Density and porosity logging in the IRDP hole, Iceland

Abstract

The results from the gamma‐gamma density log and the neutron‐neutron porosity log in the continuously cored IRDP hole are presented. As the core recovery was 99.7%, there is an unique opportunity to compare the response to logging tools in basaltic environment to laboratory measurements on the core samples. Special calibration pits for density calibration were constructed, and the logging data for densities were found to be in an agreement with laboratory measurements on core samples. For porosity, the laboratory data were found to be lower than those obtained from the logs. The reason for this was found to be that effective porosity, as measured in the laboratory, is generally significantly lower in basalt than the total porosity measured by the logs. The mean deviation in the repeatability of the density is ±160 kg/m3 and for the apparent limestone porosity index ±3%. A correlation coefficient of −0.71 is found between the matrix density and the concentration of SiO2 in the rocks, and a correlation coefficient of −0.76 is found between the matrix density and the gamma ray intensity of the rocks. It is proposed that the presence of zeolites and other secondary minerals cause the observed systematic scatter in the density‐porosity relation within individual units in the hole. Owing to a lower porosity the mean bulk density of the intrusions is higher than the mean bulk density of the flows. In contrast, the matrix densities of flows and dikes are shown to be equal within limits of error. The average density of the 1900‐m‐thick section logged is (2.85±0.27)×103 kg/m3 for the bulk density and (3.03±0.20)×103 kg/m3 for the matrix density. The average porosity is 9%. Significant variations in the matrix density with depth are shown to be associated with three phases in the volcanic activity building up to basaltic pile in Reydarfjördur. The matrix density obtained from the logs in the IRDP hole is consistent with other published determinations of the matrix density of the oceanic crust.