Method and apparatus for measuring the thermal neutron absorption rate in earth formations

• US 4,046,764 A
• Filed: 04/02/1975
• Issued: 09/06/1977
• Est. Priority Date: 04/02/1975
• Status: Expired due to Term

• Current Assignee: Western Atlas International Incorporated (Northrop Grumman Corporation)
• Original Assignee: Dresser Industries Incorporated (Halliburton Company)
• Inventors: Marquis, Gerald L.
• Primary Examiner(s): Dixon, Harold A.
• Application Number:
  US05/564,233
• Time in Patent Office: 888 Days
• Field of Search: 250/262, 250/270
• US Class Current: 250/262
• CPC Class Codes: G01V 5/102 the neutron source being of the pulsed type

Abstract

Circuitry is provided for establishing the points in time at which radiation is detected following each neutron burst in pulsed neutron logging. Means are provided to generate a signal indicative of the arithmetic mean to then compute the macroscopic thermal neutron capture cross section of the formation of interest surrounding the borehole. In one embodiment, an indeterminate number of gates provides a means of establishing the point in time of each pulse indicative of detected radiation. In another embodiment, only a predetermined number of such gates are used. In yet another embodiment, a weighting factor is applied to pulses occurring either before or after the arithmetic mean such that the number of pulses before the arithmetic mean are a multiple of the number occurring after such time.

Claims

1. Apparatus for investigating the character of earth substances and the like, comprising:

means for irradiating said substances with a burst of neutrons;

means for detecting radiation occasioned in said irradiated substances by said burst of neutrons during a preselected time interval following said burst of neutrons and for generating electrical pulses indicative of said detected radiation;

circuit means for determining the time relationship of each of said electrical pulses with respect to said time interval;

circuit means for determining the arithmetic mean of said electrical pulses as determined with respect to said time interval; and

circuit means for computing the macroscopic thermal neutron capture cross section from the said arithmetic mean based upon the following relationship;

##EQU14## where Σ

= macroscopic thermal neutron capture cross section

K₀ = a first constant

K₁ = a second constant

V = speed of thermal neutrons

t = arithmetic mean of said electrical pulses.

2. A method of well logging, comprising:

irradiating the formations surrounding a well with a burst of high energy neutrons;

detecting radiation occurring in said well resulting from said irradiation;

generating time-separated electrical pulses indicative of said detected radiation; and

generating an electrical signal representative of the arithmetic mean time of the total of said electrical pulses.

3. The method according to claim 2, including in addition thereto, the step of recording said electrical signal in correlation with an indication of depth in the well.

4. A method of well logging comprising:

pulsedly irradiating the formations surrounding a well with repetitive discrete bursts of high energy neutrons;

detecting radiation occurring in said well resulting from each of said bursts of neutrons;

generating a group of time-separated electrical pulses indicative of said detected radiation following each of said bursts; and

generating a plurality of electrical signals, each of said signals being representative, respectively, of the arithmetic mean time of the total of said electrical pulses within one of said groups of said electrical pulses.

5. The method according to claim 4, including in addition thereto, the step of recording said electrical signals in correlation with indications of depth in the well.