Study on estimation of porosity distribution using gravity anomaly or gravity gradient data

2023 Fiscal Year Final Research Report

• Project/Area Number: 20K05399
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 31020:Earth resource engineering, Energy sciences-related
• Research Institution: Kyoto University
• Principal Investigator: Kusumoto Shigekazu 京都大学, 理学研究科, 教授 (50338761)
• Project Period (FY): 2020-04-01 – 2024-03-31
• Keywords: 間隙率 / ブーゲー異常 / 地下浅部 / 密度構造 / 間隙比 / スペクトル解析 / フィルタ / 平均境界面深度

Outline of Final Research Achievements

In this study, we developed a method to estimate porosity distribution by gravity survey and an optimal filter to carry out the technique efficiently. We showed a theoretical relationship between the density structure of rock mass and gravity anomaly via porosity and applied it to the gravity anomaly observed at Jigokudani, Midagahara Volcano (Toyama Prefecture). As a result, it was shown that the low-density area caused by volcanic gas and water has a porosity of about 0.06-0.35 higher than the surrounding host rock, and it was confirmed that they are in the realistic range. In addition, we developed a method to automatically estimate the mean boundary depth and frequency band of the causative layer from the power spectrum of gravity anomaly using the degree of freedom-adjusted coefficient of determination as an index to design an optimal filter to extract gravity anomaly caused by shallow density structure objectively.

Free Research Field

固体地球物理学

Academic Significance and Societal Importance of the Research Achievements

地熱や石油・天然ガス等の資源の開発では，間隙率分布を知ることが重要である。間隙に水やガスなど低密度物質が充填された場合，間隙率の高い岩体は間隙率の低い岩体よりも低密度になる。金属鉱床のように高密度物質が充填された場合，高間隙率岩体は周囲より高密度になる。密度分布の抽出には，重力偏差を含めた広い意味での重力探査が有用であるが，重力異常を間隙率に変換して評価する手法は見当たらず，またそのためのフィルタ設計も多くない。重力異常や重力偏差データを間隙率に変換・考察する手法開発と最適フィルタ設計を行った本研究は，資源開発や地球科学・工学に一石を投じ，それらの発展に寄与できたのではないかと考えている。