Application Of Borehole Geophysical Logs To Water Resources Investigations

Borehole geophysics have been around since the late 1800's when U.S.G.S. personnel used a wireline to take temperature measurements in a borehole. Up until the early mid 1970's or SO the use of borehole geophysics were largely confined to the petroleum industry. The advancement of microelectronics has brought borehole geophysics to the water well industry by allowing the downhole probes to be smaller in diameter, lighter weight and more affordable. Borehole geophysical logs compliment other types of data collected during a site investigation such as cuttings, cores and surface geophysical data. Borehole geophysical logs have several unique features in that they can be run quickly, in small diameter mudded and/or cased holes (almost all logs can now be run in two-inch casing). Many logs can ilsee" through casing, but more importantly they provide a continuous record of various borehole parameters of interest to a hydrogeologist, particularly: bulk porosity, resistivity, density, and clay content. An experienced log analyst can take this information much in the same manner as an experienced hydrogeologist observes a core and analyzes what he sees. From this information the hydrogeologic characteristics can be used to map hydrostatigraphic units across the study area and develop a better understanding of vertical lithologic gradations and lateral facies changes essential to the development of a good understanding of the site hydrogeology. Currently, most borehole geophysical logging is used in a semiquantitative manner to assess lateral and vertical variations. Calibrated and compensated geophysical logs are available and could be used more effectively to more precisely determine: formation bulk porosity, matrix density, formation resistivity, clay content, carbonate content, water quality and even permeability and some cases vertical migration of contaminants. In summary, logs are capable of providing a continuous record of numerous parameters in a small diameter hole and in a timely manner, hydrogeologists take this data, fit it into a working hydrological model of the site to better understand the groundwater flow system.