Investigation of permeable fracture zones by tube-wave analysis

Abstract

SUMMARY In vertical seismic profiling tube waves are generally classified as noise. They have, however, useful applications: tube waves can be generated by compressional waves at fluid-filled cracks crossing the wall of the borehole. In this case the permeability of the fracture zone can be estimated from the amplitude ratio of the incident P wave and the generated tube wave. In the present study an improved model for tube-wave amplitude interpretation is presented. While former models assumed isolated cracks, now, the consideration of fracture zones of a certain porosity, permeability and width is allowed. Master curves are used to check the sensitivity of the amplitude ratio of tube and P waves with respect to the formal parameters of computation. It is shown, moreover, that mainly those parts of a fracture contribute to the tube-wave generation which are within a radius of about 1 m of the borehole. The problem of localization and characterization of fracture zones by tube waves was investigated with several complementary field experiments in shallow boreholes in crystalline rocks as vertical seismic profiling (VSP) with three-component borehole geophones and hydrophones, televiewer measurements, well tests and temperature measurements. It turned out that fracture zones were met at all depths where tube waves were generated. However, not all of them were found to be hydraulically activated during well tests and temperature measurements. This means that fractures detected by tube-wave VSP are not necessarily of hydrological importance. Transmissivity values based on tube-wave analysis are, therefore, an upper limit for the well under consideration. On the other hand, the transmissivity values derived from the well tests agreed quite well with those of the tube-wave analysis if only the ‘open’fracture zones were considered: a fact which seems to confirm our theoretical model.