Research Borehole‐Geophysical Logging in Determining Geohydrologic Properties

A research borehole-geophysical logging program was implemented to assess specific yield, aquifer mineralogy, and water resistivity more accurately and economically than can be done using conventional aquifer testing and coring procedures. The logging program was part of the Ground-Water Research Project for Abu Dhabi Emirate. A method of determining specific yield using data from a thermal decay-time (TDT) log was developed and tested. This technique relates the capture cross section of the fluid in the invaded zone to specific yield. A second new method uses data from density and neutron logs in a program “LOGAN2” to determine specific yield. Results from both methods were compared to the specific yield derived from the neutron log as well as a specific yield determined by a complex analysis of data from a set of research borehole-geophysical logs, including a gamma-gamma log for density, neutron, sonic, gamma-ray, induction, microresistivity, photoelectric, caliper, and spontaneous-potential logs. The agreement was good. A method of computer-assisted interpretation of mineralogy was developed by correlating results of direct examination of the mineralogy of sidewall cores with data from a nuclear activated spectral-gamma log (GLT). The GLT log allows the mineralogy to be assessed in a comprehensive manner. These computer interpreted results were used to develop another method of computer-assisted interpretation of holes logged not using the GLT. The second method was applied to the more than 50 other test holes that did not include GLT and TDT logs. Resistivity of the water in the formation was conceptualized using a dual-water model, which is especially suited for clayey aquifers. Water resistivities were calculated by two methods—first using data from the suite of logs without the GLT and again using the data from the research suite of logs, which included the GLT. The agreement of calculated water resistivities was excellent