Estimating Petrophysical Data From Borehole Geophysics

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