Method of and apparatus for sonic logging while drilling a borehole traversing an earth formation

Method for determining characteristics of earth formations traversed by a borehole in which borehole a hollow cylindrical drill string is positioned, the method comprising the steps of:

• connecting to one segment of said drill string a hollow drill collar, said drill collar being made of a material having a given acoustic transmission characteristic;
• emitting at a first location on said drill collar an acoustic signal having a given frequency;
• receiving at a second location on said drill collar a first direct arrival acoustic signal transmitted through said drill collar in response to the emission of said acoustic signal;
• receiving at said second location on said drill collar a second direct arrival acoustic signal transmitted through a portion of said formations surrounding said drill collar in response to the emission of said acoustic signal;
• receiving at said second location on said drill collar a third arrival acoustic signal transmitted through said drill collar in response to the emission of said acoustic signal;
• the arrival time at said second location of said first, second and third arrival acoustic signals being substantially coincident;
• effecting at a third location on said drill collar, intermediate said first and second locations, a change in the acoustic transmission characteristic of said drill collar material so that by effecting said change a portion of said third arrival acoustic signal is produced with a change in the direction of its travel relative to said first arrival acoustic signal;
• and
• effecting the emission of said acoustic signal in a manner that enhances the coupling of the energy of the emitted acoustic wave to the formations relative to the coupling of the energy of the emitted acoustic wave to the drill collar.

An apparatus for performing sonic logging while drilling with a hollow drill string having a drill bit at its lower end, comprising:

• a drill collar adaptable to be incorporated into said drill string, said drill collar having an interior fluid channel for allowing drilling fluid to pass through said hollow drill string to said drill bit;
• an acoustic transmitter mounted at a given location on said drill collar;
• said transmitter having a center frequency of emitted acoustic energy;
• a first acoustic receiver mounted on said drill collar to receive acoustic energy emitted from said transmitter;
• a second acoustic receiver mounted on said drill collar, and spaced apart from said first receiver, to receive acoustic energy emitted from said transmitter;
• and
• a stop band filter section located on said drill collar between said transmitter and said receivers;
• wherein:
• said drill collar being configured to exhibit a natural stop band about said center frequency so as to attenuate at least a portion of the acoustic energy propagating within said drill collar at said center frequency;
• and
• said stop band filter section is selected to further attenuate at least a portion of the acoustic energy propagating within said drill collar at said center frequency.

Apparatus for determining characteristics of formations traversed by a borehole in which borehole a hollow cylindrical drill string is positioned, comprising:

• an elongated hollow drill collar connected to one segment of said drill string so as to position said drill collar in said borehole;
• said drill collar being made of a material having a given acoustic transmission characteristic;
• means for emitting at a first location on said drill collar an acoustic signal;
• means for receiving at a second location on said drill collar a first direct arrival acoustic signal transmitted through said drill collar in response to the emission of said acoustic signal;
• means for receiving at said second location on said drill collar and at a given frequency a second direct arrival acoustic signal transmitted through a portion of said formations surrounding said drill collar in response to the emission of said acoustic signal;
• the arrival time at said second location of said first direct arrival acoustic signal being substantially the same as the arrival time at said second location of said second direct arrival acoustic signal;
• means for effecting at a third location on said drill collar intermediate said first and second locations, a change in the acoustic transmission characteristic of said drill collar material as to result in the energy of said first direct arrival acoustic signal being attenuated;
• and
• means for effecting the emission of said acoustic signal in a manner that enhances the coupling of the emitted acoustic energy to the formations relative to the coupling of the emitted acoustic energy to the drill collar.

