Hyun-Key Jung

Negative apparent resistivity in dipole–dipole electrical surveys

In field surveys using the dipole–dipole electrical resistivity method, we often encounter negative apparent resistivity. The term ‘negative apparent resistivity’ refers to apparent resistivity values with the opposite sign to surrounding data in a pseudosection. Because these negative apparent resistivity values have been regarded as measurement errors, we have discarded the negative apparent resistivity data. Some people have even used negative apparent resistivity data in an inversion process, by taking absolute values of the data. Our field experiments lead us to believe that the main cause for negative apparent resistivity is neither measurement errors nor the influence of self potentials. Furthermore, we also believe that it is not caused by the effects of induced polarization. One possible cause for negative apparent resistivity is the subsurface geological structure. In this study, we provide some numerical examples showing that negative apparent resistivity can arise from geological structures. In numerical examples, we simulate field data using a 3D numerical modelling algorithm, and then extract 2D sections. Our numerical experiments demonstrate that the negative apparent resistivity can be caused by geological structures modelled by U-shaped and crescent-shaped conductive models. Negative apparent resistivity usually occurs when potentials increase with distance from the current electrodes. By plotting the voltage-electrode position curves, we could confirm that when the voltage curves intersect each other, negative apparent resistivity appears. These numerical examples suggest that when we observe negative apparent resistivity in field surveys, we should consider the possibility that the negative apparent resistivity has been caused by geological structure.

Real-Time Underwater Object Detection Based on DC Resistivity Method

A new real-time underwater object detection method adopting geophysical direct-current resistivity techniques is proposed. Our final goals are real-time detection and tracking of small submarines in water depths less than 100 m, under acoustically noisy conditions. The main features of our method are as follows: a detection line, several kilometers long and semipermanently buried in the seabed, consisting of two current electrodes and multiple potential electrodes; fixed current electrode configurations for high-speed real-time detection of the target object; measurement of extremely low-level electric field signals, made possible by the shielding effect of the conductive seawater overburden; and signals from a moving object such as a submarine are extracted by differential data analysis because the target object passes through the detection area for only a short time compared with the long-term drift change of the electrical properties of seawater. We verify our method and confirm real-time detection feasibility through numerical and physical scale experiments. For this purpose, a dedicated instrument and a background data update algorithm are developed, and a differential data analysis method is used. The background data update algorithm is used for counteracting the time-varying electrical characteristics of seawater. The differential data analysis method is exploited to extract disturbed signals by objects from complex background data. Through numerical experiments, we find that the x-coordinate value of a target object corresponds to the position of a peak data point, and this result is consistent with 1:200 downscaled water tank experiments. We also confirm real-time detection feasibility using our instrument through an offshore experiment. We expect that our proposed method will complement conventional detection methods for harbor defense and surveillance systems in acoustically noisy environments.

Determination of the Strike and the Dip of a Line Source Using Gravity Gradient Tensor

Department of geophysical exploration, University of Science and Technology,Daejeon 305-350, KoreaAbstract: In this paper, the automatic determination algorithm of strike and dip of a line source using gravity gradient ona single profile is proposed. In general, the gravity gradient tensor due to a line source has only two independentcomponents because of its 2-Dimensional (2-D) characteristics. However, if the line source has the strike and dipregarding the observation profile, it comes to have five independent components. The proposed algorithm of thedetermination both strike and dip is based on the rotational transform that converts full gravity gradient tensor to reduced2-D gravity gradient tensor. The least-square method is applied in order to find optimum rotational angles that make oneof the row components minimalized simultaneously. The two synthetic cases of a line source are represented; one hasstrike only and the other has both strike and dip. This study finds that the automatic determination method using gravitygradient tensor can find directions of a line source in each case. Keywords: gravity gradient tensor, line source, strike, dip, least square method요약: 이 논문에서는 한 개의 측선에서 측정된 중력 변화율 텐서를 이용하여 선형 이상체의 주향과 경사를 자동 결정하는 알고리즘을 제시한다. 선형 이상체의 중력 변화율은 측선에 수직으로 배열되어 있는 경우 이차원 효과를 보이므로,이론적으로 두 개의 독립 성분을 제외하고는 값을 갖지 아니한다. 반면 주향과 경사를 가지는 선형 이상체는 5개의 독립 성분을 가지게 된다. 이와 같은 선형 이상체가 가지는 중력 변화율 텐서의 이차원 특성을 활용하여 5개의 독립 성분 중 3개가 동시에 최소 값이 되도록 하는 회전변환의 변환각은 곧바로 선형 이상체의 주향과 경사를 의미한다. 이논문에서는 최소자승법을 이용하여 5개의 중력 변화율 성분 중 3개를 최소로 하는 변환 행렬 각을 구하였고 이를 이용하여 선형 이상체의 주향과 경사를 자동 결정할 수 있음을 보였다. 이 논문에서는 모델 계산을 통하여 주향만 있는 경우와 주향과 경사 모두를 가지는 경우에 대하여 각각의 방향각들을 자동 결정할 수 있음을 보였다.주요어: 중력 변화율 텐서, 선형 이상체, 주향, 경사, 최소자승법

Underwater object localization using tempalte matching based on DC resistivity method

Underwater object detection and tracking is of great interest for offshore defense. Recently, a new method adopting direct-current resistivity techniques was introduced to complement conventional detection method. The new detection method needs high-speed localization of underwater target object for real-time tracking. In this paper, we propose real-time localization method using grid-based template matching. The templates are generated by numerical modeling in same condition with water tank modeling, and template matching is mainly based on correlation. Localization experiments are carried out using measured data from downscaled water tank to verify proposed method and algorithm. We show results of experiments and confirm the feasibility of this method. We expect that this study will contribute usefully to the algorithm for real-time tracking.

