Imaging Rock Density Distribution beneath Liwa Fracture Zone in the Southern Part of the Great Sumatran Fault System, Indonesia

Djedi S Widarto,Eddy Z Gaffar,Tedi Yudistira,Ikuo Katsura,Jun-Ichi Nishida,Susumu Nishimura

doi:10.4236/ijg.2016.74046

Abstract

We have imaged rock density distribution beneath Liwa fracture zone in the southern part of the the Sumatran Fault Zone by modelling and inverting Bouguer gravity data in two-and three-dimensional environments, respectively. The purpose of this study is aimed to figure out the subsurface distribution of rock densities associated with subsurface basement structure representing the evidence of trans-tensional tectonic product in the SF. In the gravity modeling, to eliminate distortions to the measured gravity values before modelling and inverting the data, Bouguer anomalies obtained in field measurements are reduced to the horizontal plane of z = +800 m as a representation of the average elevation in Liwa. For the inversion, we used algorithm implementing depth-and minimum volume weighting parameters in order to obtain a smooth model with better vertical resolution. The two-dimensional models show clearly surface topography of the basement rocks and the presence of normal faults. The reduced Bouguer anomaly of +800 m elevation shows the presence of structural lineaments extending primarily in a northwest-southeast direction, parallel to Sumatran Fault Zone and older graben faults showing a negative flower structure. From the three-dimensional inversion, the model illustrates an increase of density contrast, lower values being found near the surface and higher values in the deeper parts. The lower density contrast of 0.15 to 0.3 g/cm3 found in the rock groups at depths of 2 km and less is characteristic of relatively homogeneous and poorly compacted rocks. Rocks with moderate to high density contrast (>1.0 g/cm3) are recognized at depths of over 2 km. This model suggests a change of basement morphology as a function of depth, and delineates structural lineaments extending in northwest-southeast direction. This study supports the previous thought that Liwa area is underlain by graben structures, formed by trans-tensional tectonic events. Higher-density Tertiary volcanic breccia and lower-density Quaternary volcanic products of the Ranau Formation form the basement rocks and the overlying younger sediments, respectively.

Highlights

The gravity method is one of geophysical prospecting methods that are widely used in the exploration of hydrocarbons [1]-[3], geothermal [4]-[6], minerals [7] [8], and environmental and engineering problems [9] [10], as well as the study of the Earth’s crust structures [11] [12]
Compacted rocks generally show a lower density value than well-compacted rocks. Their differences in density vary between 0.3 and 0.7 g/cm3 [13]. In relation with those mentioned, we carried out detailed gravity survey in the Liwa fracture zone of the southern segment of the Sumatran Fault Zone
This paper will discuss the results of gravity survey in the Liwa fracture zone with reference to the images obtained by two-dimensional (2-D) modelling and three-dimensional (3-D) inversion of gravity data

Summary

Introduction

The gravity method is one of geophysical prospecting methods that are widely used in the exploration of hydrocarbons [1]-[3], geothermal [4]-[6], minerals [7] [8], and environmental and engineering problems [9] [10], as well as the study of the Earth’s crust structures [11] [12]. The method is based on the existence of rock density contrast at the subsurface that can be measured either directly or indirectly in the form of gravity anomaly. This contrast could characterize the presence of specific geological features, such as faults or layer boundary and the materials that constitute the layer, including the presence of fluids within the layer. This work is aimed to characterize the image of subsurface structures beneath an intensively fractured zone. In this respect, this survey can be used to provide subsurface features related to the faults movement or in evaluating the potential of natural resources. 3-D inversion of gravity data is performed with the purpose of obtaining detailed image of rock density distribution at the shallow depths of up to 2 km with 200 m elevation levels

Objectives

Results

Conclusion