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

Abstract

Measurement-while-drilling apparatus includes a 14 MeV neutron accelerator, a near-spaced neutron detector which primarily senses source neutrons and whose output is proportional to source strength, one or more intermediately-spaced epithermal neutron detectors eccentered against the drill collar wall and primarily responsive to formation hydrogen concentration, and a third far-spaced radiation detector, either gamma ray or neutron, primarily responsive to formation density. The intermediately-spaced and far-spaced detector outputs, normalized by the near-spaced detector output, are combined to provide measurements of porosity, density and lithology and to detect gas. A thermal neutron detector and/or a gamma ray detector may also be provided at intermediate spacings to provide additional information of interest, such as standoff measurements and spectral analysis of formation composition. Tool outputs are related to the angular or azimuthal orientation of the measurement apparatus in the borehole.

We claim: 
     
       1. 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 high energy neutron accelerator in said drill collar;   a first neutron detector in said drill collar at a first spacing from the accelerator in the lengthwise direction of the drill collar, said first neutron detector having an output that is primarily proportional to the accelerator neutron flux;   a second neutron detector in said drill collar at a second, farther spacing from the accelerator in the lengthwise direction of the drill collar, 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;   a third radiation detector in said drill collar at a third, still farther spacing from the accelerator in the lengthwise direction of the drill collar, 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;   means for recording the respective outputs of said first, second and third detectors as a function of borehole depth   and means for determining a parameter related to the formation density from the respective outputs.   
     
     
       2. The apparatus of claim 1, wherein: said second neutron detector is located closely adjacent the interior wall of the drill collar; and   said second neutron detector is back-shielded against neutrons incident thereon from the borehole.   
     
     
       3. The apparatus of claim 2, further comprising means defining a neutron window in the drill collar immediately adjacent to said second neutron detector. 
     
     
       4. The apparatus of claim 3, wherein the neutron-window defining means comprises a body of relatively low-scattering cross section material in the drill collar. 
     
     
       5. The apparatus of claim 4, wherein said body of relatively low-scattering cross section material is composed of titanium. 
     
     
       6. The apparatus of claim 5, wherein said titanium body is sheathed in boron. 
     
     
       7. The apparatus of claim 4, wherein: the exterior surface of the drill collar is surrounded by a layer of neutron absorbing material in the region of the second detector; and   said layer of neutron-absorbing material has an opening formed therein at the location of said body of relatively low-scattering cross section material.   
     
     
       8. The apparatus of claim 7, wherein said neutron-window defining means comprises a plurality of spaced-apart transverse layers of neutron-absorbing material in the drill collar in the region of the second detector. 
     
     
       9. The apparatus of claim 4, wherein said neutron-window defining means further comprises a plurality of spaced-apart lengthwise-extending layers of neutron absorbing material in the drill collar in the region of the second detector. 
     
     
       10. The apparatus of claim 2, further comprising means for processing the output of said second neutron detector to derive a measurement of the epithermal neutron slowing down time of the surrounding earth formation. 
     
     
       11. The apparatus of claim 10, wherein said processing means further derives a standoff-corrected measurement of the porosity of the surrounding earth formation. 
     
     
       12. The apparatus of claim 11, wherein said processing means further derives a measurement of standoff. 
     
     
       13. The apparatus of claim 1, wherein said first neutron detector comprises an epithermal neutron detector shielded on all sides thereof except the side facing the neutron accelerator with neutron moderating-absorbing material. 
     
     
       14. The apparatus of claim 1, wherein said first neutron detector comprises an MeV range neutron detector shielded on all sides thereof except the side facing the neutron acceleration with a high-Z material. 
     
     
       15. The apparatus of claim 14, wherein said first neutron detector is a  4  He detector. 
     
     
       16. The apparatus of claim 1, wherein said third detector comprises a gamma ray detector. 
     
     
       17. The apparatus of claim 1, wherein said third detector is an MeV range neutron detector. 
     
     
       18. The apparatus of claim 17, wherein said third detector is a  4  He detector. 
     
     
       19. The apparatus of claim 16 or 17, further comprising an intervening neutron shield located between said neutron detector and said third radiation detector. 
     
