AERO RADIOACTIVITY CHAPTER25 INTRODUCTION AVAILABLE DATA. Part IV. Regional Geophysics

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1 Part IV. Regional Geophysics CHAPTER25 AERO RAIOACTIVITY ELIZABETH R. KING U.S. Geological Survey National Center, MS 927 Reston, VA JOSEPHS. UVAL U.S. Geological Survey National Center, MS 927 Reston, VA INTROUCTION Aeroradioactivity surveying began after World War II when sensitive scintillometers that could be mounted in aircraft were developed to explore for uranium. Scintillometers measure gamma radiation produced by certain radioactive elements in the ground, primarily potassium and members of the uranium and thorium decay series. The early equipment measured only the combined signal (total counts per second) produced by all gamma-emitting elements, but the more recently developed spectral radiometry measures the signals of the different elements separately (Bristow, 1979; Grasty, 1979; Killeen, 1979). Although radiometric surveys are often made concurrently with magnetic surveys, the radiometric survey parameters are much more restrictive (Pitkin and uval, 1980). Gamma radiation is rapidly attenuated by the atmosphere, so that the maximum useful flight elevation is about 600 feet above the ground, and most surveys are flown with a nominal ground clearance of 400 feet. At 400 feet, the surface area contributing to the measurement is limited to a width of about 0.25 mile. Because spacings greater than 0.25 mile between the flight lines are often used, surveys miss anomalous sources of radioactivity between the lines. Most of the gamma radiation comes from the top 18 inches of soil and is very sensitive to the presence of water. If, however, the surface materials are representative of deeper material, the data provide a relative indication of the radioactivity of the underlying. Heavily fertilized farmland sometimes produces anomalously high signals because of radium in phosphate fertilizers. In spite of their limitations, aeroradioactivity surveys provide much valuable geologic information. AVAILABLE ATA Most of the aeroradioactivity data for Pennsylvania were collected under two national programs. The Aerial Radiological Measuring Surveys (ARMS) 329

2 330 E. R. KING AN J. S. UVAL APPALACHIAN PLATEAUS PROVINCE PP A LA CHIA N MOUNTA IN SECTI ON PIEMONT PROVINCE ATLANTIC COASTAL PLAIN PRO VI NCE PIEM ONT UPLAN AN LOWLAN SECTIONS PIEMONT PROVINCE Figure Composite aeroradioactivity image of eastern Pennsylvania north of the 40 parallel showing physiographic provinces and sections. Light areas indicate higher radioactivity. Generalized geologic units of the same area are shown on the facing page. Image compiled from NURE data; geologic units modified from Pennsylvania Geological Survey (1982); physiographic areas slightly modified from Berg and others (1989). were made for the U.S. Atomic Energy Commission to determine the background gamma radiation of rectangular areas, averaging 100 miles on a side, that surround major nuclear installations in the United States (Guillou, 1964). Two areas in Pennsylvania were flown: the Pittsburgh area, about two thirds of which is in Pennsylvania (Bates, 1964, 1966) (Figure 25-1, area A); and the Camden-elaware Valley area, a small part of which is in southeastern Pennsylvania (Guillou, 1961) (Figure 25-1, area B). Both of these surveys measured total counts per second. The entire state of Pennsylvania was surveyed by regional radiometric surveys conducted by the U.S. epartment of Energy from 1976 to 1981 to systematically assess the occurrence of uranium in the United States. In this program, the National Uranium Resource Evaluation (NURE) Program, totalcount plus uranium, thorium, and potassium channels, and aeromagnetic data were recorded in digital form for each 1- by 2-degree quadrangle. The data for the state of Pennsylvania are reported in LKB Resources (1977, 1978a- c), Texas Instruments (1978), Carson Helicopters and Texas Instruments (1980), Geodata International (1980), and High Life Helicopters and QEB ( 1982). Line spacing and direction vary from quadrangle to quadrangle and are listed in the caption of Figure Total-count gamma-ray surveys of two small areas (areas C and in Figure 25-1) in eastern Pennsylvania were made by the U.S. Geological Survey.

3 CHAPTER 25-AERORAIOACTIVITY 331 APPALACHIAN PLATEAUS PROVINCE 41 APP A LACHIAN MOUNTAIN SECTION GREAT VALLEY SECTION PIEMONT PROVINCE ATLANTIC COASTA L PLAIN PROVI NCE PIEMONT UPLA N AN LOWLAN SECT IONS PIEMONT PROVINCE SCALE FT KM EXPLANATION ~ Geologic contact Physiographic province boundary ~ Physiographic section boundary Cenozoic Mesozoic intrusive (diabase) Mesozoic sedimentary Pennsylvanian and Mississippian evonian and Silurian Ordovician and Cambrian Precambrian Figure (Continued).

