Core Spectral Gamma-Ray Logger
Core Spectral Gamma-Ray Logger
A total gamma-ray well log is a recording of the total natural gamma radiation of the formation around the well bore, whereas the spectral gamma-ray well log is a recording of the relative amounts of the three main constituents that are the sources of the natural radiation (Potassium, Uranium, and Thorium).
The Model 560 Core Spectral Gamma-Ray Logger measures the energy level and quantity of the radiation emitted from the well core and calculates the quantity of each of the elements Uranium, Potassium, and Thorium. The amounts of each of these elements and the total gamma-ray count is then plotted as a function of depth in a well log fashion.
To measure a core spectral gamma-ray log, lay the core boxes out in order of depth. Start with the most shallow depth and turn the core boxes so that the depths continue from the bottom on one box to the top of the next. Start the program by entering <SPECTRAL>. Enter the well identification information, the scale (feet or meters), Spectral or Total, and the start depth. Record the background radiation for 15 minutes or more. Place the top core on the conveyor aligning the depth with a mark on the belt. Use the foot or meter marks to maintain the core depth in sync with the belt depth. When the core reaches the tunnel, press [ESC]. When the core is at the center of the tunnel, press [S] to start incrementing the depths. The results are updated on the screen each sample interval. The final results may be plotted using the plotter routine in the operation program or the data may be exported to another report writer. When the spectral log is measured, the total log is also recorded.
The core spectral gamma-ray log is plotted on the same vertical (depth) scale as the well log so that the well and the core gamma-ray log can be overlain. Two scales are provided, 200:1 called the Metric Scale, and 240:1 the English Scale.
Unlike the total gamma-ray well log, there is no API standard for the spectral gamma-ray well log. As a result, there are no API units for the spectral log. Without an API standard, the data reduction methods used by logging companies vary and the results that are reported will, as a result, vary also. The quantity of gamma radiation, and thus the relative amounts of the constituents, is proportional to that measured in the well, although not the same because of the difference in the masses of the core and that of the earth surrounding the well bore.
The Core Spectral Gamma-Ray Logger is built around a six-inch wide by 9-foot long (15 cm X 3 M) conveyor. The conveyor belt speed is a variable speed, two range control. The speed range is controlled by the computer. The slower speed range is used for the spectral measurement and the higher is used for the total. More radiation data is required to calculate the constituents from the measured energy spectrum, thus the required slower belt speed. And in general, the slower the travel, the greater is the log definition from a small diameter core. The belt is synchronized to the gamma-ray readings with a shaft encoder interfaced to the computer. The gamma-ray detector is a 3-inch x 3- inch NaI crystal. The detector is mounted under the conveyor belt so that the distance between the core and detector will not vary with the core size. Approximately 450 lb. (204 kg) of low activity lead protect the detector from ambient radiation. The lead detector shield is heated so that its temperature is held just above ambient at a constant value, thus ensuring a more stable calibration during the core logging. The detection electronics include a High Voltage Bias supply, amplifier, 1048-channel mutichannel analyzer. The shaft encoder output (belt travel) is signal conditioned and fed into a counter. The counter is also interfaced to the computer. A menu driven program records the data. The gamma-ray log data and data identifying the core are recorded on the computer hard disk.
Measured aliquots of Sodium, Uranium, and Thorium are individually used to calibrate the radiation spectrum measured by the system. When the core log is measured, these calibration spectra and the spectrum of the core sample are first smoothed using a Tichinov routine. Then, a least-square error approximation is applied to a set of simultaneous equations representing the relationship of the standard and unknown spectra.
Available options are:
1) 50 Hz-230 volt operation at no added cost, and 8-inch wide conveyor, or
2) added length at additional cost, or
3) a digital plotter at added cost.