Apparatus for determining characteristics of earth formations traversed by a borehole in which borehole a hollow cylindrical drill string is positioned, comprising:

• means for connecting to one segment of said drill string a hollow drill collar, said drill collar being made of a material having a given acoustic transmission characteristic;
• means for emitting at a first location on said drill collar an acoustic signal having a given frequency;
• means for receiving at a second location on said drill collar a first direct arrival acoustic signal transmitted through said drill collar in response to the emission of said acoustic signal;
• means for receiving at said second location on said drill collar a second direct arrival acoustic signal transmitted through a portion of said formations surrounding said drill collar in response to the emission of said acoustic signal;
• means for receiving at said second location on said drill collar a third arrival acoustic signal transmitted through said drill collar in response to the emission of said acoustic signal;
• the arrival time at said second location of said first, second and third arrival acoustic signals being substantially coincident;
• means for effecting at a third location on said drill collar, intermediate said first and second locations, a change in the acoustic transmission characteristic of said drill collar material so that by effecting said change said third arrival acoustic signal is produced with a shift in the direction of travel of at least a portion thereof relative to said first arrival acoustic signal;
• and
• means for effecting the emission of said acoustic signal in a manner that enhances the coupling of the energy of the emitted acoustic wave to the formations relative to the coupling of the energy of the emitted acoustic wave to the drill collar.

A method for performing sonic logging while drilling comprising the steps of:

• positioning a hollow drill string having a drill bit at its lower end in a borehole passing through earth formations;
• incorporating a drill collar into said drill string, said drill collar having an interior fluid channel for allowing drilling fluid to pass through said hollow drill string to said drill bit;
• emitting at a first location on said drill collar, an acoustic signal having a center frequency of emitted acoustic energy;
• receiving at a second location on said drill collar, acoustic energy emitted from said first location;
• receiving at a third location on said drill collar, separate from said second location, acoustic energy emitted from said first location;
• locating a stop band filter on said drill collar between said first location and said second and third locations;
• configuring said drill collar to exhibit a natural stop band at said center frequency to attenuate at least a portion of the acoustic energy propagating within said drill collar at said center frequency;
• and
• selecting said stop band filter to have characteristics that further attenuate at least a portion of the acoustic energy propagating within said drill collar at said center frequency.

A method for performing sonic logging while drilling a borehole traversing an earth formation, including drilling the borehole with a drill string having a drill bit at its lower end and drilling fluid in the borehole surrounding the drill string, the steps comprising:

• a) drilling with a drill collar incorporated into the drill string;
• b) transmitting, from a location on said drill collar, acoustic energy into the surrounding earth formations;
• c) receiving, at a location on said drill collar, acoustic energy returned from the surrounding earth formations;
• d) providing an output related to the received acoustic energy;
• and
• e) determining from said output a characteristic of said earth formations.

A method for performing sonic logging while drilling a borehole traversing an earth formation to determine characteristics of said earth formation, the steps comprising:

• a) drilling a borehole with a drill collar supporting a drill bit;
• b) transmitting acoustic energy with a transmitter mounted on the drill collar;
• and
• c) receiving acoustic energy with at least one receiver mounted on the drill collar;
• d) wherein step b) includes the step of transmitting the acoustic energy about a given center frequency which allows the determination of said formation characteristics.

An apparatus for performing sonic logging while drilling a borehole traversing an earth formation to determine characteristics of said earth formation, comprising:

• a) a drill collar for supporting a drill bit during drilling;
• b) a transmitter mounted on said drill collar;
• c) at least one receiver mounted on said drill collar to receive acoustic energy emitted from said transmitter;
• and
• d) wherein said receiver is responsive to acoustic energy about a given center frequency which allows the determination of said formation characteristic.

A method for performing sonic logging while drilling a borehole traversing an earth formation to determine characteristics of said formation, the steps comprising:

• a) drilling a borehole with a drill collar supporting a drill bit;
• b) transmitting acoustic energy with a transmitter mounted on the drill collar while drilling;
• c) receiving acoustic energy with at least one receiver mounted on the drill collar while drilling;
• d) providing an output related to the received acoustic energy;
• and
• e) determining from said output a characteristic of said earth formation.