Thermal decay time logging method and apparatus

In a logging system having a fast neutron generating means and detecting means for detecting indications of thermal neutrons, a method for determining thermal neutron decay characteristics of borehole materials and the surrounding earth formation comprising the steps of

• irradiating the borehole materials and earth formations with a first burst of fast neutrons,
• detecting count rates of indications of the concentration of thermal neutrons in the formation and borehole materials following said first burst and generating first signals representative thereof,
• said first burst being of a first duration which is relatively long as compared to the expected time constant of the borehole materials, .tau..sub.B, but being relatively short as compared to the expected time constant of the formation, .tau..sub.F, such that after a first delay from the end of the first burst, the total count rate due to the borehole materials and to the formation is approximately equal to the maximum counting rate of said detecting means and the thermal neutron count rate component due to borehole materials predominates in the total thermal neutron count rate,
• irradiating the borehole materials and earth formation with a second burst of fast neutrons,
• detecting count rates of indications of the concentration of thermal neutrons in the formation and borehole materials following said second burst and generating second signals representative thereof,
• said second burst being of a second duration relatively long as compared to the expected time constant of the formation, .tau..sub.F, such that after a second delay from the end of the second burst, the total count rate due to the borehole materials and to the formation is approximately equal to the maximum counting rate of said detecting means and the thermal neutron count rate component due to the formation predominates in the total thermal neutron count rate, and
• determining from said first and second signals the thermal decay constant of the formation .tau..sub.F.

A borehole logging method for determining thermal neutron decay characteristics of the formation adjacent the borehole, comprising the steps of,

• providing a first relatively short time burst of fast neutrons at a location within the borehole thereby irradiating borehole materials and the earth formation adjacent said borehole location,
• detecting indications of the concentration of thermal neutrons in the formation and in said borehole materials following said short neutron burst and generating first signals in response thereto,
• providing a relatively long time burst of fast neutrons at substantially the same said location within the borehole thereby again irradiating said borehole materials and the earth formation adjacent said borehole location,
• detecting indications of the concentration of thermal neutrons in the formation and in said borehole materials following said long neutron burst and generating second signals in response thereto,
• determining from said first signals and from said second signals the rate of decay of thermal neutrons of said earth formation, .tau..sub.f.

Apparatus for determining thermal neutron decay characteristics of the formation, comprising,

• means for providing a first relatively short time burst of fast neutrons at a location within the borehole thereby irradiating borehole materials and the earth formation adjacent said borehole location,
• means for detecting indications of the concentration of thermal neutrons in the formation and in said borehole materials following said short neutron burst and generating first signals in response thereto,
• means for providing a relatively long time burst of fast neutrons at substantially the same said location within the borehole thereby again irradiating said borehole materials and the earth formation adjacent said borehole location,
• means for detecting indications of the concentration of thermal neutrons in the formation and in said borehole materials following said long neutron burst and generating second signals in response thereto, and
• means for determining from said first signals and from said second signals the rate of decay of thermal neutrons of said earth formation, .tau..sub.f.

In a logging system having neutron generating means and detecting means for detecting indications of thermal neutrons, a method for determining thermal neutron decay characteristics of borehole materials and the surrounding earth formation comprising the steps of:

• irradiating the borehole materials and earth formations with a first burst of neutrons,
• detecting in one or more time intervals following said first burst count rates of indications of the concentration of thermal neutrons in the formation and borehole materials and generating first signals representative thereof,
• said first burst being of a first duration, such that after a first delay from the end of the first burst, the thermal neutron count rate component due to borehole materials predominates in the total thermal neutron count rate detected in said one or more time intervals associated with said first burst,
• irradiating the borehole materials and earth formation with a second burst of neutrons,
• detecting in one or more time intervals following said second burst count rates of indications of the concentration of thermal neutrons in the formation and borehole materials and generating second signals representative thereof,
• said second burst being of a second duration such that after a second delay from the end of the second burst, the thermal neutron count rate component due to the formation predominates in the total thermal neutron count rate detected in said one or more time intervals associated with said second burst, and
• determining from said first and second signals a characteristic of thermal neutron decay of the formation and the borehole.

