Analysis of fractures detectability by borehole radar

Fracture is an important geologic phenomenon which is crucial for petroleum and geothermal exploration and exists in various scale and geometry in nature. Borehole radar is an important tool which can image single fractures several to tens meters away from borehole in thousands of meters depth. However, the detectability of various fractures is not clear. We analyzed the radar response to a thin fracture using plane wave theory, and found the primary reflection and multiple reflections cancel each other. The fact increase the difficulty of fracture detection. We use sub-cell FDTD technique to synthesize borehole radar response to fractures from 0.0005m to 0.02m wide, and filled with water or air. It is found that water-filled vertical fracture is easier to be detected than air-filled fracture, and the fracture width affect the reflected signal very much. The wider the fracture, the strong the reflected signals. Also, large dynamic range is required for weaker fracture signals detection. This kind of simulation is helpful for the fracture detection and evaluation.