Accelerator-based methods and apparatus for measurement-while-drilling

A method for measuring properties of earth formations surrounding a borehole being drilled by a drill bit at the end of a drill string, comprising:

• providing a high energy neutron accelerator in said drill string;
• providing a first neutron detector in said drill string at a first spacing from the accelerator in the lengthwise direction of the drill string, said first neutron detector having an output that is primarily proportional to the accelerator neutron flux;
• providing a second neutron detector in said drill string at a second, farther spacing from the accelerator in the lengthwise direction of the drill string, said second neutron detector being sensitive to epithermal neutrons and having an output that is primarily responsive to the hydrogen concentration of the surrounding earth formation and only secondarily responsive to the density of the surrounding earth formation;
• providing a third radiation detector in said drill string at a third, still farther spacing from the accelerator in the lengthwise direction of the drill string, said third detector having an output that is more responsive to the density of the surrounding earth formation and less responsive to the hydrogen concentration of the surrounding earth formation than is said second detector;
• and
• combining the outputs of said first, second and third detectors to derive an indication of at least density of the surrounding earth formation.

Measurement-while-drilling apparatus for measuring properties of earth formations surrounding a borehole being drilled by a drill bit at the end of a drill string, comprising:

• an elongated tubular drill collar in said drill string;
• a neutron accelerator in said drill collar for irradiating the surrounding earth formations with high energy neutrons;
• at least one radiation detector in said drill collar spaced from the accelerator in the lengthwise direction of the drill collar for detecting radiation resulting from said neutron irradiation and generating an output in response to said detected radiation, the spacing being such that the radiation resulting from said neutron irradiation is influenced by the density of the formations;
• and
• means for recording the output of said at least one detector as a function of at least one of borehole depth and azimuthal orientation within the borehole and means for determining a parameter related to the density of the earth formation.

A method for measuring the properties of earth formations surrounding a borehole being drilled by a drill bit at the end of a drill string, comprising:

• providing a neutron accelerator in said drill string for irradiating the earth formations with high energy neutrons;
• providing at least one radiation detector in said drill string spaced from the accelerator in the lengthwise direction of the drill string for detecting radiation resulting from said neutron irradiation of the earth formations and for generating an output in response to said detected radiation, the spacing being such that the radiation resulting from said neutron irradiation is influenced by the density of the formations;
• and
• recording the output of said at least one detector as a function of at least one of borehole depth and azimuthal orientation in the borehole and means for determining a parameter related to the density of the earth formation.

An apparatus for measuring properties of earth formations surrounding a borehole, comprising:

• a) a housing;
• b) a high energy neutron accelerator in the housing for irradiating the formations from within the borehole;
• c) a first neutron detector in said housing at a first spacing from the accelerator in the lengthwise direction of said housing, said first neutron detector having an output that is primarily proportional to the accelerator neutron flux;
• d) a second neutron detector in said housing at a second, farther spacing from the accelerator in the lengthwise direction of said housing, said second neutron detector being sensitive to epithermal neutrons and having an output that is primarily responsive to the hydrogen concentration of the surrounding earth formation and only secondarily responsive to the density of the surrounding earth formation;
• e) a third radiation detector in said housing at a third, still farther spacing from the accelerator in the lengthwise direction of said housing, said third detector having an output that is more responsive to the density of the surrounding earth formation and less responsive to the hydrogen concentration of the surrounding earth formation than is the second detector;
• f) means for recording the respective outputs of said first, second and third detectors;
• and
• g) means for determining a parameter related to the formation density from the respective outputs.

A method for measuring properties of earth formations surrounding a borehole, comprising:

• providing a high energy neutron accelerator for irradiating the earth formations from within the borehole;
• providing a first neutron detector at a first spacing from the accelerator in the lengthwise direction of the borehole, said first neutron detector having an output that is primarily proportional to the accelerator neutron flux;
• providing a second neutron detector at a second, farther spacing from the accelerator in the lengthwise direction of the borehole, said second neutron detector being sensitive to epithermal neutrons and having an output that is primarily responsive to the hydrogen concentration of the surrounding earth formation and only secondarily responsive to the density of the surrounding earth formation;
• providing a third radiation detector at a third, still farther spacing from the accelerator in the lengthwise direction of the borehole, said third detector having an output that is more responsive to the density of the surrounding earth formation and less responsive to the hydrogen concentration of the surrounding earth formation than is said second detector;
• and
• combining the outputs of said first, second, and third detectors to derive an indication of at least density of the surrounding earth formation.

An apparatus for measuring properties of earth formations surrounding a borehole, comprising:

• a housing;
• a neutron accelerator in said housing for irradiating the surrounding earth formations with high energy neutrons;
• at least one radiation detector in said housing spaced from the accelerator in the lengthwise direction of the housing for detecting radiation resulting from said neutron irradiation and generating an output in response to said detected radiation, the spacing being such that the radiation resulting from said neutron irradiation is influenced by the density of the formations;
• and
• means for recording the output of said at least one detector as a function of at least one of borehole depth and azimuthal orientation within the borehole and means for determining a parameter related to the density of the earth formation.

A method for measuring the properties of earth formations surrounding a borehole, comprising:

• providing a neutron accelerator for irradiating the earth formations with high energy neutrons from within the borehole;
• providing at least one radiation detector spaced from the accelerator in the lengthwise direction of the borehole for detecting radiation resulting from said neutron irradiation of the earth formations and for generating an output response to said detected radiation, the spacing being such that the radiation resulting from said neutron irradiation is influenced by the density of the formations;
• and
• recording the output of said at least one detector as a function of at least one of borehole depth and azimuthal orientation in the borehole and determining a parameter related to the density of the earth formation.