     
       20. The apparatus of claim 1, further comprising a gamma ray detector located at an intermediate spacing in the lengthwise direction of the drill collar between said first and third detectors. 
     
     
       21. The apparatus of claim 20, wherein said gamma ray detector is located at substantially the same distance from the accelerator in the lengthwise direction of the drill collar as in said second detector. 
     
     
       22. The apparatus of claim 16 or 20, further comprising means for spectrally analyzing the output of said gamma ray detector to obtain information concerning the lithology of the surrounding earth formation. 
     
     
       23. The apparatus of claim 1 wherein: a drilling fluid channel is located within said drill collar to one side of the longitudinal axis thereof; and   the accelerator and the first neutron detector are eccentered to the other side of the drill collar longitudinal axis and are substantially coaxially aligned with one another.   
     
     
       24. The apparatus of claim 23, wherein: the second neutron detector is located closely adjacent the inner wall of the drill collar; and   the third radiation detector is substantially coaxially aligned with the accelerator and the first neutron detector.   
     
     
       25. The apparatus of claim 1, wherein the lengthwise spacing between the second neutron detector and the accelerator is substantially twice the low-energy epithermal neutron slowing down length (L epi ). 
     
     
       26. The apparatus of claim 1, further comprising at least one thermal neutron detector located at an intermediate spacing in the lengthwise direction of the drill collar between the first and third detectors. 
     
     
       27. The apparatus of claim 26, further comprising means for processing the output of said thermal neutron detector to derive a measurement of at least one of standoff and the formation macroscopic cross section for capture of thermal neutrons. 
     
     
       28. The apparatus of claim 1, further comprising a plurality of said second epithermal neutron detectors located at substantially the same lengthwise position in the drill collar and spaced apart circumferentially of the drill collar to provide enhanced angular or azimuthal resolution. 
     
     
       29. The apparatus of claim 1, wherein said second detector is located within a recess formed in the wall of the drill collar and is back-shielded against borehole neutrons by a neutron moderating-absorbing material. 
     
     
       30. The apparatus of claim 1, further comprising means for recording said detector outputs as a function of the angular orientation of the drill collar within the borehole. 
     
     
       31. The apparatus of claim 1, further comprising means for recording said detector outputs as a function of the azimuthal orientation of the drill collar within the borehole. 
     
     
       32. The apparatus of claim 1, wherein: said first neutron detector is shielded against formation-origin neutrons by a high-Z material; and   said second and third detectors are shielded against source neutrons transported along the drill collar by a neutron moderating-absorbing material.   
     
     
       33. The apparatus of claim 1, further comprising means for combining the outputs of said first, second and third detectors to derive an indication of at least one of the porosity, density and lithology of or the presence of gas in the surrounding earth formation. 
     
     
       34. The apparatus of claim 33, wherein: said third detector comprises a neutron detector;   said first and third detector outputs are combined to derive a measurement of at least one of the high energy neutron slowing down length (L h ) and the low-energy neutron slowing down length (L epi );   the lengthwise spacing between the second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   said first and second detector outputs are combined to derive a measurement of hydrogen index; and   said at least one L h  measurement or L epi  measurement and said hydrogen index measurement are cross plotted to obtain information of at least one of the porosity and lithology of the surrounding earth formation.   
     
     
       35. The apparatus of claim 33, wherein: said third detector comprises a neutron detector;   the lengthwise spacing between said second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   the outputs of the second and third detectors normalized by the output of the first detector; and   the normalized outputs of the second and third detectors are cross plotted by said combining means to provide information of at least one of porosity, lithology and the presence of gas in the surrounding earth formation.   
     
     
       36. The apparatus of claim 33, wherein the output combining means combines said first detector output with the outputs of said second detector and said third detector by normalizing the outputs of said second and third detectors with said first detector output. 
     
     
       37. The apparatus of claim 36, wherein the combining means combines the normalized outputs of said second and third detectors by cross plotting said normalized outputs. 
     
     
       38. The apparatus of claim 36, wherein said combining means, in accordance with a first predetermined empirical relationship, derives a value of the hydrogen index from the normalized second detector output and, in accordance with a second predetermined empirical relationship, derives a value of the neutron slowing down length from the normalized third detector output and said value of the hydrogen index. 
     
