Anisotropy of Resistivity to Bedding Plane in Sedimentary Soft Rocks

Anisotropy of the physical properties in rocks is affected by factors such as the orientation of particles and the fracture directions. In sedimentary soft rocks, anisotropy relative to the bedding plane is developed due to preferred particle arrangement and compaction after deposition. We measured resistivity and calculated the degree of anisotropy using siltstone from the Mio–Pleistocene forearc basin. As a result, the samples exhibited anisotropy, in which the resistivity perpendicular to the bedding plane was larger compared to parallel to it. The degree of resistivity anisotropy is calculated using two parameters: anisotropic factor (AF) defined as the ratio of parallel over perpendicular value, and degree of anisotropy (DA, %) defined as the percentage difference between two directions. The anisotropy was found to be approximately 0.8 for AD and approximately -25% for DA, and tended to decrease with decreasing porosity in sedimentary soft rocks with high porosity (>35%). The anisotropy is thought to be due to the preferred orientation of mineral particles constituting the sedimentary soft rock, suggesting a layered structure. The anisotropic characteristics of resistivity are consistent with the anisotropy of P-wave velocity and thermal conductivity, and it was revealed that electrical conductivity can be detected more significantly than P-wave velocity or thermal conductivity.