Hangilro Jang

Step-off, vertical electromagnetic responses of a deep resistivity layer buried in marine sediments

A frequency-domain, marine controlled-source electromagnetic (CSEM) method has been applied successfully in deep water areas for detecting hydrocarbon (HC) reservoirs. However, a typical technique with horizontal transmitters and receivers requires large source–receiver separations with respect to the target depth. A time-domain EM system with vertical transmitters and receivers can be an alternative because vertical electric fields are sensitive to deep resistive layers. In this paper, a time-domain modelling code, with multiple source and receiver dipoles that are finite in length, has been written to investigate transient EM problems. With the use of this code, we calculate step-off responses for one-dimensional HC reservoir models. Although the vertical electric field has much smaller amplitude of signal than the horizontal field, vertical currents resulting from a vertical transmitter are sensitive to resistive layers. The modelling shows a significant difference between step-off responses of HC- and water-filled reservoirs, and the contrast can be recognized at late times at relatively short offsets. A maximum contrast occurs at more than 4 s, being delayed with the depth of the HC layer.

In-Loop Transient Electromagnetic Responses With Induced Polarization Effects of Deep-Sea Hydrothermal Deposits

A transient electromagnetic (TEM) system using a small loop source is advantageous to the development of compact autonomous instruments, which are well suited to a submersible-based survey. Since the electrical conductivity of subseafloor materials can be frequency dependent, these induced polarization (IP) effects may affect the reliability of TEM data interpretation. In this paper, we investigate the IP effects on the TEM responses of deep-sea hydrothermal mineral deposits with a thin sediment cover. Time-domain target signals are larger and appear earlier in horizontal magnetic fields than in vertical magnetic fields. The IP effects cause transient magnetic fields to enhance initially, to decay rapidly, and then to reverse the polarity. The dc conductivity and chargeability in Cole-Cole parameters influence the time of sign reversals and the enhancement of target responses, simultaneously. A damped least squares inversion scheme is applied to reconstruct a polarizable hydrothermal deposit model using synthetic TEM sounding data sampled from a time interval before the sign reversal. The test example demonstrates that the Cole-Cole parameters of a highly conductive layer can be resolved more effectively from vertical magnetic fields than from horizontal magnetic fields.

A comparative study of Hankel transform filters for marine controlled-source electromagnetic surveys

Abstract Using a closed-form solution of the Hankel transform in transverse magnetic mode for a homogeneous half-space model, we can assess the accuracy of digital linear filters for evaluating the Hankel transform. In this paper, we conduct comparative performance tests on the linear filter with known integral transforms. We examine three kinds of filters developed by W. L. Anderson, F. N. Kong and H. Mizunaga, which are known to be suitable for marine controlled-source electromagnetic (CSEM) applications. Kong’s filters perform best in the three kinds of filters over a practical range of offset distances in marine CSEM surveys. While the relative error versus distance appears as a V-shaped curve in semi-log scale, Mizunaga’s filters are the shortest in length and have a performance comparable to Kong’s filters. Anderson’s filters have a quite similar performance between the J0 and J1 filters, although these are somewhat inferior to Kong’s and Mizunaga’s filters when computing marine CSEM fields.

On the Efficient Three-Dimensional Inversion of Static Shifted MT Data

요약: MT 자료에서 3차원 전기비저항 구조와 정적효과를 동시에 구하기 위한 실용적인 역산법을 소개한다. 이 방법은감도행렬이 필요한 Gauss-Newton법을 기반으로 하고 반복과정에서 Broyden의 방식으로 감도를 수정하는 것을 기본으로하고 있다. 이 논문에서는 합성 MT 자료에 대한 역산실험을 통해 근사역산법의 성능과 정적효과에 대한 가중치에 대해검토하였다. 해석적으로 구해지는 초기감도를 Broyden의 방식으로 수정하는 역산법은 초기감도를 끝까지 쓰는 역산법보다 자료오차를 줄이는데 효과적이었다. 그리고 완전한 감도행렬을 반복 중간에서 단 한번만 사용하는 근사역산법으로서는 반복 전반부에서 완전한 감도를 사용할 때 자료오차를 가장 많이 줄이는 것으로 나타났다. 정적효과에 대한 가중치는어느 특정 한계값 이하로 선택하면 최종 자료오차에는 결정적인 영향을 주지 않는다. 합성 MT 자료에 대한 실험 결과이 역산법은 정적효과가 포함된 MT 자료로부터 3차원 전기비저항 구조를 재현하는데 효과적임을 확인하였다. 주요어: MT, 3차원, 정적효과, 역산, Gauss-Newton, Broyden의 방법Abstract: This paper presents a practical inversion method for recovering a three-dimensional (3D) resistivity model andstatic shifts simultaneously. Although this method is based on a Gauss-Newton approach that requires a sensitivity matrix,the computer time can be greatly reduced by implementing a simple and effective procedure for updating the sensitivitymatrix using the Broydens algorithm. In this research, we examine the approximate inversion procedure and the weightingfactor β for static shifts through inversion experiments using synthetic MT data. In methods using the full sensitivitymatrix constructed only once in the iteration process, a procedure using the full sensitivity in the earlier stage is usefulto produce the smallest rms data misfit. The choice of β is not critical below some threshold value. Synthetic examplesdemonstrate that the method proposed in this paper is effective in reconstructing a 3D resistivity structure from static-shifted MT data.Keywords: MT, 3D, static shifts, inversion, Gauss-Newton, Broydens algorithm

A Scheme for Computing Time-domain Electromagnetic Fields of a Horizontally Layered Earth

A computer program has been developed to estimate time-domain electromagnetic (EM) responses for a onedimensional model with multiple source and receiver dipoles that are finite in length. The time-domain solution can be obtained by applying an inverse fast Fourier transform (FFT) to frequency-domain fields for efficiency. Frequency-domain responses are first obtained for 10 logarithmically equidistant frequencies per decade, and then cubic spline interpolated to get the FFT input. In the case of phases, the phase curve must be made to be continuous prior to the spline interpolation. The spline interpolated data are convolved with a source current waveform prior to FFT. In this paper, only a step-off waveform is considered. This time-domain code is verified with an analytic solution and EM responses for a marine hydrocarbon reservoir model. Through these comparisons, we can confirm that the accuracy of the developed program is fairly high.