THE MODELING OF CONTROLLED SOURCE AUDIO FREQUENCY MAGNETOTELLURIC (CSAMT) RESPONSES

• Main Author: HILMAN MOHAMMAD (NIM : 30207009); Tim Pembimbing : Prof. Doddy Sutarno, Ph.D; Wahyu Srigutomo, IMRAN
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/16037
• Institution: Institut Teknologi Bandung

CSAMT (Controlled Source Audio Frequency Magnetotelluric) method is one of the electromagnetic method in exploration. Te method is a variant of the Magnetotelluric (MT) method. The main difference between CSAMT and MT is the use of artificial sources in CSAMT. The use of artificial source makes CSAMT data more robust to the noise, resulting in better data resolution, and hence become the main advantages of the method compared to MT. The advantage is due to artificial sources in CSAMT produces a relatively stable current than the natural source such as telluric activities, which is used by MT. The presence of artificial sources makes the distance between the source and the <br /> <br /> <br /> receiver in CSAMT is relatively very close compared to MT. In general, the response function of CSAMT modeling is based on assumption that the source of electromagnetic field (EM) used is in the form of the plane wave. The assumption can be fulfilled only when the measurement of CSAMT data is conducted on the radiation zone, which is around five times the 'skin depth' of EM wave under <br /> <br /> <br /> consideration. In practice, the assumption often can not be fulfilled due to various constraints, such as the limitations of resources and the complexity of the local <br /> <br /> <br /> topography. This conditions makes the measurement can not be conducted on the radiation zone. As the consequence, CSAMT data should be corrected to eliminate the influence of 'non-plane wave effects’. However, the correction is not possible if the influence is large enough. Thus it is very important to develop the full solution modeling of CSAMT responses. In this dissertation, the 1D CSAMT modeling and inversion and 2D CSAMT response modeling using finite element method are presented. 1D CSAMT modeling performed to calculate the CSAMT responses in both near field and far <br /> <br /> <br /> field zones. The validity of the forward modeling scheme is test using 1D Occam inversion. For the comparison, the synthetic data is also used as data in 1D MT inversion. The comparison between CSAMT and MT approaches shows the <br /> <br /> <br /> distorted interpretation of MT inversion approach, even if the modeling is done at a relatively far distance. The inversion scheme with full solution approach shows excellent results on synthetic models used. This shows the importance of a full solution approach to interpret the CSAMT data. The 2D CSAMT responses generated by horizontal electric dipole are calculated using the separation of primary and secondary field technique. The primary field <br /> <br /> <br /> is calculated using 1D analytical solution as developed before and it is used to calculate the secondary electric field in the non-homogeneous Helmholtz Equation. Calculation of Helmholtz Equation is carried out using the finite element method. Validation of this modeling is conducted by comparison of numerical results with 1D analytical response for the case of homogeneous and <br /> <br /> <br /> layered earth. The comparison of CSAMT responses are also provided for 2D cases of vertical contact and anomalous conductive body with the 2D magnetotelluric (MT) responses. The results of 2D modeling show the superiority of CSAMT method compared to MT for mapping the subsurface conductivity structure.