Using spectral approaches to magnetic field survey analyses to characterize marine sedimentary environments
Abstract
Marine sediments have historically been omitted from airborne and shipboard magnetic survey analyses because their contribution to the observed field is usually very weak, and thus difficult to distinguish from survey noise. Even when higher susceptibility sediments are present, associated anomalies are often of such limited spatial extent that they do not persist from one survey trackline to the next, and are thus filtered or removed during gridding. In such cases, the resulting magnetic field maps indicate mostly basement rock variations. However, in areas where magnetic sediments have significant lateral extent, the upper stratigraphic units can make a distinguishable contribution to the short-wavelength components of the observed magnetic field. We present a spectral approach to processing magnetic trackline data that highlights magnetic source contrasts within the shallowest sedimentary layers. We apply the approach to several areas including Chesapeake Bay, MD, where watersheds include metamorphic Piedmont rocks rich in Fe- and Ti-rich minerals such as magnetite and ilmenite; Cook Inlet, AK, where magnetite and other Fe- and Ti-rich sediments have accumulated from nearby igneous rocks in the Alaska Range; and the sections of the Oregon coast south of Newport, where magnetite-rich sediments from nearby volcanic rocks have accumulated and developed into littoral marine placer deposits. For each data trackline, we calculate the frequency spectrum for moving windows of length 50 to 600 m, with window length depending on the along-track survey sampling density. We then sum spectral power over shorter wavelengths, excluding both the highest frequencies which most likely represent survey noise, and lower frequencies representing deeper features. Areas with greatest variation in short wavelength anomalies thus exhibit the highest spectral power. Shipboard magnetic field data from Chesapeake Bay near the mouth of the Choptank River exhibit concentrations of anomalies of width < 30 m and amplitude 2-5 nT. These anomalies are clustered in shallow areas where sands dominate the seabed, including at the inflow of Parker's Creek and sections near the inflow of the Choptank River. We attribute these anomalies to the presence of heavy mineral sands that have been concentrated through wave action. At Cook Inlet, aeromagnetic data collected at an altitude of ~130 m above sea level show anomalies of width 400-1200 m and amplitude 2-4 nT clustered in areas near glacial outwash and riverine inputs. These data also exhibit numerous, similarly scaled lineations that may be attributed to folding and faulting of sedimentary layers with high magnetic susceptibilities in the upper 1 km of the seabed. Aeromagnetic data collected at 300-400 m altitude above sea level near the Oregon coast between Newport and Waldport exhibit variations of width 500-800 m, which may indicate depth variations in high-susceptibility units or concentrations of placer deposits.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMGP21B..03S
- Keywords:
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- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 3005 MARINE GEOLOGY AND GEOPHYSICS / Marine magnetics and paleomagnetics;
- 3022 MARINE GEOLOGY AND GEOPHYSICS / Marine sediments: processes and transport;
- 4235 OCEANOGRAPHY: GENERAL / Estuarine processes