     
       39. 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.   
     
     
       40. The method of claim 39, wherein said combining step comprising combining said first detector output with the outputs of said second detector and said third detector by normalizing the outputs of said second and third detectors with said first detector output. 
     
     
       41. The method of claim 40, wherein said combining step further comprises cross plotting the normalized outputs of said second and third detectors. 
     
     
       42. The method of claim 41, wherein the normalized outputs cross plotted are inverse normalized outputs. 
     
     
       43. The method of claim 40, wherein said combining step, in accordance with a first predetermined empirical relationship, derives a value of the hydrogen index from the normalized second detector output and, in accordance with a second predetermined empirical relationship, derives a value of the neutron slowing down length from the normalized third detector output and said value of the hydrogen index. 
     
     
       44. The method of claim 43, wherein said combining step further comprises combining said hydrogen index value and said slowing down length value to obtain information of at least one of porosity, lithology, and the presence of gas in the surrounding earth formation. 
     
     
       45. The method of claim 39, wherein: said third detector comprises a neutron detector;   said first and third detector outputs are combined to derive a measurement of at least one of the high energy neutron slowing down length (L h ) and the low-energy slowing down length (L epi );   the lengthwise spacing between the second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   the first and second detector outputs are combined to derive a measurement of hydrogen index; and   said at least one L h  measurement or L epi  measurement and said hydrogen index measurement are cross plotted to obtain information of at least one of the porosity and lithology of the surrounding earth formation.   
     
     
       46. The method of claim 39, wherein: said third detector comprises a neutron detector;   the lengthwise spacing between said second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   the outputs of the second and third detectors are normalized by the output of the first detector; and   the normalized outputs of the second and third detectors are cross plotted to provide information of at least one of porosity, lithology and the presence of gas in the surrounding earth formation.   
     
     
       47. The method of claim 39, wherein the combining step comprises: combining the normalized outputs of said second and third detectors to derive values of the hydrogen index and the high-energy neutron slowing down length (L h ) or the low-energy neutron slowing down length (L epi ) for the surrounding earth formation;   combining said values of the hydrogen index and L h  or L epi , in accordance with a predetermined relationship relating changes in the measured values of L h  or L epi  to changes in bulk density for a calibration formation of known bulk density, hydrogen index and elemental composition, to obtain information of the bulk density of the surrounding earth formation.   
     
     
       48. The method of claim 39, wherein the combining step comprises: determining the hydrogen index and the neutron slowing down length of the surrounding earth formation;   determining the difference between said slowing down length and the neutron slowing down length of a calibration formation of substantially the same hydrogen index and known bulk density; and   combining said neutron slowing down time difference with the density-slowing down length sensitivity ratio for the calibration formation to obtain a measurement of the bulk density of the surrounding earth formation.   
     
     
       49. 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.   
     
     
       50. 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. .Iadd.51. 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..Iaddend..Iadd.52. The apparatus of claim 51, wherein:   said neutron detector is located closely adjacent the interior wall of said housing; and   said second neutron detector is back-shielded against neutrons incident thereon from the borehole..Iaddend..Iadd.53. The apparatus of claim 52, further comprising means defining a neutron window in said housing immediately adjacent to said second neutron detector..Iaddend..Iadd.54. The apparatus of claim 53, wherein the neutron-window defining means comprises a body of relatively low-scattering cross section material in said housing..Iaddend..Iadd.55. The apparatus of claim 54, wherein said body of relatively low-scattering cross section material is composed of titanium..Iaddend..Iadd.56. The apparatus of claim 55, wherein said titanium body is sheathed in boron..Iaddend..Iadd.57. The apparatus of claim 54, wherein:   the exterior surface of said housing is surrounded by a layer of neutron absorbing material in the region of the second detector; and   said layer of neutron-absorbing material has an opening formed therein at the location of said body of relatively low-scattering cross section material..Iaddend..Iadd.58. The apparatus of claim 57, wherein said neutron-window defining means comprises a plurality of spaced-apart transverse layers of neutron-absorbing material in said housing in the region of the second detector..Iaddend..Iadd.59. The apparatus of claim 54, wherein said neutron-window defining means further comprises a plurality of spaced-apart lengthwise-extending layers of neutron absorbing material in said housing in the region of the second detector..Iaddend..Iadd.60. The apparatus of claim 52, further comprising means for processing the output of said second neutron detector to derive a measurement of the epithermal neutron slowing down time of the surrounding earth formation..Iaddend..Iadd.61. The apparatus of claim 60, wherein said processing means further derives a standoff-corrected measurement of the porosity of the surrounding earth formation..Iaddend..Iadd.62. The apparatus of claim 61, wherein said processing means further derives a measurement of   
     
