Abstract: An electrostatic sensing device for investigating a surface and/or sub-surface structure comprises a central and square electrically conductive sensor plate, and four side electrically conductive elongated and rectangular sensor plates connected together and each disposed adjacent and parallel to one edge of the central plate. The arrangement of sensor plates is applied to the surface, and the central and sensor plates are then electrically charged at different rates, the charging rate of the central plate being affected by the permittivity of the investigated structure to electric field. A differential amplifier amplifies the difference between the voltages on the central and side plates as these plates charge, and a peak detector detects the highest amplitude of this amplified voltage difference and displays it on a voltmeter.