物理探査・検層に基づく花崗岩体中の「水みち」の調査法

Abstract

Surface and borehole geophysical methods to detect hydraulic pathways in fractured granitic rock masses and its weathered surface are summarized in this paper, and some newly developed and emerging geophysical methods are explained in detail, i.e.1) precise gravity sur vey (sur vey points inter val = 1m) and precise fanshooting seismic sur vey (receiving points inter val = 0.5m) at the weathered surfaces of the Abukuma granites can detect a hydraulic pathway of the shallow groundwater system with the width of a few meters, which could not be detected by pre-conducted two-dimensional resistivity survey (electrodes interval = 5m).2) precise vertical seismic profile survey (hydrophones interval = 0.5m) of a test well in the Abukuma granites can detect open fractures (ie. candidate hydraulic pathways) with the opening width of about 0.1mm, which cannot be accounted as a hydraulic pathway by borehole camera images and fracture properties, but be proved as a hydraulic pathway by water sampling tests.To detect a small hydraulic pathway in granitic rocks needs surface and borehole geophysical methods with high spatial resolution, but few methods can satisfy the required spatial resolution. So it is important to consider the width and length of the target structures, estimate the spatial resolution to detect the structures, and adopt appropriate methods.