10:45 AM - 12:15 PM
[STT40-P03] Estimation of near-surface porosity distribution using gravity anomaly or gravity gradient data
Keywords:porosity distribution, gravity anomaly, gravity gradient
In this study, we proposed a method to estimate porosity distribution by gravity exploration in a broad sense, including gravity gradient data.
As is well known, soil and sand contain many pores, and even seemingly dense rocks have a porosity that is not significantly different from that of soil or sand. Therefore, it is important to know the porosity distribution in developing resources such as geothermal, petroleum, and natural gas. If the pores are filled with low-density materials such as water, air, and gas, the rocks having high porosity are low density and would be a source of gravity low areas. On the other hand, if the pores are filled with high-density material, the high-porosity rocks become denser than their surroundings and would be a source of gravity high areas.
Gravity exploration is one of the useful techniques for estimating density structure. However, there is no concept of porosity in the conventional density structure estimated by gravity exploration. We have discussed or interpreted subsurface structures and their conditions by comparing the estimated density with the standard density of predicted materials. In this study, we attempted to discuss the density structure by the porosity and the material filled in them.
We derived theoretical relationships between porosities and density structures estimated by gravity exploration based on the definition formula of porosity. We derived them for the following three models. 1. the host rock and the target (the object causing the gravity anomaly) are constituted by the same material, and the density contrast between the host rock and material filling pores and porosity distribution are sources of gravity anomalies. 2. the host rock and the target have different porosities, but their constituent materials are the same, and materials filling pores in the host rock and the target are the same. 3. the host rock and the target have different materials and porosities, and materials filling both pores are also different.
In the presentation, we will show these theoretical relationships between porosities and density structures and the application of model (1) to real field data.
As is well known, soil and sand contain many pores, and even seemingly dense rocks have a porosity that is not significantly different from that of soil or sand. Therefore, it is important to know the porosity distribution in developing resources such as geothermal, petroleum, and natural gas. If the pores are filled with low-density materials such as water, air, and gas, the rocks having high porosity are low density and would be a source of gravity low areas. On the other hand, if the pores are filled with high-density material, the high-porosity rocks become denser than their surroundings and would be a source of gravity high areas.
Gravity exploration is one of the useful techniques for estimating density structure. However, there is no concept of porosity in the conventional density structure estimated by gravity exploration. We have discussed or interpreted subsurface structures and their conditions by comparing the estimated density with the standard density of predicted materials. In this study, we attempted to discuss the density structure by the porosity and the material filled in them.
We derived theoretical relationships between porosities and density structures estimated by gravity exploration based on the definition formula of porosity. We derived them for the following three models. 1. the host rock and the target (the object causing the gravity anomaly) are constituted by the same material, and the density contrast between the host rock and material filling pores and porosity distribution are sources of gravity anomalies. 2. the host rock and the target have different porosities, but their constituent materials are the same, and materials filling pores in the host rock and the target are the same. 3. the host rock and the target have different materials and porosities, and materials filling both pores are also different.
In the presentation, we will show these theoretical relationships between porosities and density structures and the application of model (1) to real field data.