Semi-passive two way borehole communication apparatus and method

An apparatus and method for communicating between a surface location and a downhole location. An acoustic signal is generated at the surface, reflected by a downhole controllable reflector 208 and received at the surface by a receiver 214. The signal can be transmitted via the wellbore fluid, the drill pipe or through the Earth. Techniques such as vibrator trucks, explosives or airguns can be used to pass a signal into the Earth. The signal reflector reflects an echo, the characteristics of which are controllable by a downhole control unit 210. The reflected signal 212 can be binary (bi-level) or multi-level with distinct amplitude levels. A separate intersecting borehole (506, figure 5) can be used to communicate with the signal receiver to reduce noise.