Investigating the effect of data quality on time domain electromagnetic discrimination
Using field data and numerical simulations we investigate the effect of data quality on time domain electromagnetic discrimination. Data quality decreases when measurements contain responses not accounted for by our mathematical modelling. This can include instrument noise, inaccurately reported position and orientation information, geologic contributions to the signal, and loss of validity of the forward modelling. Survey design is critical to data quality in order to have sufficient sampling of data anomalies, and also to ensure that each target is illuminated such that both the axial and transverse components of the polarization can be excited and measured. For dipole model based discrimination algorithms, success is contingent upon the accuracy with which the components of the polarization tensor can be estimated. Field data from different survey modes are analysed to identify noise sources and provide quantitative estimates of the noise in each survey. Inversion results show that increased noise levels lead to greater spread in recovered parameters. Monte Carlo simulations are performed in order to investigate the importance of other data quality factors. Analysis of inversion results from the simulations show that anomaly size, signal to noise ratio, positioning error, line spacing and station spacing all play a role in the spread of recovered parameters. Through the analysis of our simulation results we propose a figure of merit as a means of quantifying different data quality factors with a single number and relate this number to the accuracy with which parameters can be estimated.
A Free-Air Evaluation Of Uxo Detection Instruments In The Ordnance Detection And Discrimination Study At Former Fort Ord, Ca
The Ordnance Detection and Discrimination Study (ODDS) was a phased geophysical study of ordnance detection and discrimination specific to the former Fort Ord, located near Monterey, California. The purposes of this study were to evaluate existing OE detection instruments and systems and the ability of these devices to discriminate potential unexploded ordnance (UXO) in the specific ordnance and explosives (OE), geologic, and cultural environments at Fort Ord. The first phase of the ODDS involved static, free-air measurements along three 8-foot profiles by four near-surface geophysical instruments over inert OE items placed at varying depths and orientations beneath the testing platform. The Geonics EM61, Geonics EM61- Handheld, Geophex GEM-3, and Geometrics G-858 were tested. Instrument responses were compared by spatial examination of each profile and by calculating signal-to-noise ratios (SNR) for each test. These tests resulted in four general conclusions: 1) as expected, the electromagnetic anomaly magnitudes decreased with depth faster than the magnetic; 2) the “best” orientation was different for different OE items; 3) the GEM-3 anomalies have the highest SNR between the surface and 2-3 feet depth; 4) for targets buried deeper, the G-858 SNR was the highest. Note that these results represent instrument performance under free-air conditions only. SNR values were lower in the Seeded and Field Trial Site phases of the ODDS, possibly due to vegetation and terrain conditions.
Geomagnetic Polarity Epochs: Sierra Nevada Data, 3
Available paleomagnetic and radiometric results for about 60 igneous rocks 3.6 m.y. old or less are consistent with the following geomagnetic polarity epoch time scale: Brunhes normal polarity epoch, 0.0 to 0.85±0.15 million years; Matuyama reversed polarity epoch, 0.85±0.15 to 2.4±0.1 million years; Gauss normal polarity epoch, 2.4±0.1 to 3.35±0.1 million years; Gilbert reversed polarity epoch, 3.35±0.1 million years to (?). A brief interval of normal polarity (the Olduvai event) occurs within the Matuyama reversed epoch at about 1.9 m.y., and a brief interval of reversed polarity (the Mammoth event) occurs within the Gauss normal epoch at about 3.06 m.y. Data for twelve units from eastern California and Nevada, reported here for the first time, are particularly useful in defining the Gauss-Gilbert boundary and for recognizing the Mammoth event.