Doddy Sutarno

Robust Magnetotelluric Impedance Estimation

Robust magnetotelluric (MT) response function estimators are now in standard use by the induction community. Properly devised and applied, these have ability to reduce the influence of unusual data (outliers). The estimators always yield impedance estimates which are better than the conventional least square (LS) estimation because the ‘real’ MT data almost never satisfy the statistical assumptions of Gaussian distribution and stationary upon which normal spectral analysis is based. This paper discuses the development and application of robust estimation procedures which can be classified as M‐estimators to MT data. Starting with the description of the estimators, special attention is addressed to the recent development of a bounded‐influence robust estimation, including utilization of the Hilbert Transform (HT) operation on causal MT impedance functions. The resulting robust performances are illustrated using synthetic as well as real MT data.

Resistivtiy Structure in Tangkuban Parahu Area Drived from CSAMT Data

Tangkuban Parahu is located in Bandung, West Java, one of a volcano area. Controlled-source audio frequency magnetotelluric (CSAMT) survey has been conducted in the area using transmitter located in 3-5 km from the survey area. The objective of the survey was to determine a subsurface resistivity structures that may correlate to a geothermal reservoir in unknown geothermal potential area. In the paper, we applied a two-dimensional (2D) inversion scheme to interpret of the CSAMT data to obtain several resistivity sections describing the subsurface resistivity structures. The results of 2D inversion indicated several subsurface low resistivity anomaly (1-10 ohm.m) in the depth of 1000-1500 meter that may correlate to the existing a geothermal reservoirir confirmed by several surface manifestations in the area.

The Use of Direct Solver in Vector Finite Element Modeling for Calculating 3-D Magnetotelluric Responses

In this work, we seek numerical solution of 3-D Magnetotelluric (MT) using edge- based finite element method. This approach is a variant of standard finite element method and commonly referred as vector finite-element (VFE) method. Nonphysical solutions usually occurred when the solution is sought using standard finite element which is a node based element. Vector finite element attempt to overcome those nonphysical solutions by using the edges of the element as vector basis. The proposed approach on solving second order Maxwell differential equation of 3-D MT is using direct solver rather than iterative method. Therefore, divergence correction to accelerate the rate of convergence for its iterative solution is no longer needed. The utilization of direct solver has been verified previously for correctness by comparing the resulting solution to those given by analytical solution, as well as the solution come from the other numerical methods, for earth layered model, 2-D models and COMMEMI 3D-2 model. In this work, further verification resulted from recent comparison model of Dublin Test Model 1 (DTM1) is presented.

VLF -MT Survey around Nakadake crater at Aso Volcano

Shallow resistivity structure around Nakadake craters has been revealed by extensive VLF- MT survey from August to October 2006. This structure comprises a varied resistivity value at the upper and bottom layer of the western flank of craters; a conductive upper layer at the fourth Crater; and a resistive upper layer at southern part of craters. The conductive value of second layer at a line in the western flank and southern part of craters at several ten meter depth, interpreted as water saturated, and high resistivity at second line of western flank of craters as sheet of lava and at fourth crater is interpreted as lava rock. At western flank of craters, we found the resistivity value and the depth of second layer decrease in depth with decreasing of distance to craters line.

Development on electromagnetic impedance function modeling and its estimation

Today the Electromagnetic methods such as magnetotellurics (MT) and controlled sources audio MT (CSAMT) is used in a broad variety of applications. Its usefulness in poor seismic areas and its negligible environmental impact are integral parts of effective exploration at minimum cost. As exploration was forced into more difficult areas, the importance of MT and CSAMT, in conjunction with other techniques, has tended to grow continuously. However, there are obviously important and difficult problems remaining to be solved concerning our ability to collect process and interpret MT as well as CSAMT in complex 3D structural environments. This talk aim at reviewing and discussing the recent development on MT as well as CSAMT impedance functions modeling, and also some improvements on estimation procedures for the corresponding impedance functions. In MT impedance modeling, research efforts focus on developing numerical method for computing the impedance functions of three dimensionally (3-D) earth resistivity ...

