracite coal is a type of coal that has the highest wave speed among other types of coal.This is also supported by research from Ibrahim et al. that anthracite has the highest shear wave speed, which means that anthracite coal is very dense and not easily broken [15].Based on the interpretation results between 2D stratigraphy of microtremor data and borehole data, it is known that there are differences in the thickness and top coal of the coal layer.Likewise with other research points, the depth and thickness of the coal seam is slightly different from the data from the borehole, but the difference is not too significant.After the microtremor data is correlated with data from the borehole, it can be seen that in cross section E-E' the coal seam thickens to the southwest.Similarly, with the correlation results of other microtremor cross sections, the cross section shows that the coal layer is thickened and continuous to the west or southwest.This is reinforced by the existence of boreholes as data correlation.
The coal seams at this research point are bituminous to sub-bituminous coal seams that are prospective for mining, so the microtremor method is suitable for conducting initial exploration of coal estimation.Heryanto and Suyoko mentioned that in the Sebayur area there are three coal layers that can be found in the Lemau Formation [16].However, microtremor measurements can only identify coal in the first layer.Therefore, it is necessary to explore coal in the same location as this research using other geophysical methods so that it can be correlated and validated.

Conclusion
Based on the interpretation between 1D stratigraphy and borehole, it is known that the Vs value of coal is in the range of 613.37 m/s -887.70 m/s with various types of coal and different water content.There are other constituent materials found in the research area such as soil, claystone, sandstone, carboneseus.Based on the Vs value of the 1D model, it is known that the research area shows the distribution of coal at a depth of 53.5-76 meters.The coal layers in the study area are estimated to be bituminous to sub-bituminous coal layers.After correlation with the borehole, it is known that the distribution of coal seams spreads and is continuous to the West or Southwest.The accuracy of the microtremor method is less when compared to the borehole, but the microtremor method can be used as a preliminary survey in determining the presence of coal seams.

Figure 2 .
Figure 2. H/V curve processing results at point A1.

Figure 3 .
Figure 3. H/V inversion result curve at study point 8.

Figure 4
Figure 4 above shows that the types of constituent materials in the research area are dominated by soil, claystone, sandstone, and coal based on the shear wave velocity values that propagate in the research area.According to the results of the 1D stratigraphic interpretation of microtremor data in the research area, it is known that the Vs value of the coal layer is in the range of 613.37 m/s -887.70 m/s with various types of coal and different water content.From the shear wave velocity, it is estimated that the type of coal seam in the research area is bituminous or sub-bituminous.The inversion results show variations in coal depth at each research point that are influenced by different shear wave velocities (Vs).However, the depths are not significantly different from each other.

Figure 5 .
Figure 5. cross section A that intersects the study point vertically from north to south.

Figure 6 .
Figure 6.Cross-sections of three boreholes near the study point.