Signal analysis of cyclicity in Maastrichtian pelagic chalks from the Danish North Sea

Abstract

Low field bulk magnetic susceptibility has been determined on Maastrichtian chalk samples from a drill core from the Dan Field in the Danish North Sea. Fast Fourier Transformations (FFT) have been used to detect possible cycles in the magnetic susceptibility data. Power spectra from the complete section and from sub-sections of the magnetic susceptibility reveal two cyclicities of ca. 0.4 cycles/m and ca. 1.7 cycles/m, which are present on a 90% confidence level. Signal analysis of natural gamma-ray wire-line log data supports these findings. Sedimentation rate estimates place the cycles in the Milankovitch frequency band. Artificial time series are used to study the applicability of the FFT to identifying cyclicity in chalks. Expected geological distortions (e.g., hiati and sedimentation rate variations) are introduced into the time series to investigate the response of the frequency spectra. Different methods of handling missing data intervals are also examined.