Marine towed electrical resistivity survey using sequential dual-transmission and low-noise sensor

Marine towed electrical resistivity survey is widely used for various purposes such as underground resources exploration, coastal engineering, archaeological detection, and UXO detection. To improve data quality and resolution, we propose enhanced in-house developed marine towed electrical resistivity survey system. First, hollow-type carbon fiber submersible electric field sensors were developed; second, sequential dual-transmission routine implemented by three-phase bridge power circuit. To implement sequential dual-transmission routine, our new streamer cable has three graphite current electrodes, and they are connected to the three-phase bridge power circuit. The sensor was made by a bundle of carbon fibers, generally 7 micro meter diameter. The routine was verified using the numerical experiment, and the sensors were verified using comparison tests in the water tank filled with seawater. Through the field surveys in the sea, we confirmed that our whole system worked properly and the dual transmission routine was effective to distinguish separated objects in the seabed. We expect that this system can contribute to improve detection probability and efficiency in the field.

Experiment and response analysis of marine monostatic EM instrument toward mapping seafloor sulfide massive deposit

We performed numerical and physical experiment with a marine monostatic electromagnetic (EM) survey system, which is developed primarily for exploration of seafloor mineral deposits. The system is a multi-frequency EM system housed to endure high pressure in deep sea up to 2000 m. To keep stable altitude of the system from the seafloor in rough seafloor topography condition, the system is connected rigidly to ROV (Remotely Operated Vehicle). We performed a test with the new loop EM system by keeping altitude of ROV 2 m above from the seafloor at a depth of 300 m sea water. Experimental results showed that electromagnetic anomaly was clearly identified at the location of metallic object, and the pattern of the EM responses matches well with the theoretical responses from a 3-D integral equation EM modeling code. With the off-shore test and the numerical analysis, we confirmed that the method of operation of the EM survey system by ROV is readily feasible, and the system are expected to detect actual seafloor sulfide massive deposits in the highly conductive seafloor environment.

Preliminary study of underwater object detection using direct-current (DC) resistivity method

Underwater object detection has been always issued problems. Acoustic methods are mainly used, however, detection using acoustic methods in shallow sea is difficult problem. Therefore, we carry out preliminary study adopting direct-current (DC) resistivity method, which use constant current as an active source. For the verification of our method, numerical and water tank experiments are conducted. An isolated one-channel measurement system is developed and used for detecting stationary underwater objects. A differential data analysis is used for investigating electric field distortion characteristics. The preliminary study for confirming detection feasibility is successful. Therefore, future studies are focused on real-time detection under real sea environmental conditions.

A Study on the Development of Multifuntional Real-Time Inclination and Azimuth Measurement System

In geophysics and geophysical exploration fields, we can use information about inclination and azimuth in various ways. These include borehole deviation logging for inversion process, real-time data acquisition system, geophysical monitoring system, and so on. This type of information is also necessarily used in the directional drilling of shale gas fields. We thus need to develop a subminiature, low-powered, multi-functional inclination and azimuth measurement system for geophysical exploration fields. In this paper, to develop real-time measurement system, we adopt the high performance low power Micro Control Unit (made with state-of-the-art Complementary Metal Oxide Semiconductor technology) and newly released Micro Electro Mechanical Systems Attitude Heading Reference System sensors. We present test results on the development of a multifunctional real-time inclination and azimuth measurement system. The developed system has an ultra-slim body so as to be installed in 42mm sonde. Also, this system allows us to acquire data in real-time and to easily expand its application by synchronizing with a depth encoder or Differential Global Positioning System.

Reply by the authors to the discussion by Cho and Kim

Hyun-Key Jung Dong-Joo Min Hyo Sun Lee Seokhoon Oh Hojoon Chung Exploration Geophysics and Mining Engineering Department, Korea Institute of Geoscience & Mineral Resources, 92 Gwahang-no, Yuseung-gu, Daejeon, 305-350, Korea. Department of Energy Systems Engineering, Seoul National University, 599 Gwanak-ro, Seoul, 151-744, Korea. Department of Geosystem Engineering, Kangwon National University, 192-1 Hyoja-Dong, Chuncheon, Gangwon-do, 200-701, Korea. Human & Earth Inc., 513-22, Sangdaewon-dong, Jungwon-gu, Sungnam, Kyeonggi, 462-120, Korea. Corresponding author. Email: spoppy@snu.ac.kr