Carol A. Peterson

In 1978 a 1920 m continuously cored hole was drilled at Reydarfjordur in eastern Iceland. Comparisons between the geophysical logs and core measurements were used to evaluate the success of the logs in predicting the physical properties and lithology.

Two hundred and fifty density and porosity samples were taken from various lithologies. Average dike and flow densities and porosities were not significantly different. Good correlation was obtained between the density log and the core measurements. The poor correlation between porosity measurements and the log is due to the presence of hydrous minerals.

U, Th and K2O were analyzed in 300 samples, including all major clastic units. The average natural gamma ray radiation for the tholeiites is 39 API. There is good correlation between the radioactive elements and the natural gamma ray log.

The clastics, icelandites and ignimbrites can be identified from the geophysical logs. Tholeiitic dikes and flows are not distinguishable using crossplots of logs, spectral analysis or by plotting unit boundaries. Flow units tend to exhibit larger variance than dikes and partitioning the variance at various frequencies enhances the separation. A moving window, based on these variance partitions, produced a synthetic log which agreed with the core log in 64% of the intervals. If large dikes ( 20 cm) can be visually identified and removed from analysis the agreement is 82%. Many more complete core and log studies are required before this method can be applied to the geophysical logs in the uncored holes.

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Supervisor: James Hall