4 332 E. R. KING AN J. S. UVAL Area C is part of a survey of the Reading Prong, in which the radioactivity survey was an adjunct to an aeromagnetic survey (Boynton and others, 1966a, b). Area is a strip 25 miles long that was surveyed in 1954 to investigate occurrences of uranium in eastem Pennsylvania. MAPS OF RAIOMETRIC ATA The older radiometric surveys were recorded in analogue form and compiled as sets of profiles or as maps of levels or ranges of counts per second (cps). These levels, which could be identified on a number of adjacent profiles, were used to divide the maps into zones that could be correlated with the geology of the area (Guillou, 1964; Pitkin and others, 1964). The data for areas A, B, and on Figure 25-1 were compiled into maps of radioactivity levels. Eventually, such maps were replaced by contour maps, which are able to provide much greater detail. The contour maps of area C (Figure 25-1) and the other maps of this survey in adjacent New Jersey show coherent patterns that can be related to the mapped geology. Both the older maps of radioactivity levels and the newer contour maps show a strong correlation of radiometric data with geologic units. For example, in area A, the Mississippian carbonate on the flanks and crests of anticlines in the southeast corner of the mapped area have an excellent correlation with elongate areas of lower radioactivity (Bates, 1966). Figure 25-2 (p. 330) shows a black-and-white version of a color composite of the three channels (uranium, thorium, and potassium) in which the three sets of data, each assigned a different color, are superimposed to give a single image (uval, 1983). The data are from the NURE surveys for the Williamsport, Harrisburg, Scranton, and Newark 1- by 2-degree quadrangles (Figure 25-1, areas F and G), where the flight-line spacing was 3 miles. In Figure 25-2, the light areas indicate where all three components are present in higher amounts, and the dark areas indicate where they are lower. Geologic units are shown on the second map in Figure 25-2 (p. 331) to illustrate the correlation of lithologies with the radiometric patterns. The shaly of the Great Valley stand out as a broad belt having higher radioactivity than the adjacent lithologies. A similar light pattern coincides with the clastic of the Newark basin and with the metamorphic terrane farther south, which includes abundant phyllite. Separating these two light areas is a narrow, dark belt of lower radioactivity over parts of diabase intrusions and coarser Mesozoic clastic of the Gettysburg basin and over some units in the Blue Ridge province. The Appalachian fold belt is characterized by northeast-trending areas of alternating higher and lower radioactivity, and the plunging folds coincide with interfmgering light and dark areas. Because these data are regional and measure less than 10 percent of the surface area (Pitkin and uval, 1980), they do not provide precise correlations with the geologic units, but more closely spaced data would provide better correlations with the geology. RELATIONSIP OF ATA TO URANIUM EPOSITS Exploration for uranium deposits has provided the main impetus for aeroradioactivity surveys. There are known deposits (K.lemic, 1962; Rose, 1970) in the eastern part of the state in Upper evonian to Mississippian and Upper Triassic clastic and in some of the gneisses and skarns of the Reading Prong (see Chapter 39). A small strip, area on Figure 25-1, was flown over a cluster of occurrences in Carbon County, but only one has an associated radiation anomaly, and no other promising anomalies were detected in spite of the 0.25-mile flight-line spacing. The NURE data, which were collected for the entire state at intervals of 3 to 6 miles, delineate the Reading Prong and indicate several anomalous locations in eastern Pennsylvania in areas underlain by clastic, especially those of continental origin. However, known. uranium deposits in three areas in the Williamsport 1- by 2-degree quadrangle (Figure 25-l, north half of area F) did not have associated aeroradioactivity anomalies in the survey data (Smith and Hoff, 1984; LKB Resources, 1978c), and others could have been missed because of the wide flight-line spacing. PROBLEMS AN FUTURE RESEARCH Many areas of research could benefit from more closely spaced aeroradiometric data. etailed contour maps, gray-scale images, and composite-color maps of such data can provide information on faults, facies changes, and other lithologic features in sedimentary and assist in unravelling the complex structures of the igneous-metamorphic terrane of southeastern Pennsylvania, particularly gneisses and mafic intru-

5 CHAPTER 25-AERORAIOACTIVITY 333 sions. Radon-pollution studies have recently generated great interest (see Chapter 55B) and would benefit from detailed aeroradioactivity data. Another very interesting area of research is in petroleum exploration (Saunders and others, 1987), as some investigators see a correlation of radioactivity with oil fields. Such studies might have applications in western Pennsylvania. RECOMMENE FOR FURTHER REAING Adams, J. A. S., and Gasparini, Paolo (1970), Gamma-ray spectrometry of, Amsterdam, Elsevier, 295 p. Adams, J. A. S., and Lowder, W. M., eds. (1964), The natural radiation environment, University of Chicago Press, I,069 p. obrin, M. B. (1960), Introduction to geophysical prospecting, 2nd ed., New York, McGraw-Hill, p Hood, P. J., ed. (1979), Geophysics and geochemistry in the search for metallic ores, Geological Survey of Canada Economic Geology Report 31, 811 p. Telford, W. M., Geldart, L. P., Sheriff, R. E., and Keys,. A. (1976), Applied geophysics, Cambridge, England, Cambridge University Press, p