A comparative analysis of time–depth relationships derived from scientific ocean drilling expeditions

Abstract

Time–depth relationships (TDR) are required for correlating geological information from drill sites with seismic reflection profiles. Conventional time–depth domain conversion is implemented using P-wave velocity data, derived from downhole sonic logs, calibrated with vertical seismic check-shots. During scientific ocean drilling expeditions, immediate seismic correlation is carried out using laboratory velocities measured on recovered core material. As these three velocity measurements vary significantly in signal frequency, resolution and acoustic pathways, they carry potential for substantial TDR differences and consequent miscorrelation to seismic profiles. Our analytical work uses the comprehensive scientific ocean drilling dataset to quantify these differences in core-seismic integration. TDRs are calculated and compared at sites where check-shot, sonic log, and laboratory velocity measurements cover the same depth segments of the drill hole. We find that the maximum differences between the TDRs (TDR $diff_{max}$) reach up to 55%, which can cause fundamental errors in the seismic correlation. No direct relationship to porosity and bulk density of the cored material is observed. Instead, higher TDR variability is found at sites with carbonate content > 70%, particularly with coarser grain texture. Sites containing primarily igneous and siliciclastic sequences show less than 10% TDR $diff_{max}$. This semi-quantitative criterion indicates that downhole logging should be conducted during drilling expeditions, especially at sites with carbonate sequences, or low core recovery, to ensure accurate core-seismic integrations.