Magnetotelluric data analysis using 2D resistivity modelling in Gondang region, Bojonegoro

Abstract

Previous studies in Gondang Subdistricthave identified underground mud reservoirnear a rock intrusion, while another study near Gondang Subdistrict has identified deep fault structures. This study will identify the distribution and characteristics of possible geological features in the area using magnetotelluric (MT) method to further describe the relationship between the geological features related to local geology. MT data measurements were conducted on 7 stations alonga north-south line, then modelled in 2D using nonlinear conjugate gradient algorithm. The model was used to describe the subsurface resistivity distribution and to identify the geological features. The results show 5 resistive zones (20–1250 Ω⋅m) and 4 conductive zones (≤10 Ω⋅m). The former consist of 4 vertical zones, 1 vertical zone deeper than 5 km, and 1 horizontal zone near the surface. 2 conductive zones surround a resistive zone, while 2 others stretch below the horizontal resistive zone. The vertical resistive zones are interpreted as andesite intrusions, and the horizontal one as volcanic breccia. The conductive zones are interpreted consisting of tuff and marl with possible saline water content. 3 vertical intrusions are thought to have the same source, and all vertical intrusions are suspected to co-occurr with the Pandan Volcano intrusion.

Identification of Shallow Fault with Seismic Refraction Modelling in Gondang Area, Bojonegoro

“…Geological structures such as faults in the local area and the presence of strong enough pressure make hot mud be found above the surface [6]. Now, if we look at several previous studies, with other methods such as gravity and magnetotelluric, it shows that there are andesite intrusions around Mount Pandan, where the presence of andesite in an area could be an indication of past volcanic activity, which could potentially be associated with geothermal anomalies [7] [8]. But it is worth noting that the presence of andesite does not necessarily mean that there are currently active geothermal, it needs more additional geological and geophysical investigation, which is what we are doing here trying to identify shallow faults by using seismic refraction modeling.…”

Geomagnetic Data Analysis to Identify Subsurface Condition of Gondang Area, Bojonegoro

“…The upward continuation have a process to filter magnetic field anomaly towards the altitude which functions to correct magnetic field measurements and eliminate noise effects on the surface of the measurement site [11]. A value of 100m datum is applied to the total magnetic anomaly to eliminate residual effects without losing the geological information (Figure 3) [14].…”

“…In this inversion, the matrix inversion computation employs the Singular Value Decomposition (SVD) method, that is steadier. Certain values can be completely disregarded or regarded zero in this method and will not be included in solution computation [11]. The formula calculates both the model norm and the data norm, with both norms (ʎ) being attempted for the finest value [14].…”

“…The magnetization induction process, which produced positive and negative magnetic field anomalies locally, influenced this variation [10]. In geology, for the purpose of locating magnetic anomalies investigation [11]. The primary goals of the magnetic research are to identify geothermal potential structure, which including structure existence distribution from reservoir to manifestation, as well as to examine the continuity of the Gondang geothermal system [11,13].…”

Microtremor analysis around Jari mud volcano, Bojonegoro

“…Previous research study using other geophysical method has been done around this area in particular Gondang area, such as gravity method to give a proper subsurface model for making 3D model to identify subsurface using 3D inversion [5]. Besides that, using magnetotelluric data to determine distribution of resistivity which is translated in the form of 2D model [6].…”