Ocean Floor Magnetic Property Measurement System (OFMPMS): An In Situ Detection Instrument for Obtaining Remanence Curves of Bottom Sediments

Marine geomagnetic survey plays a pivotal role in the study of submarine tectonics, but existing technology lacks the ability to acquire the magnetic mineral composition of marine bottom sediments, which is crucial for the accurate analysis and interpretation of magnetic anomalies. To address this problem, we have developed an ocean floor magnetic property measurement system (OFMPMS), which is capable of in situ measurement of isothermal remanent magnetization (IRM) acquisition curves and alternating field demagnetization (AFD) curves in deep-sea environments. The subsea unit of the OFMPMS consists of three modules: a magnetic field generator, a system controller, and a measuring unit, all of which are enclosed in three sealed cabins to withstand the high-pressure environment of the deep sea. The OFMPMS is equipped with five anisotropic magnetoresistance (AMR) magnetometers, enabling it to effectively detect the magnetic inhomogeneity of samples. Two sea trials were completed in South China Sea. During the first sea trial, the OFMPMS performed in situ detection of marine bottom sediments at a depth of 1385 m. However, no valuable IRM and AFD data were obtained because the sediments in this area were predominantly paramagnetic. During the second sea trial, at a depth of 4500 m, an iron ore sample was subjected to testing. The test results showed that in situ data matched laboratory data under high applied fields but differed significantly under low applied fields, which was attributed to the pressure exerted by seawater on the sample. This discovery highlights the limitations of ex situ measurement methods (measurements taken at normal pressure on land) for analyzing marine bottom sediment magnetic properties. OFMPMS provides an approach for in situ identification of magnetic minerals in marine bottom sediments and is expected to provide an accurate interpretation of submarine magnetic anomalies.