Near-bottom Magnetic Study Of Hydrothermal Fields On The Southwest Indian Ridge

Mid-ocean ridges(MORs)are mostly far away from land and are characterized by complicated geological structures and the existence of various types of hydrothermal systems with complex controlling factors.It is a brand-new task to explore seafloor sulfide resources for industrial or scientific circles.With the advent of deep-towed geophysical surveys,near-bottom magnetic prospecting is considered to be an efficient method for investigating activity/inactivity hydrothermal fields and studying hydrothermal system spatial structure.Autonomous underwater vehicles(AUVs)are advanced near-bottom platforms for magnetometer analysis of the seafloor;however,the vehicle itself can cause interference with magnetic measurements that must be corrected and data processing flow and interpretation methods must be established during sea trials.Qianlong-II as the first AUV equipped with a three-component magnetometer in China.On this basis,we established the integration of near-bottom magnetic processing technology and research methods from four aspects: theoretical model,data processing,inversion analysis and comprehensive interpretation.We made related analyses and research by using near-bottom magnetic data in the Longqi and Duanqiao hydrothermal fields.The main research achievements are as follows.1.Theoretical model.To understand the magnetic structure features of hydrothermal sulfide deposits at mid-ocean ridges,we started from the formation mechanism and structural characteristics of hydrothermal sulfides,stated the possible reasons for magnetic anomalies in hydrothermal fields,and built 3D forward models for both mafic-and ultramafic-hosted hydrothermal sulfide deposits to simulate the near-bottom magnetic field.Our modeling results showed a low-amplitude magnetic anomaly above mafic-hosted hydrothermal deposits,but a high-amplitude magnetic anomaly above ultramafic-hosted deposits.Integrating existing research,we summarized the progress of survey and exploration hydrothermal sulfides by near-bottom magnetic methods,and discussed and analyzed key problems that are unresolved or merit further investigation.These can provide a theoretical framework for investigation and data interpretation.2.Data correction.On the basis of the compositions and causes of near-bottom magnetic anomalies,we derived a functional relationship,termed as triangle function with a five-parameter correction formula,between magnetic measurements and AUV heading,and established a method to correct tri-axial magnetometer interference caused by the AUV.Using our method,we obtained good results to correct magnetic data collected by Qianlong-II AUV in China and ABE(Autonomous Benthic Explorer)in America.We analyzed the effect of the AUV's main components on magnetic measurement results in detail to determine the optimum position for the magnetometer and that data led to installing it on a 1.3-m-long extension on the AUV's stern.Then,the difference of magnetic readings was reduced to only ±400 n T and the accuracy was within ±30 n T during AUV spinning.3.Data processing and inversion analysis,according to the characteristics of near-bottom magnetic data,we built procedures for data processing and evaluation and quality control of magnetic data and introduced some magnetic anomaly processing and conversion methods.Thereinto,the ISDV method is very effective at defining the boundaries of geological bodies and is well suited for near-bottom magnetic data.If the ratio of the boundary distance between geological bodies and the investigation height is greater than or equal to 1.5,the boundary can be distinguished.The tilt-depth method obtains convinced spatial parameters such as location and ore body depth,but geological body top depth results are not directly viewable.To reduce the topography,we obtained magnetic field distribution by cutting their topography to the same level.We tried to get the longitudinal apparent magnetization of geologic bodies using a 2D imaging inversion method and transverse magnetization using a Fourier transform method.Moreover,we obtained perfect inversion results using three similar and upright hexahedra located at different depths.4.Comprehensive interpretation.Based on our interference correction and data processing methods,we obtained high-resolution near-bottom magnetic data with the Qianlong-II AUV in the Longqi and Duanqiao hydrothermal fields.In the Longqi hydrothermal field,we infer a 120 m wide and >2 km long N–S fault along the discovered hydrothermal vents by removing the influence of terrain and using the ISDV method.It is consistent with precise topography,which was collected by the Jiaolong submersible during a previous dive.The fault line may be connected to a known detachment fault and constituted one part of the hydrothermal channel.We subsequently outline the hydrothermal alternation zone according the ISDV method results,and believe that the demagnetization was induced by hydrothermal alternation.We attempted to show the transverse boundary and the top depth of field sources in the Longqi hydrothermal field,but the quantitative value should be estimated by the distance between the 45° and-45° contour lines.In the Duanqiao hydrothermal field,we found an obvious low magnetic anomaly and magnetization on the known hydrothermal vent,and a lower magnetic anomaly and magnetization on the new hydrothermal vent,where hydrothermal anomalies were observed.We delineated the boundary of the low-longitudinal apparent magnetization in the Duanqiao hydrothermal field using a 2D imaging inversion method.The low apparent magnetization area was inferred to be a fluid conduit,with hydrothermal alteration leading to the low apparent magnetization.