Hyoungrea Rim

Single-borehole imaging using gravity gradiometer data

SUMMARY We present a feasibility study for interpretation of gravity gradient surveys in a single borehole using both an analytic and inverse-based approach. We analytically solve for distance and direction angles to a point source using the components of the gravity gradient tensor from a set of observations in a single borehole. We then generalize this approach to include the response due to a prismatic source. By inverting multiple sets of tensor components for 3D density contrast distributions, we show that as few as two independent gravity gradient components in a single borehole can be used to locate and image subsurface anomalous bodies. However, gradient measurements in a single borehole are insufficient to resolve multiple closelyspaced compact bodies.

Detecting Steel Pile Using Bore-hole 3-components Fluxgate Magnetometer

A steel pile often utilized to bear heavy loads of the upper sub-structure, e. g., bridge constructions and tall buildings. As the steel piles are driven in the underground, it is not easy to detect the depth of the existing pile foundation when there is no detailed foundation information available. However, accurate informations of the depths of piles becomes critical required when reinforcing the existing structures or constructing new ones at the adjacent stage to assure the safety of existing structures. In this study, we tested the applicability of the three components borehole fluxgate magnetometer for detecting the depths and locations of steel piles which are commonly used in civil engineering. Results showed that the information of location as well as the depth of steel piles could be obtained by using data from the three components borehole fluxgate magnetometer.

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

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.

4D inversion of borehole gravity data for monitoring fluid fronts

Monitoring fluid movement is an important component in enhanced oil recovery (EOR) and CO2 sequestration. The newly available slim-hole gravimeter operating at high temperature offers a new avenue for such monitoring efforts because of the direct sensitivity to the change in the density distribution. We present a time-lapse gravity inversion algorithm for recovering the front of injected fluid using borehole gravity measurements. We assume that the horizontal extent of the fluid can be represented by a polygon with known but variable thickness and density contrast due to fluid substitution. We represent the evolution of the front as a 4D function of the spatial position and time since the initiation of the injection. The inversion can be carried out either independently at discrete time points or as a single inversion simultaneously over all time points. We demonstrate that the latter approach is superior in that it is more stable and offers improved capability in detecting break-through events at later times. In this paper, we will describe the details of the two inversion approaches, including two different model objective functions in polar coordinates and the nonlinear solution strategies. We will illustrate the advantages and drawbacks of independent and simultaneous 4D inversions using numerical examples

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.

Compression of Terrain Data using Integer Wavelet Transform (IWT) and Application on Gravity Terrain Correction

Terrain data is one of important basic data in various areas of Earth science. Recently, finer DEM data is available, which necessary to develop a method that deals with such huge data efficiently. This study was conducted on the lossless compression of DEM data and efficient partial reconstruction of terrain information from compressed data. In this study, we compressed the wavelet coefficients of DEM, obtained from integer wavelet transform (IWT) by entropy encoding. CDF (Cohen-Daubechies- Feauveau) 3.5 wavelet showed the best compression ratio of about 45.4% and the optimum decomposition level was 3. Results also showed that a small region of terrain could be restored from the inverse wavelet transform with a part of the wavelet coefficients that are related to such region instead of whole reconstruction. We discussed the potential applications of the terrain data compression for precise gravity terrain correction.

Conversion of 3-Component Magnetic Data into Geodetic Coordinate System by Using Clinometer and GNSS Compass

This study proposes a method to correct inclination of instrument during exploration with a biaxial clinometer and GNSS compass. In 3-component magnetometry, measured vectors are ordinarily described in randomly inclined observation coordinate system due to movement, vibration, and shaking of instrument. Therefore, rotation angles of observation plane are needed to transform it into geodetic coordinate system. In this study, we measured inclination angles of observation plane by using 2-axis clinometer and GNSS compass, and derived proper parameters for rotational transform from them. We applied the conversion method to on-board 3-component magnetometry, and then transformed raw data into proper values on geodetic coordinate system.

Separation of potential field data using spatial filtering

Summary In this paper, we propose a spatial filtering scheme using factorial kriging, a kind of geostatistical filtering method in order to separate regional and residual gravity anomaly. Spatial filtering assumes that regional anomalies have longer distance relation and residual anomalies have effected on smaller range. We decomposed gravity anomalies into two variogram models with long and short effective ranges by means of factorial kriging. And decomposed variogram models produced the separated regional-residual anomalies. This algorithm was examined with a synthetic gravity data, and also applied to a real microgravity data to figure out abandoned mineshaft