Estimating Petrophysical Data From Borehole Geophysics

Abstract

A study of the ability of ground penetrating radar and natural gamma logging to indicate porosity, lithology and permeability in the unsaturated zone of a sandstone aquifer is described. The aquifer (Sherwood Sandstone, UK) consists of a series of fluvially derived sequences fining upwards from medium to fine sandstone and siltstone. Laboratory measurements on core samples of porosity, grain size, mineralogy and hydraulic conductivity are reported. Vertical hydraulic conductivity is lower than horizontal hydraulic conductivity, probably as a result of preferential sand grain orientation and fine/medium sand lamination. Clay is the primary influence on hydraulic conductivity, although porosity also has some effect. Data for hydraulic conductivity, porosity and clay content are compared to dielectric constant values from zero-offset 50 MHz cross borehole radar profiles and natural gamma activity. Natural gamma activity increases with increasing clay content and reducing hydraulic conductivity. Dielectric constant averaged over six months also correlates with clay content, despite scatter due to variations in the moisture content during this period. Dielectric constant (unlike gamma activity) is very strongly influenced by moisture content because of the very high dielectric constant of water compared with those of mineral solids and air. However its correlation with clay content is preserved in the unsaturated zone because clay rich layers also have high moisture retention. Dielectric constant increases with reducing hydraulic conductivity when the profile is relatively wet, but not when it is drier. However low hydraulic conductivity layers may be detectable using ground penetrating radar because they show large seasonal fluctuations in dielectric constant.