Errata‐based correction of marine geophysical trackline data

Abstract

[1] We announce the completion of 5,230 errata correction tables which remove extreme, obvious errors from the National Geophysical Data Center's marine geophysical trackline archive. Along with a range of error types correctable using along-track analysis, we determined that $62% of gravity surveys omit raw measurements and that $89% of magnetic anomalies are outdated and require recomputing. These errata tables reduce median global crossovers from 27.3 m to 24.0 m (bathymetry), 81.6 nT to 29.6 nT (magnetic anomalies) and 6.0 mGal to 4.4 mGal (free air gravity). We consider these methods gentler and more fundamental than the predominant crossover approach in that along-track corrections affect only data flagged as erroneous by a reviewer; the vast majority of data are left unchanged. Removal of obvious errors is an important initial step that should precede crossover analysis, interpolation, and gridding, thereby strengthening the scientific analysis.Components: 9300 words, 9 figures, 4 tables.

The Seagoing Scientist's Toolbox: Integrated Methods for Quality Control of Marine Geophysical Data at Sea

“…We have applied the skills developed over the course of our many days at sea and many data sets processed into a set of steps that performs these tasks in a timely fashion. If this approach is adopted by the community, after the initial period of assessing each vessel's different data structure issues, conversion into standard data exchange formats such as MGD77T (Hittleman et al, ), the Generic Mapping Tools (Wessel et al, ) mgd77 table format (herein referred to as GMT DAT), or the NetCDF compliant MGD77+ format (Wessel & Chandler, ), enables thorough data quality assessment and control using along‐track (Chandler & Wessel, , ) and crossover analysis. Crossover analysis is supported in that the mgd77 formats are supported by GMT's crossover analysis toolkit, x2sys (Wessel, ), for cases in which sufficient ship track intersections occur.…”

“…Along‐track analysis, detailed extensively by Chandler and Wessel (, ), is a technique by which marine geophysical track line files are examined internally for various types of errors. For instance, data values should range within reasonable limits.…”

“…Although some were reduced using nearby observatories, ideally these surveys will be resubmitted along with total field values but they can likely be utilized, as is, given further isolation of the crustal field by each data user and for each affected survey. A host of other data quality issues occur such as data reported in the wrong units (e.g., fathoms instead of meters), data inappropriately scaled by factors of ten by archivists, unreconciled gravimeter drift and tare artifacts (e.g., Figure c), vessel motion artifacts, a systemic tendency to omit observed gravity from the majority of archive files (62% of gravity legs lack observed gravity measurements), ∼7,500 m depth offsets stemming from erroneous digitization of two‐way travel time echograms, inaccurate navigation especially among the many older data sets, excessive speed due to temporary loss of satellite or radio navigation signals or inappropriate filtering across geographic discontinuities, navigation passing over land, and the presence of outliers and unrealistic data gradients (Chandler & Wessel, , ; Smith, ; Wessel & Watts, ). These and a plethora of other data problems warranting our diligent supervision (e.g., hardware malfunctions, operator errors, and software crashes) continue to occur at sea.…”

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.…”