Gamma-Gamma Density Logging

Basic Concept

Gamma-gamma density (GGD) borehole logging is an active-nuclear method that is used to determine the bulk-formation densities of borehole-intersected formations. The GGD method measures the response of subsurface materials to the gamma radiation emitted from a controlled source that is housed within the tool. The GGD method is, thereby, considered an active-source method as opposed to a passive method, some of which can measure naturally occurring radioactivity (e.g., natural- and spectral-gamma).

Gamma-gamma density sondes (i.e., tools) respond to variations in electron density, which is directly proportional to the bulk-formation density in most subsurface environments. Along with grain- and fluid density information, bulk-density data permits formation porosity calculations. Thus, GGD logs can be used to qualitatively or quantitatively estimate porosity variations with depth as well as assist interpretations of lithology and fluid content (Alger and Raymer, 1963).

Theory

Gamma radiation is one of the three types of radiation emitted by the unstable nuclei of radioactive isotopes during radioactive decay. Essentially, gamma radiation is a stream of gamma photons, which are elementary particles that exhibit characteristics of particles and high-frequency waves. Gamma-gamma density tools contain a radioactive source (e.g., Cobalt-60, Cesium-137) that emits a focused stream of high-energy gamma photons (or “rays”) that readily penetrate into borehole-intersected materials.

Gamma rays undergo various reactions (e.g., pair production, photoelectric effect, and Compton scattering) as they propagate through the subsurface. As gamma rays bombard subsurface materials, collisions with formation electrons scatter and slow the photons. This Compton scattering effect obstructs the gamma-ray path such that photons are “back scattered” and redirected toward the tool. Gamma-gamma density tools measure the diffusion of gamma photons via scattering that occurs within the formation between the tool source and -detector.

Gamma-gamma density tools detect gamma radiation using a scintillation detector, which is a crystal composed of sodium- or cesium iodide. The scintillation crystal is shielded such that it only measures gamma photons that have undergone Compton scattering and disregards those directly from the source. As the crystal is struck by photons, it becomes ionized and releases electronic light pulses, the rate per time of which is recorded by the tool.

Through space, gamma photons would travel directly from the source to the detector, and the maximum count rate would be recorded. As the electron concentration increases, so does the number of scattered photons, and, therefore, electron density is inversely proportional to gamma counts. Additionally, electron density is proportional to the bulk-formation density of common rock-forming materials. Thus, gamma counts can be calibrated directly to bulk density (ρ_b), which is typically how data are output (Collier, 1993).

The bulk density of porous rocks can be defined in the following relationship ρ_b = φρ_f + ( 1 - φ) ρ_ma , where, ρ_b is the bulk-formation density, φ is the porosity, ρ_f is the fluid density,and ρ_ma ;is the matrix density.

Thus, when certain material properties are known, values of porosity can be calculated. In near-surface investigations, fluid density is often assumed to be 1 gram per cubic centimeter (i.e., ρ_freshwater = 1.0 g/cm³). If unknown, matrix density can be resolved using the gamma-gamma density method in conjunction with neutron or sonic logs. Though, it is more common to apply the standard, lab-tested matrix-density values for sandstone, limestone, and/or dolomite.

Applications

Quantitative interpretation of gamma-gamma density data, which includes porosity estimates, requires accurate tool calibration, and an uncalibrated GGD tool is limited to providing relative porosity estimates. Accurate calibration of GGD data to bulk-density values occurs in a thermally stable and controlled environment where measurements of materials with known bulk densities are collected. GGD tool calibration is typically conducted by the manufacturer in a test well or man-made test pit composed of common earth materials.

There exist a few slimhole gamma-gamma density sondes designed for environmental applications. Typically, the source is situated at the bottom of the tool at a known distance away from one or two scintillation detectors. Compensated (i.e., dual detector) sondes determine density by comparing gamma counts measured by both detectors. Omnidirectional tools have spherical volumes of investigation and are typically run with a centralizer. Sidewall tools collect directional measurements and require contact with the borehole sidewall.

Because gamma photons are highly penetrative, gamma-gamma density data can be collected in air-, water-, or mud-filled boreholes that are open or cased with PVC or steel. However, material properties affect photon penetration and attenuation, and resolving inaccuracies allows for accurate and qualitative data analysis. Factors that can affect GGD data include borehole diameter, mud thickness, borehole-fluid density, contact between tool and sidewall, decay of radioactive-source strength, and natural-gamma emissions (U.S. Bureau of Reclamation, 2001).

