Abstract: A method for use in or for detecting a downhole feature in a well comprises transmitting an electromagnetic signal from a first position located substantially at or adjacent to surface through a first space to the downhole feature. The method further comprises receiving an electromagnetic signal at a second position located substantially at or adjacent to surface after reflection of the transmitted electromagnetic signal from the downhole feature and after propagation of the reflected electromagnetic signal through a second space. The method may comprise sealing the well before transmitting the electromagnetic signal. Such a method may be used to detector determine a distance from surface to a downhole feature such as a fluid interface in a completed production, injection or observation well.

Abstract: A structure defines an elongate space and comprises an apparatus installed within the structure. The apparatus includes a signal generator and detector arrangement, and at least one antenna coupled to the signal generator and detector arrangement. The at least one antenna is coupled to the elongate space for transmitting an electromagnetic signal into the elongate space and for receiving the electromagnetic signal from the elongate space after reflection of the electromagnetic signal from a feature within the elongate space. The apparatus may be used to monitor the feature within the elongate space.