María José Jurado 2 and Belén Alonso 3


This study is an original application of the log inversion methods on logging-while-drilling (LWD) measurements to clay mineral estimation in an unusual geologic setting: the Barbados accretionary prism. LWD was used for the first time by the Ocean Drilling Program during Leg 156 at Sites 947 (Hole 947A) and 948 (Hole 948A). At Site 948 the décollement zone was logged and cored. Hole 948C, located close to Hole 948A, was cored and logged with open-hole (conventional) logging tools. A model of the main mineralogical components identified at Hole 948C has been obtained from the inversion of LWD data from Hole 948A, using a methodology that is commonly applied on conventional logs. Density (RHOB), photoelectric factor (PEF), neutron porosity (TNPH), spectral natural gamma ray (SGR), and the spectral natural gamma ray Th/K ratio (Th/K) LWD data were used in the inversion. Hole 948C core sample analyses indicative of a clay-dominated "complex lithology" have been used to constrain the model and to test the quality of the results obtained. The inversion of log data provides a good approximation to the actual mineralogy, and the relative abundance of clay, and the main clay types. The deviation observed between the model and Hole 948C sample analyses results can be mostly explained by the presence of minerals not included in the inversion model. The porosity values calculated are close to shipboard "bulk water content" values. This model shows that the application of conventional log inversion techniques on a limited set of LWD data to quantitatively estimate trends in mineralogical changes can yield satisfactory results. The inversion model gives a good approach to the results obtained from X-ray diffraction for Hole 948C Units II and III sediments (420-580 mbsf) and constitutes the first inversion model for the composition of sediments obtained from the first LWD data of the Barbados accretionary prism. LWD data can be used to model mineralogical composition and porosity.

1 Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results,156: College Station, TX (Ocean Drilling Program).
2 Geophysikalisches Institut, Universität Fridericiana Karlsruhe, Hertzstrasse 16, 76187 Karlsruhe, Germany. mjjurado@ija.csic.es
3 Instituto de Ciencias del Mar, CSIC, Paseo Joan de Borbó s/n, 08039 Barcelona, Spain.