Method and apparatus for determining a granularity property of a subsurface formation around a borehole

An apparatus for deriving a subsurface formation parameter which characterizes a granularity property of the subsurface formation around a borehole comprising

• means for producing like pulses of ultrasonic energy from inside the borehole at the subsurface formation with frequency bandwidths selected to produce acoustic backscatter from grains in the subsurface formation, the range of acoustic frequencies in said pulses being selected such that the acoustic backscatter from inside the formation includes frequencies which attenuate in a manner which is characteristic of a granularity property of the subsurface formation and detecting the acoustic backscatter produced by said pulses;
• means responsive to signals representative of said detected acoustic backscatter for generating frequency spectra thereof with frequency component signals whose amplitudes represent said frequency dependency;
• and
• means for deriving said frequency dependency from said frequency spectra.

An acoustic investigation method for obtaining a measurement of a granularity property of a subsurface formation with an ultrasonic investigation tool located in a borehole penetrating the formation, comprising the steps of

• directing pulses of ultrasonic energy from inside the borehole at the subsurface formation with a frequency bandwidth selected to produce acoustic backscatter from grains inside the formation with wavelengths of the order of the mean diameter of grains in the subsurface formation;
• detecting said acoustic backscatter produced by said pulses of ultrasonic energy;
• forming a frequency spectrum of the detected acoustic backscatter produced by said pulses;
• deriving from said backscatter frequency spectrum an indication of said granularity property of the subsurface formation.

An acoustic investigation method for obtaining a measurement of a granularity property of a subsurface formation with an ultrasonic investigation tool located in a borehole penetrating the formation, comprising the steps of

• directing pulses of ultrasonic energy from inside the borehole at the subsurface formation with a frequency bandwidth selected to produce acoustic backscatter from grains inside the formation with wavelengths of the order of the mean diameter of grains in the subsurface formation;
• detecting said acoustic backscatter produced by said pulses of ultrasonic energy;
• forming a frequency spectrum of the detected acoustic backscatter produced by said pulses;
• modifying said frequency spectrum to remove the effect of the frequency response of the tool within the selected frequency bandwidth;
• and
• deriving an indication of said granularity property from said modified frequency spectrum.

An acoustic investigation method for obtaining a measurement of a granularity property of a subsurface formation with an ultrasonic investigation tool located in a borehole penetrating the formation comprising the steps of

• directing pulses of ultrasonic energy from a transducer inside the borehole at the subsurface earth formation with a frequency bandwidth selected to produce acoustic backscatter from grains inside the formation and detecting said acoustic backscatter produced by said pulses of ultrasonic energy with said transducer;
• forming frequency spectra of the detected acoustic backscatter produced by said pulses;
• averaging spectra attributable to backscatter from adjacent regions of the subsurface formation to form an averaged spectrum;
• modifying said frequency spectrum to remove the effect of the frequency response of the transducer within said selected frequency bandwidth;
• detecting a peak in said modified frequency spectrum;
• determining the frequency of said peak;
• and
• deriving from the determined peak frequency an indication of the grain sizes of said subsurface formation.

An apparatus for obtaining an indication of a granularity property of a subsurface formation with an ultrasonic investigation tool comprising

• means for producing pulses of ultrasonic energy from inside the borehole at the subsurface formation with a frequency bandwidth selected to produce acoustic backscatter from grains inside the formation with wavelengths of the order of the mean diameter of grains in the subsurface formation and detecting said acoustic backscatter produced by said pulses of ultrasonic energy;
• means for producing a frequency spectrum of the detected backscatter produced by said pulses of ultrasonic energy;
• and
• means responsive to said frequency spectrum for generating a signal representative of said granularity property of the subsurface formation.