Typically, the depth of investigation (DOI) of a gamma-gamma density tool is approximately ½ the source-to-detector spacing (i.e., ≈10-15 cm) and decreases as bulk density increases. If a relatively significant amount of low-density drilling mud separates the tool from the sidewall or has invaded the formation, gamma counts are often insufficiently counted. The best quality data are collected within uncased boreholes where sidewall contact is well maintained by a decentralizing arm, which may also collect supplementary caliper data.

The vertical resolution of a gamma-gamma density tool is a function of logging speed, sampling rate, and source-to-detector spacing. Though GGD sondes are able to log at or around standard logging speeds, data quality increases as speed decreases. Additionally, gamma counts are subject to statistical variations such that, though the average rate over a long period is constant, observed short-term rates vary. Thus, logging at slower speeds increases the resolution and improves the statistical accuracy of data.

The major disadvantage of the gamma-gamma density method is that the tool contains a radioactive source material. Special permits and licenses may be required for use, and some field sites may prohibit the tool altogether in attempts to avoid any possible groundwater contamination. However, if available, the gamma-gamma density method has proven to be a valuable addition to numerous geophysical studies, some of which include the following:

Calculation of porosity values as a function of depth
Differentiation/correlation of lithology/stratigraphy
Determination of formation mechanical properties
Evaluation of gravel pack and grout distribution
Interpretation of surface gravity and seismic data
Evaluation of matrix conditions

Examples/Case studies

Alger, R.P and Raymer Jr., L.L., 1963, Formation Density Log Applications in Liquid-Filled Holes: Journal of Petroleum Technology, v. 15, no. 3, p. 321-332, doi:10.2118/435-PA.

Abstract: The formation density logging tool provides a radioactivity measurement that yields formation densities in situ. The relationship between bulk density and porosity is well understood. With knowledge of grain and fluid densities, porosity may be computed from the indicated formation bulk densities in shale-free formations. The porosities thus determined may be used with a resistivity log for water-saturation determinations. One technique utilizes a density vs resistivity plot. A plot of density vs Sonic-log transit time is used for porosity determinations in shaly sands. This method is particularly suited to wells drilled with oil-base or salt muds. In shale-free formations, comparisons of density values and neutron-log readings are used to identify lithology and, thus, to select appropriate values of grain density for porosity computations. Through the use of Formation Density, Sonic and neutron logs, the interpretation problems caused by complex matrix lithologies are simplified. Other applications of the Formation Density log are found in the identification of minerals in evaporate deposits and in yield determinations of "oil shales".

Borsaru, M., Charbucinski, J., Eisler, P.L., and Youl, S.F., 1985, Determination of ash content in coal by borehole logging in dry boreholes using gamma-gamma methods: Geoexploration, v. 23, no. 4, p. 503-518, doi:10.1016/0016-7142(85)90077-8.

Abstract: Investigations carried out in the laboratory and in a field trial demonstrated that spectrometric gamma-gamma methods are suitable for borehole logging measurements of coal ash content carried out in rough dry boreholes. The laboratory investigations established that an appropriate probe configuration included a probe separation of 29 cm, an angle of collimation at the source of 90 to the probe axis, and a 137Cs source of about 1 mCi strength. 133Ba was a suitable alternative primary source to 137Cs, but 75Se was found to be unsuitable. Alternative spectral parameters to the Pz factor were found with less sensitivity to variations of chemical composition in coal ash. Accuracies achieved for ash content measurements in the laboratory were about 1.5% ash for samples of uniform ash composition, and 2.4% ash for samples of variable ash composition. In the field trial, the accuracy achieved was 2.2% ash for ash contents between 7 and 28% ash.

Chatfield, M., Trofimczyk, K., Harney, D., and Kachigunda, T., 2009, Downhole Wireline Density Versus Drill Core Density Measurements in Porous and Vuggy Rocks, in Proceedings, 11th SAGA Biennial Technical Meeting and Exhibition: Swaziland, South Africa, European Association of Geoscientists and Engineers, doi:10.3997/2214-4609-pdb.241.chatfield_paper1.

