Abstract: A method of logging an underground formation from within a borehole includes the steps of emitting an electromagnetic signal from a first location in a borehole, determining a time at which the signal is emitted, detecting a direct component of the signal which has passed from the first location to a second location without being reflected, determining a time at which the direct component arrives at the second location and determining the speed of the signal in the underground formation from the time at which the signal is emitted, the time of arrival of the direct component and the spacing of the first and second locations. The speed of the signal can be used for radar logging of the formation.

Abstract: A borehole logging tool such as a wireline logging tool or a logging-while drilling tool includes a tool body; a transmitting antenna located in the body; a pulser for causing the transmitting antenna to transmit an electromagnetic signal; a receiving antenna, located in the body, which includes an array of receiver elements disposed around the longitudinal axis of the tool body; and electronics for measuring the difference in signals detected by pairs of elements in the array. The receiver elements can be linear dipole elements and the electronics for analyzing the signals measures the difference in signals measured by pairs of elements so as to determine the direction of a reflecting structure from the tool.