Improving the quality of marine geophysical track line data: Along‐track analysis

Abstract

[1] We have examined 4918 track line geophysics cruises archived at the U.S. National Geophysical Data Center (NGDC) using comprehensive error checking methods. Each cruise was checked for observation outliers, excessive gradients, metadata consistency, and general agreement with satellite altimetry-derived gravity and predicted bathymetry grids. Thresholds for error checking were determined empirically through inspection of histograms for all geophysical values, gradients, and differences with gridded data sampled along ship tracks. Robust regression was used to detect systematic scale and offset errors found by comparing ship bathymetry and free-air anomalies to the corresponding values from global grids. We found many recurring error types in the NGDC archive, including poor navigation, inappropriately scaled or offset data, excessive gradients, and extended offsets in depth and gravity when compared to global grids. While $5-10% of bathymetry and free-air gravity records fail our conservative tests, residual magnetic errors may exceed twice this proportion. These errors hinder the effective use of the data and may lead to mistakes in interpretation. To enable the removal of gross errors without over-writing original cruise data, we developed an errata system that concisely reports all errors encountered in a cruise. With such errata files, scientists may share cruise corrections, thereby preventing redundant processing. We have implemented these quality control methods in the modified MGD77 supplement to the Generic Mapping Tools software suite.Citation: Chandler, M. T., and P. Wessel (2008), Improving the quality of marine geophysical track line data: Along-track analysis,

Errata‐based correction of marine geophysical trackline data

“…The problem occurred fairly often when navigation systems lacked sufficient satellites or stations to maintain consistent vessel location. Extreme cases of this problem have speeds upward of 10 7 m/s [ Chandler and Wessel , 2008], producing globe spanning ship tracks that impede the utilization of these data.…”

“… a Along‐track analysis outlier thresholds from Chandler and Wessel [2008]. Error checking thresholds for data values and gradients (per km) were calibrated using the 99th percentile or known physical limits. …”

“…The existing set of marine geophysical trackline measurements archived at the National Geophysical Data Center (NGDC) reflects an estimated USD 6.5 billion taxpayer investment [e.g., Chandler and Wessel , 2008] and is becoming increasingly valuable as costs for acquisition of new data continue to rise. These publicly available data are widely used in constraining bathymetric charts, ground‐truthing satellite altimetry‐derived gravity, inferring seafloor magnetization for plate kinematic studies, performing grid compilations, and in many other investigations of the seafloor, crust, and upper mantle.…”

Community infrastructure and repository for marine magnetic identifications

“…Magnetic anomaly identifications are an interpretation of data, with errors stemming from a variety of sources: the original data itself; the interpretation technique; the way the information has been preserved. Source data errors have largely been addressed through error corrections applied to the NGDC data [ Chandler and Wessel , ], but the errors originating from the source data remain as these corrections have not been propagated through to magnetic anomaly identifications made from the uncorrected data. Sources of error may derive from errors in the location of the measurements, particularly for old, pre‐GPS data; large skewness angles due to magnetization and the ambient geomagnetic field directions; nonvertical magnetic boundaries within the magnetic source layer.…”

“…The majority of marine magnetic anomaly data, collected through marine ship track, aeromagnetic, and helicopter surveys, have been made available to the scientific community through the GEODAS (GEOphysical Data System) archive, developed by the US National Geophysical Data Center (NGDC) [ Sharman et al ., ]. A subset of these data, which have been error‐checked for observational outliers, excessive gradients, metadata consistency, and agreement with satellite altimetry‐derived gravity and bathymetry grids [ Chandler and Wessel , ] is available through the MGD77 supplement to the Generic Mapping Tools software suite [ Wessel et al ., ]. Experts in marine geophysical data interpretation compare these magnetic anomaly data against synthetic crustal magnetic models and the geomagnetic reversal time scale to create a set of so‐called magnetic anomaly identifications—a spatiotemporal representation of the magnetic anomalies themselves.…”