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USGS2005

A moving hum filter to suppress rotor noise in high-resolution airborne magnetic data

A unique filtering approach is developed to eliminate helicopter rotor noise. It is designed to suppress harmonic noise from a rotor that varies slightly in amplitude, phase, and frequency and that contaminates aero-magnetic data. The filter provides a powerful harmonic noise-suppression tool for data acquired with modern large-dynamic-range recording systems. This three-step approach - polynomial fitting, bandpass filtering, and rotor-noise synthesis - significantly reduces rotor noise without altering the spectra of signals of interest. Two steps before hum filtering - polynomial fitting and bandpass filtering - are critical to accurately model the weak rotor noise. During rotor-noise synthesis, amplitude, phase, and frequency are determined. Data are processed segment by segment so that there is no limit on the length of data. The segment length changes dynamically along a line based on modeling results. Modeling the rotor noise is stable and efficient. Real-world data examples demonstrate that this method can suppress rotor noise by more than 95% when implemented in an aeromagnetic data-processing flow. ?? 2005 Society of Exploration Geophysicists. All rights reserved.

Geophysics↗

USGS1993

Preliminary results of a helicopter electromagnetic and magnetic survey of the Oak Ridge Reservation, Tennessee for environmental and geologic site characterization

The 35,252 -acre Oak Ridge Reservation (ORR), in the western portion of the Appalachian Valley and Ridge province in Tennessee, has been a nuclear production and development facility for 50 years. Contaminants in the many waste sites on the ORR include heavy radioactive isotopes as well as many organic and inorganic compounds. The locations, geometry, and contents of many of these waste sites are reasonably well known, while others are poorly known, and some may be unknown. To better characterize the known sites and locate and characterize additional environmentally hazardous sites, a two-phase aerial survey of the ORR was begun in April, 1992. Phase I, which began in April, 1992, consisted of aerial radiation, multispectral scanner, and photographic surveys. Phase II, which began in November, 1992 and is described in this presentation, consisted of a helicopter electromagnetic (HEM), magnetic, and radiation survey. Targets of the survey were both man-made (drums, trench boundaries, burn pits, well heads) and geologic (fractures, faults, karst features, geologic contacts). The Phase II survey has three components: testing, reconnaissance, and high-resolution data acquisition. To date, the testing and reconnaissance data acquisition have been completed, and part of the data have been processed. They indicate that: 1) magnetic and HEM data are complementary, and do not always highlight the same anomaly; 2) under favorable circumstances, helicopter magnetometer systems are capable of detecting groups of 4 or more 55-gallon drums at detector altitudes of 15 m or less, 3) HEM data provide data which compare favorably with surface data collected over burial trenches, 4) well casings seem to be related to magnetic monopole anomalies, as would be expected, 5) Changes in EM and magnetic anomaly character are related to lithologic changes and might be used to track contacts between known outcrops.

Tennessee↗