     
        standoff..Iaddend..Iadd. 3.  The apparatus of claim 51, wherein said first neutron detector comprises an epithermal neutron detector shielded on all sides thereof except the side facing the neutron accelerator with neutron moderating-absorbing material..Iaddend..Iadd.64. The apparatus of claim 51, wherein said first neutron detector comprises an MeV range neuron detector shielded on all sides thereof except the side facing the neutron acceleration with a high-Z material..Iaddend..Iadd.65. The apparatus of claim 64, wherein said first neutron detector is a  4  He detector..Iaddend..Iadd.66. The apparatus of claim 51, wherein said third detector comprises a gamma ray detector..Iaddend..Iadd.67. The apparatus of claim 51, wherein said third detector is an MeV range neutron detector..Iaddend..Iadd.68. The apparatus of claim 67, wherein said third detector is a  4  He detector..Iaddend..Iadd.69. The apparatus of claim 66 or 67, further comprising an intervening neutron shield located between said neutron detector and said third radiation detector..Iaddend..Iadd.70. The apparatus of claim 51, further comprising a gamma ray detector located at an intermediate spacing in the lengthwise direction of the housing between said first and third detectors..Iaddend..Iadd.71. The apparatus of claim 70, wherein said gamma ray detector is located at substantially the same distance from the accelerator in the lengthwise direction of the housing as is said second detector..Iaddend..Iadd.72. The apparatus of claim 66 or 67, further comprising means for spectrally analyzing the output of said gamma ray detector to obtain information concerning the lithology of the surrounding earth formation..Iaddend..Iadd.73. The apparatus of claim 51, wherein the length wise spacing between the second neutron detector and the accelerator is substantially twice the low-energy epithermal neutron slowing down length (L epi )..Iaddend..Iadd.74. The apparatus of claim 51, further comprising at least one thermal neutron detector located at an intermediate spacing in the lengthwise direction of the housing between the first and third detectors..Iaddend..Iadd.75. The apparatus of claim 74, further comprising means for processing the output of said thermal neutron detector to derive a measurement of at least one of standoff and the formation macroscopic cross section for capture of thermal neutrons..Iaddend..Iadd.76. The apparatus of claim 51, further comprising means for recording said detector outputs as a function of the angular orientation of the housing within the borehole..Iaddend..Iadd.77. The apparatus of claim 51, further comprising means for recording said detector outputs as a function of the azimuthal orientation of the housing within the borehole..Iaddend..Iadd.78. The apparatus of claim 51, wherein: said first neutron detector is shielded against formation origin neutrons by a high-Z material; and   said second and third detectors are shielded against source neutrons transported along the housing by a neutron moderating-absorbing   
     
