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2GDA-1000 Density / Guard Resistivity / Caliper

The 2GDA-1000 DX series density probe section is a versatile tool that can be used in a wide variety of logging applications. The tool section measures compensated density (dual density), button guard resistivity, and caliper. Since the tool is part of the DX family of probes, it can be operated with any DX probe section. In this manner, additional measurements (such as natural gamma) can be added to the tool string.

The tool section is offered in several different versions. The standard 2GDA-1000 is a dual detector tool optimized for use with a 100 mCi 137Cs source. The 2GDA-1000 source to detector offsets are 20 and 35 cm. The 2GDA-1000-S is optimized for use with smaller sources in the range of 100 µCi 137Cs to 75 mCi 137Cs . The 2GDA-1000-S source-to-detector offsets are 4 and 11 cm. The 2GDA-1000-4 has four possible detectors, one at each source to detector offset (4, 11, 20, and 35 cm). Customers select the desired pair for their application. These standard tools all measure back-scattered radiation in the Compton energy band. Each of these is available with a photoelectric window and a Compton window so that Z/A ratios can be measured. The model number for the tools has ZA appended as a suffix (i.e. 2GDA-1000-ZA, 2GDA-1000-SZA, and 2GDA-1000-4ZA).

Theory of Operation

The compensated density measurement is accomplished using a radioactive source and two radiation detectors. The detectors are designed for use with a 137Cs source (emits 662 KeV gamma particles). They are set to respond to gamma particles having energies of 200 KeV or higher (the Compton energy band). The radiation detectors are comprised of two CsI(Th) scintillator crystals coupled to two photomultiplier tubes. The near detector scintillator is 0.5 inches long and 0.5 inches in diameter; the far scintillator is 1.5 inches long and 0.5 inches in diameter. The detectors are shielded so that they only respond to gamma radiation from the same side of the tool that source energy radiates from. Radiation emanating from the source travels into the formation (into the side of the borehole opposite the bowspring), and is backscattered by Compton scattering. The detectors sense this backscattered radiation. Compton scattering results from gamma radiation interacting with the electrons in the formation, so electron density is measured rather than bulk density.

The detector response is a function of density and the detector offset from the source. For low densities (typically less than 1 g/cc) and close offsets (typically less than 8 cm for 137Cs), the detector response increases with increasing density. The intuitive explanation of this phenomenon is that when density and spacing are small, more electrons cause more backscattering and more radiation at the detectors. As density and detector offset continue to increase, more of the radiation is absorbed and the response decreases with increasing density and spacing. The detector response at 4 cm from the source is on the positive slope of the curve below; that is, the response increases with increasing density. The response for detectors at 11cm or more from the source has a negative slope.

2GDA-1000 Specifications

Maximum pressure 3000 PSI
Operation temperature range -20 to 70 degrees C
Storage temperature -40 to 100 degrees C
Density range 1 - 4 g/cc
Density accuracy: 0.1 g/cc using a 100 mCi 137Cs source
Density resolution: 0.05 g/cc using a 100 mCi 137Cs source
Guard resistivity range 0-1000 ohm-m
Guard resistivity accuracy 1% of full scale
Guard resistivity measurement resolution 0.5 ohm-m
Guard resistivity vertical resolution <2 cm
Caliper range 1.75 - 10 inches (4.5-25 cm)
Caliper accuracy 0.1 inch (2.5 mm)
Caliper resolution 0.025 inch (0.64 mm)
2SMA-1000 Modem Section  
Length (assembled) 24.625 inches (62.55 cm)
Diameter 1.5 inches (3.81 cm)
Weight 6 lbs (2.7 kg)
2GDA-1000 Density Section  
Length (when connected to a 2SMA-1000) 126 inches (320 cm)
Diameter 1.75 inches (4.45 cm)
Weight  38 lbs (17.3 kg)