Magnetostratigraphy from downhole measurements in ODP holes

The “Geological High-resolution Magnetic Tool” (GHMT) was used on 17 ODP Legs to obtain downhole logs of magnetic field strength and magnetic susceptibility. The remanent magnetization of the sediment around the borehole is derived from these measurements, and used to produce magnetostratigraphies. This review of the ODP GHMT data indicates that reliable magnetostratigraphies can usually be obtained from reasonably well-magnetized (above approximately 10−3 A/m) fine-grained sediments. Additionally, the GHMT data indicates that a drill-string overprint, common in data collected on cores, does not significantly affect the sediments surrounding the borehole. This allows us to use the remanent and induced magnetization data to obtain estimates of relative paleointensity, and when averaged over time in homogenous sediment, the paleomagnetic recording efficiency of that sediment.