A rapid, accurate, and continuous method for measuring porosity or bulk density of cores from boreholes has been developed. The method is based on gamma-ray scattering.

The principal technical advantage of this porosity-measuring device is that variations in porosity encountered by a moving, pencil-size, gamma-ray beam are recorded continuously, whereas other methods yield only an average porosity for a particular core piece.

Basically, this device consists of a variable-speed drive system to move geologic material between a shielded gamma-ray source and a shielded detector, an optical caliper to measure the sample thickness, and a computer to calculate density and porosity from the measured parameters. The measured thickness and the computed density and porosity of the sample are recorded on a strip chart. Because the sample and the recorder chart are driven at the same speed, a direct comparison between the recorded parameters and the actual sample is possible.

Excellent agreement exists between porosity values of common sedimentary rocks measured by this method and by other conventional methods.

The system is designed for either field or laboratory use and only one operator is required. The device handles cores 1-4½ in. in diameter, conventional core plugs, and slabbed cores at drive speeds more than 3 in./min. At this speed, 100 ft. of core can be analyzed for density and porosity in about 6 hrs.

The device was designed primarily for density and porosity evaluation of sedimentary rocks obtained in oil exploration and development. Other applications of this system are possible. They include porosity measurements of unconsolidated recent sediment samples, determining oil content of oil-shale samples, measuring the ore content of metalliferous deposits, and estimating acoustic wave velocities or average density for geophysical purposes. Porosity profiles measured by the gamma-ray device for a variety of rock types illustrate uses of these results.

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