Numerical solution of 3-D magnetotelluric using vector finite element method

Magnetotelluric (MT) is a passive electromagnetic (EM) method which measure natural variations of electric and magnetic vector fields at the Earth surface to map subsurface electrical conductivity/resistivity structure. In this study, we obtained numerical solution of three-dimensional (3-D) MT using vector finite element method by solving second order Maxwell differential equation describing diffusion of plane wave through the conductive earth. Rather than the nodes of the element, the edges of the element is used as a vector basis to overcome the occurrence of nonphysical solutions that usually faced by scalar (node based) finite element method. Electric vector fields formulation was used and the resulting system of equation was solved using direct solution method to obtain the electric vector field distribution throughout the earth resistivity model structure. The resulting MT response functions was verified with 1-D layered Earth and 3-D2 COMMEMI outcropping structure. Good agreement is achieved for b...

Development of CSAMT impedance modeling and its estimation

Accurate modeling and estimation of impedance functions is essential for the correct interpretation of Controlled Source Audio Magnetotelluric (CSAMT) measurements. Non plane wave effect of CSAMT source and noises are inevitably encountered when CSAMT observations are conducted and, consequently, impedance estimates are usually based on least-squares (LS) approximation, and the resulting estimates need to be corrected for the non plane wave field fraction. However, estimation procedure based on LS would not be statistically optimal, as outliers (abnormal data) are frequently superimposed on a normal ambient CSAMT noise field. In this situation, the estimation can be seriously misleading, while plane wave correction has also limited application, as the non plane wave field fraction is reasonably strong. This paper briefly discus the recent development of alternative methods for the CSAMT impedance modeling and its estimation, those are efficient in nature. The means for accomplishing the non plane wave pro...

Resistivity structure of Sumatran Fault (Aceh segment) derived from 1-D magnetotelluric modeling

Sumatran Fault Zone is the most active fault in Indonesia as a result of strike-slip component of Indo-Australian oblique convergence. With the length of 1900 km, Sumatran fault was divided into 20 segments starting from the southernmost Sumatra Island having small slip rate and increasing to the north end of Sumatra Island. There are several geophysical methods to analyze fault structure depending on physical parameter used in these methods, such as seismology, geodesy and electromagnetic. Magnetotelluric method which is one of geophysical methods has been widely used in mapping and sounding resistivity distribution because it does not only has the ability for detecting contras resistivity but also has a penetration range up to hundreds of kilometers. Magnetotelluric survey was carried out in Aceh region with the 12 total sites crossing Sumatran Fault on Aceh and Seulimeum segments. Two components of electric and magnetic fields were recorded during 10 hours in average with the frequency range from 320 H...

Integrated geophysical measurements for subsurface mapping at Papandayan volcano, Garut, Indonesia (preliminary result)

Papandayan volcano is an active volcano situated in Garut, West Java Indonesia. In 2002 eruption, it released pyroclastic volcanic material to the surface covering the 4 km2 area on the summit to the north part of the volcano and formed new craters on the summit. We have undertaken several geophysical measurements in period of 2008 - 2011 including magnetotelluric, gravity, magnetic and DC resistivity in order to delineate subsurface structure of the volcano. Magnetotelluric measurement was designed using a grid system with the 30 sites and interval station of approximately 250m covering the summit area. Frequency range from 320 Hz to 1 HZ were used in MT measurement by recording two components of electric and magnetic fields using phoenix MTU-5 system. Magnetic and gravity data were acquired in the summit area overlapping with the MT survey to determine basement of the volcano structure. The shallow resistivity structure was constrained by two lines DC resistivity survey that was carried out in east-west...

Topographic effect modeling of 2D MT responses using boundary element method

Magnetotelluric data acquisition sometimes should be taken on topographic terrain, for example on the mountainous zones. The variety of topography around the point of measurement yields additional effects on the data obtained. Therefore, the modeling of MT responses in the topographic terrain is very necessary to get a complete understanding of topographic effects on MT measurement. This paper will discuss the modeling of topographic effects on 2D MT response function by using the boundary element method. Boundary element method was chosen because of its simplicity in domain discretization, low cost memory and time consuming modeling, and its accuracy to obtain good precision results.