Apparatus and method for downhole measurement of formation parameters

An apparatus for downhole measurement of formation parameters utilises a sonde (10) with a microwave transmitting antenna (30) and receiving antenna (32), with measuring means (52) for measuring signals transmitted between the antennae and through the formations, and means (24) for determining formation parameters from the measurements. In a method of measuring the formation parameters, the measurements are used to determine an equivalent circuit representative of the formation between the transmitter antenna and receiver antenna; the equivalent circuit encodes valuations of resistivity, dielectric constant and magnetic permeability of the formation material. Multiple measurements can be made in a given formation, and the sonde can be moved steadily or periodically to make measurements at selected locations. Formation electrical parameters in adjacent formations can then be determined.