Wide bandwidth borehole dipole source for acoustic logging

An acoustic logging tool for making measurements of a substrate surrounding a borehole comprising:

• a body insertable in the borehole, the body comprising an acoustic detector and a dipole source each disposed along an axial length of the body, the dipole source comprising:
• a cylindrical shell;
• a center beam having a bushing therein;
• a frame coupling the center beam to the cylindrical shell, the frame being directly coupled to the center beam;
• a movable projector disposed inside the bushing to impact the center beam in a direction along a dipole axis of the dipole source, the dipole axis being perpendicular to an axis of the center beam;
• a plurality of springs directly coupling the projector to the center beam to restore the projector to an initial position after impacting the center beam;
• and
• a cavity surrounding the frame to acoustically insulate an interior of the cylindrical shell from an exterior of the cylindrical shell;
• and
• a control unit operatively coupled to the acoustic detector and the dipole source to provide power thereto and to process data collected from the acoustic detector to obtain information about the substrate surrounding the borehole.

A method for generating an acoustic wave in a spatial pattern having a dipole symmetry axis comprising:

• providing a current to a coil for a first selected time interval;
• propelling a projector through a bushing of a center beam to thereby impact the center beam using a magneto-motive force generated by the current during the first selected time interval, the impact of the projector with the center beam comprising a force and a change in time of the force, wherein:
• a direction of the force and a direction of the change in time of the force are substantially the same, along a dipole symmetry axis;
• and
• providing a restoring force to the projector for a second selected time interval, wherein:
• the projector, the coil, and the center beam are disposed inside a frame coupling the center beam to an interior of a cylindrical shell, the frame being directly coupled to the center beam;
• at least one spring directly coupling the projector to the center beam to restore the projector to an initial position;
• and
• a cavity surrounding the frame and filled by a fluid which acoustically insulates the interior of the cylindrical shell from an exterior of the cylindrical shell.

A method for measuring properties of a substrate using an acoustic logging tool having a dipole source, the method comprising:

• using the dipole source, generating an acoustic wave in a pattern substantially symmetric about an axis of the dipole source, the dipole source being positioned in a borehole;
• detecting the acoustic wave using an acoustic detector placed along an axial length of the acoustic logging tool, the acoustic detector being positioned in the borehole;
• and
• measuring a velocity of the acoustic wave through the substrate, wherein generating an acoustic wave in a pattern substantially symmetric about an axis of the dipole source comprises:
• providing a current to a coil for a first selected time interval;
• propelling a projector through a bushing of a center beam to thereby impact the center beam using a magneto-motive force generated by the current during the first selected time interval, the impact of the projector with the center beam comprising a force and a change in time of the force, wherein a direction of the force and a direction of the change in time of the force are substantially the same;
• and
• providing a restoring force to the projector for a second selected time interval, along a dipole symmetry axis, wherein:
• the projector, the coil, and the center beam are disposed inside a frame coupling the center beam to an interior of a cylindrical shell, the frame being directly coupled to the center beam;
• and
• a cavity surrounding the frame and filled by a fluid which acoustically insulates the interior of the cylindrical shell from an exterior of the cylindrical shell,
• wherein providing the restoring force to the projector for the second selected time interval comprises using a plurality of springs directly coupling the projector to the center beam.