Abstract: The determination of in situ rock density is an important process in most opencast mining operations where an accurate estimate of total resource tonnage allows forecasting of metal production and life-of-mine. At the Skorpion zinc mine in Namibia, various techniques have been tried to improve density measurements, which are used to estimate mined tonnage and predict ore recovery based on laboratory measurements of ore yield per rock mass. Hitherto, the Archimedean submersion technique has been the basis of measurement. Downhole wireline density logging was put forward as a technology that could assist in getting accurate density determinations, particularly in the porous and vuggy arkose host rock. A borehole logging trial was conducted where a dual spaced gamma-gamma density sonde and other complementary downhole measurements, such as the photo electric density and optical televiewer, were surveyed for comparison with core density measurements. The wireline logs were run successfully in both wet and dry borehole conditions and a high degree of precision was achieved. The accuracy of the logs was based on industry standard calibration and borehole compensation with some quality assurance using core data in non-porous sections of the borehole sample. Results showed good agreement between core and wireline density in the non-porous rocks, but the wireline logs measured somewhat higher density in porous and vuggy zones. Analysis of the Archimedean technique used at Skorpion showed that the precision achieved was good, but accuracy was compromised by unsaturated and unmeasurable drill core porosity. The extreme geological environment and predominantly dry boreholes highlights the limitations of the gamma-gamma density measurement and demonstrates the critical requirement for sound data quality control and adherence to rigorous calibration standards. The availability of complementary measurements is found to be important to developing the right understanding of the wireline density response in an unfamiliar geological environment. Skorpion mine has recognised the differences in density measured by the Archimedean and the wireline logging techniques, particularly in porous and vuggy formations and supports further test work for quantitative calibration of the response. Following the successful outcome of that work, the wireline method could be deployed on an ongoing basis to supplement and enhance the accuracy of the core-based measurements.

Dworak, D., Woźnicka, U., Zorski, T., and Wiącek, U., 2011, Numerical modeling of the gamma–gamma density tool responses in horizontal wells with an axial asymmetry: Applied Radiation and Isotopes, v. 69, no. 1, p. 268-274, doi:10.1016/j.apradiso.2010.08.015.

Abstract: A signal of a spectrometric gamma–gamma density tool in specific borehole conditions has been numerically calculated. Transport of gamma rays, from a point 137Cs gamma source situated in a borehole tool, through rock media to detectors, has been simulated using a Monte Carlo code. The influence of heterogeneity of the rock medium surrounding the borehole on the signal of the detectors has been examined. This heterogeneity results from the presence of an interface between two different geological layers, parallel to the borehole wall. The above conditions may occur in horizontal logging, when the borehole is drilled along the boundary of geological layers. It is possible to assess the distance from the boundary on the basis of the responses of the gamma–gamma density tool, using the classic interpretation “spine & ribs” procedure. The effect of different densities of the bordered layers on the tool response has been analyzed. The presented calculations show the wide possibilities of numerical modeling of the complex borehole geometry and solving difficult interpretation problems in nuclear well logging.

Minette, D.C., Hubner, B.G., Koudelka, J.C., and Schmidt, M., 1986, The Application Of Full Spectrum Gamma-Gamma Techniques To Density/Photoelectric Cross Section Logging, in Proceedings, SPWLA 27th Annual Logging Symposium: Houston, Texas, Society of Petrophysicists and Well-Log Analysts, 12 p.

Abstract: The Compensated Z-Densilog instrument is a new full spectrum gamma-gamma logging instrument, measuring the density and photoelectric cross section (Pe) of the formation. This instrument represents an advancement over present instruments in that the observed gamma spectrum is sent to the surface as a full 256-channel spectrum instead of count rates in a few energy gates. This advancement is made possible by the use of high speed digital electronics which are protected from the high temperatures observed downhole by custom-built flasks. In this paper, the new advanced electronics will be considered in some detail. This consideration will include a comparison of full spectrum data acquisition with the multiple discriminator window technique, which is the foundation of the "few gate" method. Both of these methods will be critiqued based upon electronic stability and the information obtained. The full spectrum provided by high speed electronics facilitates a much more sophisticated data analysis. This data analysis, based on a phenomenological model of the instrument (Minette, 1984), includes real time compensation for changes in detector gain and crystal resolution. It also includes a "four-dimensional" rib-spine plot, which separates the compensation for mudcake density from the compensation for the Pe of the formation and the mudcake. Also included in the software are real time error minimization and real time decision-making techniques. These techniques maximize the sensitivity of the instrument to the density and Pe of the formation and minimize error due to statistical fluctuations. Gain and resolution compensation completes the data analysis. This compensation virtually eliminates density and Pe errors caused by changing gain and resolution. The application of the full spectrum technique will be illustrated in several test well examples. These examples include a sample of the Compensated Z-Densilog presentation (including the new data confidence curve, STAB), the repeatability of the Compensated Z-Densilog instrument, and comparison of the density obtained with the Compensated Z-Densilog instrument with density measurements made by a borehole gravitometer (BHG) and a Densilog instrument. In this manner, the advantages and accuracy of full spectrum density logging will be demonstrated.

