Forward modelling of the physical properties of oceanic sediments: constraints from core and logs, with palaeoclimatic implications

Abstract

Abstract A new methodological approach based on the analysis of core data, logs and high-resolution electrical images of borehole surfaces (FMS) is developed in order to improve the study of oceanic sediments from physical properties. This approach is tested on data obtained in the context of the Ocean Drilling Program (Japan Sea, Leg 128, Hole 798B). The downhole measurements and FMS images exhibit a cyclic pattern reflecting variations in oceanic surface productivity combined with continental aeolian supply due to palaeoclimatic changes. On the basis of m-scale physical measurements, cm-scale FMS images and measurements on core, the objective is to deconvolve the variations in sedimentary supply of oceanic and continental components through time and to compute the intrinsic formation factor versus depth. The latter topic is approached in two ways: first by conventional log analysis, then with a new iterative forward modelling method. In the second case, the low frequency electrical resistivity log (SFL) is modelled using a numerical modelling code (Resmod2D © ) in order to obtain an accurate formation electrical resistivity model (Rt), where individual beds are derived from FMS images. An analytical routine is also used to model the natural gamma-ray measurement (CGR). While the conventional log analysis allows deconvolution of the sedimentary supplies, the forward modelling leads to a greater resolution and accuracy in more precise sediment characterization, such as that obtained from the derivation of the formation factor.