Geophysical borehole logging

Abstract

Introduction Geophysical borehole logging involves measuring the physical properties of surrounding rocks with a sensor located in a borehole. The record of measurements as a function of depth is called a borehole log (or well log). Many of the methods of geophysical surveying (described in previous chapters), with some modifications, can be adapted for use in borehole logging. Downhole geophysical measurements greatly increase the value of the hole to the interpreter by their ability to provide more direct information about formation thickness and lithology, dip of strata, porosity, formation fluid content, permeability, and temperature. In addition, they provide a means of correlating geological formations from one borehole to another. Geophysical logging techniques have been widely used in the investigation of wells drilled for hydrocarbon exploration, as they provide important in-situ properties of possible reservoir rocks. This chapter focusses on the application of geophysical logging techniques that are of special relevance to environmental investigations. These are related mainly to the current and future stability of the site and the possible migration of contaminants in the groundwater. In particular, environmental impact studies at proposed landfill sites and potential nuclear waste repositories have concentrated much attention on detecting fractures as possible conduits for rapid transport of toxic chemicals. Conventional logging techniques (such as those used in exploratory wells for oil/gas and water) can be applied to define the hydrogeological regime at hazardous waste sites, but special interpretation procedures must be applied in those areas where the logging zone of interest is above the water table, or where the rock has an extremely low permeability.