Background correction method and apparatus for pulsed neutron logging of earth boreholes

A method for well logging, comprising the steps of:

• irradiating the formations surrounding a well with bursts of neutrons;
• detecting gamma radiations emanating from said irradiated formations and generating electrical pulses indicative of said detected radiations;
• generating a first measurement functionally related to a time distribution of said detected radiations, said time distribution measurement including an exponential characteristic and a background characteristic;
• generating a second measurement functionally related to said background characteristic;
• and
• correcting said time distribution measurement with said background characteristic measurement yielding a corrected time distribution measurement for said exponential characteristic.

A method for determining characteristics of earth formations traversed by a borehole, comprising the steps of:

• pulsatingly irradiating said earth formations with repeated bursts of high energy neutrons;
• detecting secondary radiations emanating from said irradiated earth formations and generating time-sequenced electrical pulses indicative of said detected radiations;
• generating a first measurement functionally related to a time distribution of said electrical pulses within a preselected time interval;
• generating a second measurement functionally related to a time distribution of the relative background radiation within said preselected time interval;
• correcting said time distribution measurement of said electrical pulses with said time distribution of said relative background radiation;
• and
• computing the macroscopic neutron absorption capture cross section for said irradiated formations from said corrected time distribution measurement.

Apparatus for determining characteristics of earth formations surrounding a borehole, comprising:

• means for irradiating said formations with bursts of high energy neutrons;
• means for detecting radiations occasioned in said irradiated formations by said burst of neutrons and for generating time-separated electrical pulses indicative of said detected radiations;
• circuit means for determining a time relationship of said electrical pulses within a preselected time interval;
• and
• means for correcting said time relationship to compensate for background representative information.