In a logging system having a neutron generating means and detecting means for detecting indications of thermal neutrons, a method for determining thermal neutron decay characteristics of borehole materials comprising the steps of:

• irradiating the borehole materials and earth formations with a first burst of neutrons,
• detecting in one or more time intervals following said first burst count rates of indications of the concentration of thermal neutrons in the formation and borehole materials and generating first signals representative thereof,
• said first burst being of a duration chosen such that after a first delay from the end of the first burst, the total count rate due to the borehole materials and to the formation does not exceed the maximum counting rate of said detecting means and the thermal neutron count rate component due to borehole materials predominates in the total thermal neutron count rate detected in one or more of said intervals associated with said first burst, and
• determining from said first signals a characteristic of the thermal decay of the borehole.

In a borehole logging system having a neutron generating means and detecting means for detecting indications of neutrons, a method for determining neutron decay characteristics, where the neutron decay is characterized by the sum of components which decay at different rates, comprising the steps of:

• irradiating the borehole materials and earth formations with a first burst of neutrons,
• detecting in one or more time intervals following said first burst by a short delay, count rates of indications of the concentration of neutrons in the formation and borehole materials and generating first signals representative thereof,
• said first burst being of a duration which is relatively short such that the total count rate due to the borehole materials and to the formation does not exceed the maximum counting rate of said detecting means and the neutron count rate component due to a selected decay component decaying at a rapid rate dominates in the total neutron count rate detected in said one or more time intervals, and
• determining from said first signals a characteristic related to said selected decay component.

A logging system for determining thermal neutron decay characteristics of borehole materials and the surrounding earth formation comprising:

• neutron generating means,
• means for detecting indications of thermal neutrons,
• means for irradiating the borehole materials and earth formations with a first burst of neutrons,
• means for detecting in one or more time intervals following said first burst count rates of indications of the concentration of thermal neutrons in the formation and borehole materials and generating first signals representative thereof,
• said first burst being of a first duration, such that after a first delay from the end of the first burst, the thermal neutron count rate component due to borehole materials predominates in the total thermal neutron count rate detected in said one or more time intervals associated with said first burst,
• means for irradiating the borehole materials and earth formation with a second burst of neutrons,
• means for detecting in one or more time intervals following said second burst count rates of indications of the concentration of thermal neutrons in the formation and borehole materials and generating second signals representative thereof,
• said second burst being of a second duration such that after a second delay from the end of the second burst, the thermal neutron count rate component due to the formation predominates in the total thermal neutron count rate detected in said one or more time intervals associated with said second burst, and
• means for determining from said first and second signals a characteristic of thermal decay of the formation and the borehole.

A logging system for determining thermal neutron decay characteristics of borehole materials comprising:

• neutron generating means,
• means for detecting indications of thermal neutrons,
• means for irradiating the borehole materials and earth formations with a first burst of neutrons,
• means for detecting in one or more time intervals following said first burst count rates of indications of the concentration of thermal neutrons in the formation and borehole materials and generating first signals representative thereof,
• said first burst being of a duration chosen such that after a first delay from the end of the first burst, the total count rate due to the borehole materials and to the formation does not exceed the maximum counting rate of said detecting means and the thermal neutron count rate component due to borehole materials predominates in the total thermal neutron count rate detected in one or more of said intervals associated with said first burst, and
• means for determining from said first signals a characteristic of the thermal decay of the borehole.

A borehole logging system for determining thermal neutron decay characteristics, where the neutron decay is characterized by the sum of components which decay at different rates, comprising:

• neutron generating means,
• means for detecting indications of neutrons,
• means for irradiating the borehole materials and earth formations with a first burst of neutrons,
• means for detecting in one or more time intervals following said first burst by a short delay, count rates of indications of the concentration of neutrons in the formation and borehole materials and generating first signals representative thereof,
• said first burst being of a duration which is relatively short such that the total count rate due to the borehole materials and to the formation does not exceed the maximum counting rate of said detecting means and the neutron count rate component decaying at a rapid rate due to a selected decay component dominates in the total neutron count rate detected in said one or more time intervals, and
• determining from said first signals a characteristic related to said selected decay component.