Electrical Techniques
Spectral Induced Polarisation
Detail
Spectral IP surveys involve measurement of the magnitude and relative phase of the polarisation voltage that results from the injection of an alternating current into the ground. The polarisation voltage essentially results from the ability of the ground to store charge (i.e. to become polarised) in a similar manner to an electrical capacitor.
Two main mechanisms are known to be responsible for the IP effect although the exact causes are still poorly understood. The main mechanism in rocks containing metallic conductors is electrode polarisation. This results from the build up of charge on either side of conductive grains within the rock matrix as they block the flow of current. On removal of this current the ions responsible for the charge slowly diffuse back into the electrolyte (groundwater) and the potential difference across each grain slowly decays to zero. The second mechanism, membrane polarisation, results from a constriction of the flow of ions around narrow pore channels. It may also result from the excessive build up of positive ions around clay particles. This cloud of positive ions similarly blocks the passage of negative ions through pore spaces within the rock. On removal of the applied voltage the concentration of ions slowly returns to its original state resulting in the observed IP response.
In spectral IP surveys measurements of the relative phase shift between the transmitted current and the measured signal, and the magnitude of the polarisation voltage, are taken over a range of different frequencies, typically between 0.125 and 9000Hz. This results in a distinct IP response spectrum or 'dispersion' at each measurement position that can be used to determine various parameters of the subsurface materials such as relaxation time and chargeability.
Spectral IP was initially used in trying to determine the type and texture of mineralisation in exploration work but is being increasingly applied to the detection of subsurface contamination. Zetica Ltd is currently investigating the use of the technique in hydrocarbon mapping in conjunction with a research project into the use of ground penetrating radar with Shell Research.
