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Electrical resistivity measurements made on recovered core samples, by drill-hole logging, and by surface sounding have been used to estimate the porosity of carbonate sediments composing Midway and Kure atolls in the western part of the Hawaiian Islands. The porosities within the first few tens of feet of the surface generally exceed 50 percent by volume but decrease by a factor of two or three at a depth of 500-1,000 feet. The use of resistivity measurements seems to provide a better estimate of porosity than porosity measurements made directly on recovered core samples because of difficulties in collecting an unbiased sample of high-porosity materials. INTRODUCTION AND ACKNOWLEDGMENTS The drilling of two holes through reef sediments into underlying basalt on Midway atoll, Hawaii, has been described by Ladd, Tracey, and Gross (1967). In addition to the other scientific studies carried out in conjunction with this drilling project, electrical resistivity measurements were made of the carbonate sediments. Some measurements were made on the sediments in place using surface-based and drill-hole measurement techniques, and other measurements were made oil the samples recovered from the drill holes. The purpose was to provide information on the possible applicability of electrical resistivity surveys to the study of reef sedimentation. The electrical properties of limestones, and in particular of reef limestones, have been the subject of several studies related to petroleum exploration. Archie (1952) indicated that the relation between electrical resistivity and water-filled porosity in carbonate rocks is essentially the same as that in sandstones, except that the complex pore structures in carbonates cause a greater variability in the resistivity. McCrossan (1961) studied resistivities from electric logs made in calcareous interreef deposits in Alberta and found that resistivity values seemed to be related to the proximity to reef structures. Pirson (1963) pointed out that the variety of pore structures in carbonate rocks should have some effect on the behavior of electrical properties. He indi1 Colorado School of Mines, Golden, Colo. cated that it is important to distinguish between matrix porosity and structural porosity. All the work reported in the literature, however, has dealt with ancient carbonates, where it is reasonable to expect that recrystallization and diastrophism may have modified the original pore structures markedly. The drilling at Midway atoll offered the opportunity to study electrical properties of reef rocks having a minimum of such modifications. I am indebted to Harry S. Ladd, U.S. Geological Survey, for making these studies possible, and to the Hawaii Institute of Geophysics, University of Hawaii, under whose auspices the program in the Midway area was carried out. The work was supported by the National Science Foundation (grant No. GP4728) in cooperation with the U.S. Geological Survey and the U.S. Office of Naval Research. SAMPLE MEASUREMENTS Two holes were drilled on Midway atoll: one on Sand Island, which reached basalt at a depth of 516 feet, and the other just inside the reef on the north side of the lagoon, which reached basalt at a depth of 1,261 feet (Ladd and others, 1967). Cores were cut with a diamond drill with a rubber-sleeve core barrel, which preserved even the friable material for recovery at the surface. Most cores were 3 inches in diameter. A sea-water base drilling mud was used. Approximately 160 small samples were selected from the 1,700 feet of carbonate core material for studies of electrical properties. The samples were air dried and cut to regular shapes for electrical properties studies. Samples were resaturated with a 0.5 normal solution of sodium chloride by vacuum boiling; that is, the samples were immersed in the saturating solution and placed in a vacuum for 12 hours. The vacuum chamber was then opened to atmospheric pressure so that the saturating solution would enter the pore structures of the samples. After saturation, the samples were stored in the 0.5 normal solution of sodium chloride for 2 months to permit the extra salts remaining in the pore structure from the original water content to redistribute by diffu-

Electrical resistivity measurements, Midway and Kure atolls