Sundberg, J., Back, P.-E., Ericsson, L.O., and Wrafter, J., 2009, Estimation of thermal conductivity and its spatial variability in igneous rocks from in situ density logging: International Journal of Rock Mechanics and Mining Sciences, v. 46, no. 6, p. 1023-1028, doi:10.1016/j.ijrmms.2009.01.010.

Abstract: Characterisation of thermal conductivity of rock and its spatial variability by laboratory measurements is costly and time-consuming. There is an incentive to find more cost-effective and rapid methods. A new empirical relationship between density and thermal conductivity for igneous rocks has been found. This paper explains that the relationship is based on the rock forming processes which in turn generate typical mineral compositions. Based on this relationship, thermal conductivity can be estimated from geophysical density loggings. Experience has shown that this indirect method of determining the thermal conductivity can be used to characterise the spatial variability of thermal conductivity for certain rock types. The spatial data can be used in the modelling of thermal rock domains. Applications of the statistical relationship are presented and illustrative examples are given of how spatial variability of thermal conductivity can be estimated, based on the site investigations in a crystalline bedrock environment in Sweden. Possible deficiencies of the method are identified, such as uncertainties associated with different rock forming minerals. Methods of improvement are also discussed.

Vonder Mühll, D.S. and Holub, P., 1992, Borehole logging in alpine permafrost, upper Engadin, Swiss Alps: Permafrost and Periglacial Processes, v. 3, no. 2, p. 125-132, doi:10.1002/ppp.3430030209.

Abstract: In two boreholes through Alpine permafrost, borehole geophysical measurements were performed. At the borehole through rock glacier Murtèl‐Corvatsch, where cores were taken, the main difficulty was to fill up the hole with water. Consequently, only some of the logging methods could be applied. Nevertheless, gamma‐gamma log (and qualitatively also gamma‐ray) made it possible to obtain density and to calculate ice content. At the Pontresina‐Schafberg drill site, a complete set of logs was run in a water‐filled hole. The bedrock was reached at a depth of approximately 16 m. No cores were obtained by the air‐lift percussion technique. Logging measurements were done to determine thickness of the perennially frozen debris layer above the bedrock and to investigate P‐wave velocity, resistivity and ice content of the whole permafrost body. The boreholes at both drill sites show a high ice content layer (80–95 % by volume), although temperature characteristics are quite different.

References

Alger, R.P and Raymer Jr., L.L., 1963, Formation Density Log Applications in Liquid-Filled Holes: Journal of Petroleum Technology, v. 15, no. 3, p. 321-332, doi:10.2118/435-PA.

Collier, H.A., 1993, Porosity Tools, in Borehole Geophysical Techniques for Determining Water Quality and Reservoir Parameters of Fresh and Saline Water Aquifers in Texas: Austin, Texas, Texas Water Development Board, v. 1, p. 289-343.

Dworak, D., Woźnicka, U., Zorski, T., and Wiącek, U., 2011, Numerical modeling of the gamma gamma density tool responses in horizontal wells with an axial asymmetry: Applied Radiation and Isotopes, v. 69, no. 1, p. 268-274, doi:10.1016/j.apradiso.2010.08.015.

Mussett, A.E. and Khan, M.A., 2000, Well Logging and Other Subsurface Geophysics, in Looking into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, p 285-305.

U.S. Bureau of Reclamation, 2001, Borehole Geophysical and Wireline Surveys, in Engineering Geology Field Manual - Second Edition: Washington D.C, U.S. Department of the Interior, Bureau of Reclamation, v. 2, p. 37-81.