항공 TEM 을 이용한 천해지역에서의 퇴적층 두께 및 기반암 심도 원격탐사에 관하여 Towards remote sensing of sediment thickness and depth to bedrock in shallow seawater using airborne TEM원문보기
선행된 연구에서의 성공적인 수심도 작성 예에 뒤이어, 항공전자탐사를 이용한 해저면 특성파악 가능성이 고찰되었다. 헬리콥터에 탑재된 시간영역전자탐사 (TEM) 장비에서 얻어진 자료의 1D 역산으로부터 추정된 퇴적층의 두께가 해양 탄성파 연구에 기초하여 얻어진 추정치와 비교되었다. 일반적으로, 해수의 깊이가 대략 20 m이고 퇴적층의 두께가 40 m 미만이면 퇴적층의 두께 즉 비전도성 기반암까지의 깊이는 두 경우에 있어서 타당한 범위 내에서 일치됨을 보였다. 잡음이 섞인 합성자료의 역산은 초기 모형이 실제모형과 차이가 나는 경우에도 수직 전자탐사 유일성 이론과 일치하게 역산 후 실제모형과 매우 닮은 결과를 보여주었다. 잡음이 섞인 합성자료로부터 얻어진 천해 해수 깊이에 관한 표준편차는 대략 깊이의 ${\Box}5\;%$ 정도였으며, 이는 실제자료의 역산 시 대략 ${\pm}1\;m$ 정도의 오차를 우발할 수 있다. 이에 상응하는 기반암 깊이 추정의 불확실성은 대략 ${\pm}10\;%$에 이른다. 잡음이 포함된 합성자료로부터 얻어진 해수와 퇴적층의 평균 역산 두께는 대략 1 m 정도의 정밀도를 나타냈고, 중합에 의해 정밀도가 향상되었다. 주의 깊게 보정된 항공 TEM 자료를 이용하면 퇴적층의 두께와 기반암의 지형을 조사할 수 있다는 가능성을 알 수 있었으며, 천해에서의 해저면 저항치를 알아내기 위한 방법으로서의 가능성도 보여 주었다.
선행된 연구에서의 성공적인 수심도 작성 예에 뒤이어, 항공전자탐사를 이용한 해저면 특성파악 가능성이 고찰되었다. 헬리콥터에 탑재된 시간영역전자탐사 (TEM) 장비에서 얻어진 자료의 1D 역산으로부터 추정된 퇴적층의 두께가 해양 탄성파 연구에 기초하여 얻어진 추정치와 비교되었다. 일반적으로, 해수의 깊이가 대략 20 m이고 퇴적층의 두께가 40 m 미만이면 퇴적층의 두께 즉 비전도성 기반암까지의 깊이는 두 경우에 있어서 타당한 범위 내에서 일치됨을 보였다. 잡음이 섞인 합성자료의 역산은 초기 모형이 실제모형과 차이가 나는 경우에도 수직 전자탐사 유일성 이론과 일치하게 역산 후 실제모형과 매우 닮은 결과를 보여주었다. 잡음이 섞인 합성자료로부터 얻어진 천해 해수 깊이에 관한 표준편차는 대략 깊이의 ${\Box}5\;%$ 정도였으며, 이는 실제자료의 역산 시 대략 ${\pm}1\;m$ 정도의 오차를 우발할 수 있다. 이에 상응하는 기반암 깊이 추정의 불확실성은 대략 ${\pm}10\;%$에 이른다. 잡음이 포함된 합성자료로부터 얻어진 해수와 퇴적층의 평균 역산 두께는 대략 1 m 정도의 정밀도를 나타냈고, 중합에 의해 정밀도가 향상되었다. 주의 깊게 보정된 항공 TEM 자료를 이용하면 퇴적층의 두께와 기반암의 지형을 조사할 수 있다는 가능성을 알 수 있었으며, 천해에서의 해저면 저항치를 알아내기 위한 방법으로서의 가능성도 보여 주었다.
Following a successful bathymetric mapping demonstration in a previous study, the potential of airborne EM for seafloor characterisation has been investigated. The sediment thickness inferred from 1D inversion of helicopter-borne time-domain electromagnetic (TEM) data has been compared with estimate...
Following a successful bathymetric mapping demonstration in a previous study, the potential of airborne EM for seafloor characterisation has been investigated. The sediment thickness inferred from 1D inversion of helicopter-borne time-domain electromagnetic (TEM) data has been compared with estimates based on marine seismic studies. Generally, the two estimates of sediment thickness, and hence depth to resistive bedrock, were in reasonable agreement when the seawater was ${\sim}20\;m$ deep and the sediment was less than ${\sim}40\;m$ thick. Inversion of noisy synthetic data showed that recovered models closely resemble the true models, even when the starting model is dissimilar to the true model, in keeping with the uniqueness theorem for EM soundings. The standard deviations associated with shallow seawater depths inferred from noisy synthetic data are about ${\pm}5\;%$ of depth, comparable with the errors of approximately ${\pm}1\;m$ arising during inversion of real data. The corresponding uncertainty in depth-to-bedrock estimates, based on synthetic data inversion, is of order of ${\pm}10\;%$. The mean inverted depths of both seawater and sediment inferred from noisy synthetic data are accurate to ${\sim}1\;m$, illustrating the improvement in accuracy resulting from stacking. It is concluded that a carefully calibrated airborne TEM system has potential for surveying sediment thickness and bedrock topography, and for characterising seafloor resistivity in shallow coastal waters.
