Gamma
The tool detects natural gamma radiation emanating from the subsurface rocks. Gamma rays are very high frequency electromagnetic radiation and are derived from the radioactive decay of various elements. In borehole applications the most common radioactive elements are radioisotopes of potassium, thorium and uranium.
Gamma Ray logs provide a clear indication of variations in lithology and also accurately define bed thickness.
The Gamma Ray log is used for identification of coal, as coal typically exhibits a low counting rate. In this application a high sensitivity Gamma tool generally provides best results.In petroleum applications the higher gamma activity of the clay size fraction is due to the potassium contained in clay minerals, fine micas and feldspars. This is not only useful information in its own right, but is a crucial parameter in the accurate interpretation of Density and Neutron logs.
Gamma Rays are an obvious choice in uranium exploration and development. The relationship between uranium content and count rate may be used so ore grades may be determined in the borehole.
Groundwater and engineering applications utilise the Gamma Ray log for lithology indicators and strata thickness' as well as clay content, to determine permeability and rock strength.
The SP and PR logs are acquired on a separate run.
Resistivity
Resistivity logs may be usefully applied in a number of areas.
In Ground-water studies, electrical resistivity logs have been regarded as vital for many years. Apart from general geological / stratigraphic information, resistivity values provide excellent information on two important hydro-geological aspects: aquifer location and water quality In sedimentary sequences (consolidated or otherwise) aquifers are usually much more resistive than aquicludes such as clay layers and shale. In igneous and carbonate sequences, secondary aquifers formed by fracturing or dissolution are often less resistive than the adjacent massive rocks.
In Coal studies, electrical resistivity logs indicate that the seam has been coked by an igneous intrusion, or has been affected by oxidation.
PR (point resistance) measurements are used to ascertain depths and thicknesses of strata. These measurements will often clearly define the strata boundaries.
The SP log measures small, naturally occurring spontaneous potential generated by electrochemical differences between differing rock types, water and drilling fluids. Occasionally there is another component called streaming potential created by fluids entering of leaving the borehole. Recognition of this is important in some engineering applications.
Specifications
Detector Sensor Nal Crystal and Electrode Range N/A for Gamma
2 to - 2mV/1 to 1000 ohmsCalibration Primary Calibration Pits or API Jig
Millivolt Values / Resistance ValuesUnits of measure CPS or API / mV & ohms Communication Duplex Digital RS232 Nominal Current 30 Power at Tool (VDC) 28 Pressure Rating (KPg/PSI) 21000/3000 Nominal Current Consumption 80 mA Temp Rating (Degrees C) 0-60 Diameter (mm) 45 Length (mm) 1600 Weight (Kg) 12.5
Auslog