Detection of permeable fractures in granitic rock by hydrophone VSP. Case study for granitic rock, Gifu prefecture, Japan. Pt. 2

The multi-offset hydrophone VSP experiment was carried out at Shomasama site in order to estimate the capability of this method to detect and characterize permeable fractures. From the VSP experiment using weight drop as seismic source, 19 tube wave generations are detected from 20m to 559m depth. We confirm that VSP experiment using weight drop is effective for detecting permeable fractures as well as one using dynamite explosion. We calculate fracture dip, strike and permeability at 15 depths from VSP data using dynamite explosion as source. Most of the calculated permeability are of the order of 100md. By using BTV image, fractures are detected at all depths where tube waves are generated. Fracture dip and strike estimated by tube wave analysis are consistent with the results of BTV image at some depths. However, there are some cases that dip and strike by tube wave analysis are different from those by BTV image. The reason why this discrepancy between tube wave analysis and BTV image has occurred is not clear at the present time. Permeability estimated by tube wave analysis will be discussed in 'part 3' report. (author)