Following a successful bathymetric mapping demonstration in a previous study, the potential of airborne EM for seafloor characterisation has been investigated. The sediment thickness inferred from 1D inversion of helicopter-borne time-domain electromagnetic (TEM) data has been compared with estimates based on marine seismic studies. Generally, the two estimates of sediment thickness, and hence depth to resistive bedrock, were in reasonable agreement when the seawater was ${\sim}20\;m$ deep and the sediment was less than ${\sim}40\;m$ thick. Inversion of noisy synthetic data showed that recovered models closely resemble the true models, even when the starting model is dissimilar to the true model, in keeping with the uniqueness theorem for EM soundings. The standard deviations associated with shallow seawater depths inferred from noisy synthetic data are about ${\pm}5\;%$ of depth, comparable with the errors of approximately ${\pm}1\;m$ arising during inversion of real data. The corresponding uncertainty in depth-to-bedrock estimates, based on synthetic data inversion, is of order of ${\pm}10\;%$. The mean inverted depths of both seawater and sediment inferred from noisy synthetic data are accurate to ${\sim}1\;m$, illustrating the improvement in accuracy resulting from stacking. It is concluded that a carefully calibrated airborne TEM system has potential for surveying sediment thickness and bedrock topography, and for characterising seafloor resistivity in shallow coastal waters.
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문제 정의
In this paper we assume that the seawater depth and conductivity are known, and investigate the degree of improvement in definition of sediment thickness and conductivity. Therefore the seawater/sediment interface is held fixed, and the depth of the sediment/bedrock interface is adjusted via inversion first; if necessary, the conductivity of the sediment is altered in a second stage of inversion.
This study shows that AEM has the potential to be used for remote sensing of sediment thickness and for delineating coarse scale features of the bedrock topography, in areas of shallow seawater.
제안 방법
Inversion of layer interface depths was performed to determine the accuracy of inverted seawater and bedrock depths. The starting depth, and upper and lower bounds (relative to the sea surface), for the seawater-sediment interface were 20, 1, and 75 m respectively for all 36 synthetic decays.
Inversion of layer interface depths, using synthetic data (noise-free, and with added random noise), was performed to illustrate the accuracy of recovered seawater depth and sediment thickness. The noise-free synthetic data represented a suite of two-layer models with sediment thicknesses of 10 and 30 m, water depths spanning 5 to 50 m, and survey altitudes of 20, 35, and 45 m.
Water depth residuals represent the difference between inverted and true (model) seawater/sediment interfacial depths. Residuals were obtained for the following 36 models: water depths of 5, 10, 20, 30, 40, and 50m each overlying a 10m and 30m sediment layer, with altitudes of 20, 35, and 45 m above sea level. Each water depth group (identified by the coordinate value along the x-axis) refers to a set of 6 models (i.
0001 Sm~' respectively. Setting the sediment thickness to 10m, HoistEM decays were calculated for water depths of 5, 10, 20, 30, 40, and 50 m, each at flight altitudes of20, 35, and 45 m. Synthetic data were also computed for a second set of 18 models, with a sediment thickness of 30 m.
Similarly, the starting depth, and upper and lower bounds, for the sedimentbedrock interface were set to 55, 1, and 80 m respectively for all decays, except for 30 m sediment cases with water depth 40 and 50 m, in which case the lower bound was set to 95 m. The inversion for all 36 synthetic transients was performed assuming a 0.1% relative error (en =0.001on, equation 1), appropriate for noise-free synthetic data, c.f. the 2% relative error assumed for inversion of real AEM data in this study. Inversion ceases once the LI-misfit for the computed TEM response drops below unity.
The purpose of inverting synthetic data was to determine the accuracy of recovered interfacial depths and to check if this accuracy was consistent with inversion of real data. Noise-free synthetic data were computed for a suite of two-layer models comprising seawater and sediment layers overlying resistive basement.
The interpretation of marine seismic data from Sydney Harbour only provides an estimate of the depth to bedrock which has not been independently confirmed in any of the locations within the survey areas. The results of the comparison are encouraging because they highlight the potential use of AEM methods as a remote sensing tool to estimate sediment thickness (hence depth to bedrock) and conductivity (hence sediment type). This finding was supported by inversion of synthetic data computed for a suite of realistic seawater plus sediment plus basement models.
대상 데이터
Water depth residuals represent the difference between inverted and true (model) seawater/sediment interfacial depths. Residuals were obtained for the following 36 models: water depths of 5, 10, 20, 30, 40, and 50m each overlying a 10m and 30m sediment layer, with altitudes of 20, 35, and 45 m above sea level. Each water depth group (identified by the coordinate value along the x-axis) refers to a set of 6 models (i.
Setting the sediment thickness to 10m, HoistEM decays were calculated for water depths of 5, 10, 20, 30, 40, and 50 m, each at flight altitudes of20, 35, and 45 m. Synthetic data were also computed for a second set of 18 models, with a sediment thickness of 30 m.
The noise-free synthetic data represented a suite of two-layer models with sediment thicknesses of 10 and 30 m, water depths spanning 5 to 50 m, and survey altitudes of 20, 35, and 45 m. Random noise was added to a subset of the two-layer models to demonstrate inversion in the presence of noise.
01oπ, equation 1). Three models were used: 10m seawater overlying 10m of sediment (Model A); 30m seawater overlying 10m sediment (Model B); and 40 m seawater overlying 10m sediment (Model C). Transmitter altitude was 35m in all cases.
성능/효과
Generally, qualitative agreement between bedrock peaks and troughs was good. The quantitative correspondence between AEM and seismic bedrock depth estimates was reasonable; in areas where seawater was shallower than 20 m and sediment thickness was less than 40 m, differences in predicted depth typically ranged between 5 and 15 m.
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