     
        material..Iaddend..Iadd.79.  The apparatus of claim 51, further comprising means for combining the outputs of said first, second, and third detectors to derive an indication of at least one of the porosity, density and lithology of or the presence of gas in the surrounding earth formation..Iaddend..Iadd.80. The apparatus of claim 79, wherein: said third detector comprises a neutron detector;   said first and third detector outputs are combined to derive a measurement of at least one of the high energy neutron slowing down length (L h ) and the low-energy neutron slowing down length (L epi );   the lengthwise spacing between the second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   said first and second detector outputs are combined to derive a measurement of hydrogen index; and   said at least one L h  measurement or L epi  measurement and said hydrogen index measurement are cross plotted to obtain information of at least one of the porosity and lithology of the surrounding earth formation..Iaddend..Iadd.81. The apparatus of claim 79, wherein:   said third detector comprises a neutron detector;   the lengthwise spacing between said second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   the outputs of the second and third detectors normalized by the output of the first detector; and   the normalized outputs of the second and third detectors are cross plotted by said combining means to provide information of at least one of porosity, lithology and the presence of gas in the surrounding earth formation..Iaddend..Iadd.82. The apparatus of claim 79, wherein the output combining means combines said first detector output with the outputs of said second detector and said third detector by normalizing the outputs of said second and third detectors with said first detector output..Iaddend..Iadd.83. The apparatus of claim 82, wherein the combining means combines the normalized outputs of said second and third detectors by cross plotting said normalized outputs..Iaddend..Iadd.84. The apparatus of claim 83, wherein said combining means, in accordance with a first predetermined empirical relationship, derives a value of the hydrogen index form the normalized detector output and, in accordance with a second predetermined empirical relationship, derives a value of the neutron slowing down length from the normalized third detector output and said valued of the hydrogen index..Iaddend..Iadd.85. 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..Iaddend..Iadd.86. The method of claim 85, wherein said combining step comprising combining said first detector output with the outputs of said second detector and said third detector by normalizing the outputs of said second and third detectors with said first detector output..Iaddend..Iadd.87. The method of claim 86, wherein said combining step further comprises cross plotting the normalized outputs of said second and third detectors..Iaddend..Iadd.88. The method of claim 87, wherein the normalized outputs cross plotted are inverse normalized outputs..Iaddend..Iadd.89. The method of claim 40, wherein said combining step, in accordance with a first predetermined empirical relationship, derives a value of the hydrogen index from the normalized second detector output and, in accordance with a second predetermined empirical relationship, derives a value of the neutron slowing down length from the normalized third detector output and said value of the hydrogen index..Iaddend..Iadd.90. The method of claim 89, wherein said combining step further comprises combining said hydrogen index value and said slowing down length value to obtain information of at least one of porosity, lithology, and the presence of gas in the surrounding earth formation..Iaddend..Iadd.91. The method of claim 85, wherein:   said third detector comprises a neutron detector;   said first and third detector outputs are combined to derive a measurement of at least one of the high energy neutron slowing down length (L h ) and the low-energy slowing down length (L epi );   the lengthwise spacing between the second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   the first and second detector outputs are combined to derive a measurement of hydrogen index; and   said at least one L h  measurement or L epi  measurement and said hydrogen index measurement are cross plotted to obtain information of at least one of the porosity and lithology of the surrounding earth formation..Iaddend..Iadd.92. The method of claim 85, wherein:   said third detector comprises a neutron detector;   the lengthwise spacing between said second detector and the accelerator is substantially twice the low-energy neutron slowing down length (L epi );   the outputs of the second and third detectors are normalized by the output of the first detector; and   the normalized outputs of the second and third detectors are cross plotted to provide information of at least one of porosity, lithology and the presence of gas in the surrounding earth formation..Iaddend..Iadd.93. The method of claim 85, wherein the combining step comprises:   combining the normalized outputs of said second and third detectors to derive values of the hydrogen index and the high-energy neutron slowing down length (L h ) or the low-energy neutron slowing down length (L epi ) for the surrounding earth formation; and   combining said values of the hydrogen index and (L h ) or (L epi ) in accordance with a predetermined relationship relating changes in the measured valued of (L h ) or (L epi ) to changes in bulk density for a calibration formation of known bulk density, hydrogen index and elemental composition, to obtain information of the bulk density of the   
     
     
        surrounding earth formation..Iaddend..Iadd.94.  The method of claim 85, wherein the combining step comprises: determining the hydrogen index and the neutron slowing down length of the surrounding earth formation;   determining the difference between said slowing down length and the neutron slowing down length and the neutron slowing down length of a calibration formation of substantially the same hydrogen index and known bulk density; and   combining said neutron slowing down time difference with the density-slowing down length sensitivity ration for the calibration formation to obtain a measurement of the bulk density of the surrounding earth formation..Iaddend..Iadd.95. 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..Iaddend..Iadd.96. 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..Iaddend.