Coastal clay deposits are an increasingly important focus of environmental and geotechnical studies, and to date, geophysical techniques for mapping them have almost exclusively relied on bulk conductivity measurements. However, direct correlation of bulk conductivity data is difficult, as it is a non-unique parameter. A potential alternative geophysical mapping tool is the complex conductivity technique, where both in-phase and out-of-phase conductivity are measured concurrently. Using similar principles, the induced polarisation field technique measures the charge polarisation parameter chargeability. Studies that utilise these techniques have almost exclusively concentrated on the charge polarisation properties of sediments with low clay contents, and these sediments have intuitively credible positive chargeability values. However, the few studies that include high-clay-content sediments like those encountered in coastal clay deposits have often reported negative chargeabilities. No mechanism has been presented that accounts for this effect. Similar negative chargeability effects have been noted in other sciences, but often remain unexplained. Negative chargeability is shown here to be a non-linear property of clay gels unique to sediments with bulk membrane properties and a potential new clay mapping parameter. A new clay polarisation model is presented which accounts for negative chargeability. Clay sediments are considered as a clay gel membrane, as opposed to traditional models of clay zones within a solid substrate. In the revised model, non-linear ionic gradients occur within the sample, due to the bulk membrane properties of clay gels. Charge dispersion is described in terms of anomalous diffusion Warburg impedances, which create currents in the opposite sense to the applied potential. Negative chargeability is then a unique feature of sediments with bulk membrane properties. Laboratory electrical impedance spectroscopy (EIS) data are presented. The experimental data covers a variety of clay types at a variety of water contents. For pure clay samples, negative chargeability is shown to occur when the clay s water content is such that it is in a gel state. The threshold water content is shown to be a function of the clay s liquid limit. This trend is more complicated for natural clays. Field measurements of negative chargeability are presented. The Conductivity Cone Penetrometer (CCPT) is utilised as a chargeability-logging tool at a field site at Hexham, New South Wales. In the first study of its kind, negative chargeability CCPT logging is shown to be effective in mapping a coastal clay deposit.