The equivalent U3O8 content of a zone may be calculated manually from the gamma-
log, or by computer methods which record the counts per second at one-half foot intervals and then calculate the equivalent U3O8 over mineralized intervals. The manual
method most commonly used is described in detail by Dodd and Eschliman (1972), and Bailey and Childers (1977e). A typical calculation form based on this method is shown by Fig. 2.4.10. The method is based on a relationship GT = KA where:
Figure 2.4.10. G = grade in percent eU3O8
T = ore thickness in feet
K = instrument constant determined in test holes
A = area of curve based on counts per second by 0.15 meter (0.5 ft) increments.
First, an upper and a lower bed boundary are determined to the nearest 0.3 m (1 ft) at a point about halfway between background and the peak of the gamma-ray curve Fig. 2.9. A reading of the counts per second (CPS) is made at each of these boundaries and termed E1 and E2. These are added and the total multiplied by 1.38 to provide for tail
effect (shown by Fig. 2.4.10). Consecutive readings in CPS are then recorded for each
0.15 (0.5 ft) interval from the upper to the lower boundary. Each of these readings as well as the one for E1 + E2 × 1.38 is corrected for instrument dead time by the formula
N = corrected counts per second n = observed counts
t = unit dead time in seconds.
Most logging units now being used make this correction electronically and effectively have a zero dead time. The counts per second are totaled and multiplied by a water factor, if any. The result is termed the corrected area [which is CPS × 0.15 m (0.5 ft) increments]. The corrected area is then multiplied by the instrument K-factor. This result is divided by thickness to determine percent eU3O8. A printout of the CPS values
by 0.15 m (0.5 ft) intervals is also provided by most logging units (Fig. 2.4.11). In addition, the digital data may be obtained on computer tape and the tape processed by the logging company or a company-owned computer to provide thickness and grade of ore intercepts at various cutoff grades.
Figure 2.4.11.
It is important that the probe be calibrated with a known source before logging each hole. This is usually printed at the bottom of the geophysical log (Fig. 2.4.9). When ore zones are being encountered, it is also important that the logging unit periodically probe a standard test hole with known thickness and grade in percent U3O8. In the western
purposes and to determine the instrument K-factor. It is good practice to request copies of test logs on a weekly or biweekly basis and file them within the sequence of ore holes drilled.
DISEQUILIBRIUM
Disequilibrium is a term for the disparity between uranium and its naturally occurring radioactive daughter products which are measured by the gamma log (Nininger, 1954a). Generally, checks are made for disequilibrium when drill-indicated reserves reach approximately 45 360 to 226 800 kg (100,000 to 500,000 lb) of contained U3O8. In new
areas, disequilibrium is checked after the first few ore holes. The procedure consists of coring a mineralized intercept and also probing the hole upon completion. The core is split along intervals bounded by changes in mineralization, based on scanning the core with a geiger counter. As a practice, sample intervals are held to less than 1.5 m (5 ft). Contiguous mineralized samples may later be calculated to a composite by a weighted averaging method. Split core samples are then bagged, labeled, and forwarded to an assay laboratory. Each sample is crushed and a portion split for pulverizing to prepare a pulp which is then assayed. Two separate assays are requested: a scaler-radiometric or closed-can radiometric assay, and a wet chemical assay (Grimald, 1956). The two assay methods are performed on exactly the same pulp. The ratio between the two assays is the disequilibrium factor, which is generally reported in the form chemical:radiometric, and should be so stated when discussing disequilibrium. This factor, which is of obvious economic importance, may be in favor of either the chemical or the radiometric assay. The chemical assays are then compared to the gamma log results for another check. However, in no case should this comparison be used for determining disequilibrium. The gamma-probe detects radioactivity through a 0.76 m (2.5 ft) diameter cylindrical area centered about the drill hole (Dodd and Eschliman, 1972). The core samples a cylindrical area of 4.45 cm (1.75 in.) diameter.
Choice of an assay laboratory is very important because there is a wide variation in quality. It is a good practice to seek recommendations from several of the larger and well-known consulting engineering and geology firms with a highly regarded reputation for experience in evaluating uranium deposits.
After an ore-bearing drill hole is logged, it is a good practice to drift survey the hole to provide an accurate location of ore zone intercepts (Lipton, 1963). Although it is not common, a drill hole 122 m (400 ft) deep may deviate from collar to bottom as much as 9 or 15 m (30 or 50 ft). Hole deviation is principally caused by variation in rock type and driller ability.
The gamma-ray resistivity and SP probe is removed from the cable and a drift tool attached. The drift tool takes consecutive readings of azimuth and inclination at intervals of 15 or 30 m (50 or 100 ft) The results are tabulated in X, Y, Z coordinates at recorded depths and most logging companies provide a computer-generated map which plots the drift (Fig. 2.4.12).
Geophysical logging units with operator may be contracted from a number of available logging companies. The units are commonly truck-mounted, but portable units are available for helicopter or backpack transport to remote areas. Typical costs for a logging truck on a monthly basis are in the range of $6,000 per month plus expenses and $.10 per ft (0.3 m) for a gamma-ray, resistivity, and SP log. Neutron density, caliper, dry-wall resistivity, and temperature logs typically cost an additional $.15 to $.20 per ft (0.3 m) each. In areas near their district offices, logging trucks may be obtained on a call-out basis. Costs are charged for mileage, setup, and the footage logged.