Yeong-Sue Park

3D gravity inversion with Euler deconvolution as a priori information

It is difficult to obtain high-resolution images by 3D gravity inversion, because the problem is extremely underdetermined – there are too many model parameters. In order to reduce the number of model parameters we propose a 3D gravity inversion scheme utilising Euler deconvolution as a priori information. The essential point of this scheme is the reduction of the nonuniqueness of solutions by restricting the inversion space with the help of Euler deconvolution. We carry out a systematic exploration of the growing body process, but only in the restricted space within a certain radius of the Euler solutions. We have tested our method with synthetic gravity data, and also applied it to a real dataset, to delineate underground cavities in a limestone area. We found that we obtained a more reasonable subsurface density image by means of this combination between the Euler solution and the inversion process.

Leveling the Gamma-ray Spectrometric Data using Baseline Survey

KIGAM has surveyed most of the Korean territory since 1982 using airborne gamma-ray spectrometry, and complete the nationwide scale map in the near future. However, since the duration of survey is too long and the conditions of survey is not consistent, the data does not have physical consistency. In addition, the window counts (count/sec) were recorded instead of potassium, uranium and thorium radioelement concentrations. Thus, the data could not be registered to the International Atomic Energy Agency (IAEA) radioelement datum. This limits the usefulness of the data and it is not possible to easily combine surveys into regional compilations or make quantitative interpretations between different survey areas. To solve these problems, we undertook a test baseline survey over Jincheon-Eumseong area, to level the different two sets of data and to map radioelement concentrations. This survey confirms to IAEA radioelement baseline. The method and procedures of data leveling prepared by this study improve the usefulness and usability of the radiometric data, and make it enable to compile the nationwide scale radioelement concentration maps.

Case Study on a Low-enthalpy Geothermal Exploration in Pohang Area, Korea

ABSTRACT Korea Institute of Geoscience and Mineral Resources (KIGAM) launched a new project to develop the low-enthalpy geothermal water in the area showing high geothermal anomaly, north of Pohang city, for large-scale space heating from Korea Research Council of Public Science & Technology(KORP) funding. Surface geologic and geophysical surveys including Landsat TM image analysis, gravity, magnetic, magnetotelluric (MT) and controlled-source audio-frequency MT (CSAMT) and self-potential (SP) methods have been conducted and the possible fracture zone was found that would serve as deeply connected geothermal water conduit. By the end of 2003, two test wells of 1 km depth were drilled, various kinds of borehole survey along with additional MT measurements and sample analysis will follow and then the detailed subsurface condition is to be characterized. Next step would be drilling the production well of 2 km depth and all further steps remain to be determined depending upon the results of the test well studies.

A Comparative Analysis of Edge Detection Methods in Magnetic Data

Many edge detection methods, based on horizontal and vertical derivatives, have been introduced to provide us with intuitive information about the horizontal distribution of a subsurface anomalous body. Understanding the characteristics of each edge detection method is important for selecting an optimized method. In order to compare the characteristics of the individual methods, this study applied each method to synthetic magnetic data created using homogeneous prisms with different sizes, the numbers of bodies, and spacings between them. Seven edge detection methods were comprehensively and quantitatively analyzed: the total horizontal derivative (HD), the vertical derivative (VD), the 3D analytic signal (AS), the title derivative (TD), the theta map (TM), the horizontal derivative of tilt angle (HTD), and the normalized total horizontal derivative (NHD). HD and VD showed average good performance for a single-body model, but failed to detect multiple bodies. AS traced the edge for a single-body model comparatively well, but it was unable to detect an angulated corner and multiple bodies at the same time. TD and TM performed well in delineating the edges of shallower and larger bodies, but they showed relatively poor performance for deeper and smaller bodies. In contrast, they had a significant advantage in detecting the edges of multiple bodies. HTD showed poor performance in tracing close bodies since it was sensitive to an interference effect. NHD showed great performance under an appropriate window.

