Apparatus and method for acoustically investigating a casing set in a borehole

An apparatus for investigating a borehole having a borehole wall with a casing having a casing wall penetrating an earth formation comprising:

• a tool shaped to move along the borehole, said tool having an axis which is generally parallel to the wall of the borehole;
• a substantially continuous array of acoustic transducers concentrically mounted on the tool around the tool axis, said array being oriented to transmit and then detect acoustic energy traveling along a path generally parallel to the tool axis;
• a substantially radially continuous acoustic reflection surface which is generally concentric with the tool axis and is axially spaced from the array and is interposed in the path of acoustic energy transmitted from and to be detected by the array;
• means for selectively actuating the transducers in the array to produce circumferentially discrete pulsed beams of acoustic energy from the array with the beams distributed around the tool axis and being directed onto the reflection surface;
• said reflection surface being inclined at such angle relative to the tool axis so as to reflect transmitted beams primarily onto discrete circumferential casing segments of the casing wall with an angle of incidence that is substantially normal to the casing wall and with a circumferentially curved acoustic wavefront;
• the acoustic transducers that are energized to produce said pulsed beams being of a number so selected that the casing segments onto which the beams are incident circumferentially overlap;
• and
• means for deriving from acoustic energy detected by the transducers an indication of a characteristic of the casing wall at said casing segments.