Dependence Of Sandstone Dielectric Behaviour On Moisture Content And Lithology

An investigation of the radar frequency dielectric behaviour of Sherwood Sandstone from<br>Yorkshire, England is reported. The equipment used, a large one port co-axial cell, and the method of<br>data interpretation are described. The co-axial cell measures the frequency dependent complex<br>reflection co-efficient of drill core samples. The dielectric properties are extracted from the complex<br>reflection co-efficient, the cell calibration data, and specimen geometry. Proof testing of the dielectric<br>cell is described. The cell works well over the frequency range of 75 to 1000MHz for saturated<br>specimens, and over the frequency range of 200 to 1000MHz for dry specimens.<br>The frequency dependent dielectric behaviour of sandstone specimens with mean grainsize<br>ranging from 300 to 125 microns over the full range of saturation levels is described (samples were<br>progressively wetted by addition of de-ionised water, saturated by soaking, and then dried by<br>evaporation). The results of the study show that the real component of the dielectric constant, Kr, is<br>independent of frequency and lithology for between 350MHz and 1000MHz. Below 350MHz, dielectric<br>behaviour is lithology dependent, with finer grained specimens showing higher Kr than medium grained<br>specimens.<br>The behaviour of Kr with moisture content is compared with that predicted by the Topp equation<br>and the Complex Refractive Index Method (CRIM). The Topp equation, and CRIM with Ks between<br>3.5 and 5 provide a good match to the data for all lithologies at 500MHz. They are also likely to provide<br>a good match at 100MHz for the medium grained sandstone lithology. The results indicate that<br>determination of moisture content/porosity is best done at higher frequencies (e.g. using Time Domain<br>Reflectometry) where a unique relationship exists. In contrast, characterisation of lithology from<br>dielectric behaviour is best carried out at lower frequencies (i.e. in 50 or 100MHz radar surveys), where<br>the dielectric constant is lithology dependent.