Calculation of gravity due to a vertical cylinder using a spherical harmonic series and numerical integration

We developed a spherical harmonic series that represents the gravitational potential and its gravity field due to a buried right vertical cylinder. This series can be used at far- and intermediate-regions, and is fast and accurate, using only a few terms. We compared the values of the fields acquired by this new spherical harmonic series, with ones computed by direct numerical integrations, using a fine-mesh structure for a vertical cylinder. Results of the calculations are shown and performances of the two different methods are compared. Faithfulness of the spherical harmonic series is tested with an inversion example. In this paper, the spherical harmonic series of the gravitational potential and its gravity field due to a right vertical cylinder was developed. This series can be used at far and intermediate regions, and it is fast and accurate, using only a few terms.

Separation-sounding Filter for Potential Data

요약: 특정한 지질 구조에 의한 이상을 분리하는 문제, 특히 깊이가 다른 샘들에 의하여 중첩된 이상을 수직적으로 분리하는 문제는 중력과 자력탐사 자료 해석에서 매우 필수적이며 중요한 과정이라 할 수 있다. 측정된 퍼텐셜 자료를 단순하게 광역 효과와 남은 이상의 합으로 가정할 수도 있지만, 수직적으로 여러 층인 구조인 경우로 확장하여 특정한 층에 의한 효과를 분리해 내거나 분리된 이상을 깊이에 관하여 정량적으로 규정할 수 있으면, 이것은 깊이 분리라고 부를수 있을 것이다. 깊이 분리 필터로서 로그 파워 빛띠 분석을 이용한 진동수 필터를 제안한다. 이것을 수치 모델링과 흥해 일대에서 얻은 실제 중력탐사 자료에 적용하였으며, 2 배 깊이만큼 위로 연속 필터와 비교, 검토해 보았다. 수치 모델링과 실제 중력 자료에 적용한 바에 의하면, 이 논문에서 제안한 깊이 분리 필터는 지구물리적인 의미가 있는 중력 불연속면까지의 중력 효과를 잘 벗겨냄으로써 불연속면의 중력 분포를 추정하여 지질 구조를 해석하는 데에 유용한 도구임을보여주었다.주요어: 퍼텐셜, 깊이 분리, 진동수 필터, 위로 연속Abstract: One of the most critical and essential procedures in the interpretation of gravity and magnetic data is to separatethe anomaly due to the specific geologic structure from the summation of effects from a broad variety of geologic sources,especially those of different depths. Separation of the residual anomaly from the regional field is the most simple caseof the vertical separation. If the anomaly due to a layer of specific depth can be separated or the depth of the separatedlayer can be quantitatively determined, it may deserve the separation-sounding. We suggest a wavelength filter whosecutoff frequency is determined by log-power spectrum analysis, as a separation-sounding filter. We applied this filter bothto synthetic and real gravity data acquired at Heunghae area, and compared the results with those of Jacobsens upwardcontinuation filter. These showed that the proposed separation-sounding filter could be a useful tool for interpretation ofthe vertical geologic structure by stripping the gravity effects of geologic sources down to the desired depth. Keywords: potential, separation-sounding, frequency filter, upward continuation

Cavity mapping and grout monitoring: A microgravity case history in Korea

Muan has suffered from subsidence, thought to be related to cavities developed in Paleozoic limestone. Microgravity survey was executed at a rice field in Muan for subsurface cavity mapping. The data were collected using a Scintrex CG-3 gravimeter at about 800 stations by 5 m interval along paddy paths, which provided a semi-grid data set. The residual gravity anomaly was interpreted by Euler deconvolution, and 2-D and 3-D inversion. The density distribution of the profiles was drawn by two dimensional inversion based on the minimum support stabilizing functional, which generated better focused images of density discontinuities. Three-dimensional density distribution was imaged by growing body inversion. We devised an inversion scheme utilizing Euler deconvolution as a priori information in order to reduce the inherent non-uniqueness of gravity inversion. The essential point of the scheme is to restrict the model space with help of Euler deconvolution, which pointed plausible locations of anomaly sources. The three-dimensional density image showed that the cavities were dissolved, enlarged and connected into a cavity network system. It was generally coincided with the result of resistivity survey and supported by drill hole logs. A time-lapse microgravity survey on a newly widen road passing through the site for monitoring the change of the density distribution before and after grouting. The comparison of density distributions imaged by minimum support inversion showed the change and development of density structure during the lapsed time, which implies the effects of grouting.