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KIRTLAND AIR FORCE BASE ALBUQUERQUE, NEW MEXICO ...

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APPENDIX B<br />

Waveforms obtained at each receiver can be recorded digitally every 4 micro-seconds for approximately<br />

1 millisecond (255 records after a predetermined delay time (e.g., 110 micro-seconds after the transmitter<br />

fires). The uphole instrumentation automatically picks the time between the first compressional P wave<br />

arrival at each receiver and an amplitude of the first arriving P wave. This delta T log is in microseconds<br />

per ft and is directly related to compressional wave velocity. The amplitude log is related to the signal<br />

strength of the first compressional wave arrival. Amplitude is relative as displayed from 0 to 255 (8-bit<br />

resolution) with 0 being the lowest amplitude and 255 the highest amplitude. Digitized waveforms from<br />

each receiver can be displayed and reprocessed for subsequent waveform analysis. Ideally, the first<br />

arriving P wave, the shear S wave, and the tube or Stoneley wave are all recorded in 1-millisecond<br />

record.<br />

Discontinuities such as weathering or fracturing affect waveforms arriving at the receiver. These effects<br />

are due to increased travel time (lower velocity), reduced amplitude, and distorted waveforms. VDLs in<br />

these zones show ‗chevron‘ type features propagating along the entire waveform. The thickness of the<br />

discontinuity can be measured by subtracting the transmitter/receiver spacing from the recorded vertical<br />

distortion. For example, if the recorded vertical distortion is 48 inches on the 36-inch<br />

transmitter/receiver VDLs, the rock discontinuity is approximately 12 inches.<br />

In its simplest form, the FWS log can be used as a cement bond log with a VDLs display. In more<br />

sophisticated applications, waveform analysis can provide compressional and potentially shear velocities,<br />

and information about the Tube or Stoneley wave amplitudes.<br />

Gamma-Gamma Density Measurement<br />

The principle behind density logging is detecting Compton-scattered gamma rays originating form a 0.1<br />

curies radioactive Cesium-137 source. The dual collimated gamma ray detectors are located at discrete<br />

distances from the source. This arrangement results in a directional-oriented tool which obtains<br />

measurements while flush against the borehole wall. Gamma ray intensity (measured at the detectors in<br />

counts per second) for a specific density range is inversely proportional to bulk density of the adjacent<br />

material.<br />

The dual detector probe is designed to quantitatively measure bulk density at two different radii which<br />

vary depending upon the density of the medium and source strength. Using observed gamma-gamma<br />

intensities and the caliper reading, a compensated density log can be calculated compensating for near<br />

borehole effects.<br />

Empirical measurements of depths of investigation and apparent densities for the dual detector probe<br />

have been made in the laboratory on various configurations of casing completions. These results are<br />

used to support interpretation of field data. The distinct density differences of grout, alluvium, air, and<br />

water allows a means of monitoring well completion evaluation when the density logs are run in cased<br />

wells.<br />

Neutron Measurement<br />

The neutron measurement is a single function radiation probe which detects thermal neutrons using a<br />

Helium-3 (He-3) detector. An Americium-241 (Beryllium-activated Americium-Beryllium-241 [AmBe-<br />

241]) neutron source emits high energy (fast) neutrons into the formation. These neutrons diffuse<br />

through the formation and collide with the atoms present. Collisions with atoms nearest the mass of<br />

neutrons, such as hydrogen, results in an exchange of energy. Thus, these neutrons are slowed down to<br />

thermal energies which can be detected by the He-3 detectors. Since slowing is primarily due to<br />

Kirtland AFB<br />

SOPs for Field Investigations B-31 April 2004

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