Patent ID: 7639562

Claim:
An apparatus configured to evaluate an earth formation having a cased borehole, the apparatus comprising: (a) an acoustic transmitter on a logging tool configured to be conveyed in the borehole, the transmitter configured to generate an acoustic signal that propagates through casing (casing signal) and through the formation (formation signal); (b) at least one receiver on the logging tool configured to be responsive to the casing signal and the formation signal; and (c) a device on the logging tool disposed between the transmitter and the at least one receiver configured to use a measurement of the casing signal to actively suppress the casing signal propagating to the location of the at least one receiver, the device including: (i) a first sensor disposed between the transmitter and the at least one receiver, the first sensor configured to make the measurement of the casing signal, (ii) a transducer disposed between the first sensor and the at least one receiver, the transducer configured to produce an output that substantially suppresses the casing signal using the measurement of the first sensor.