Natural‐field and very low‐frequency tipper profile interpretation of contacts

Geophysics ◽  
1987 ◽  
Vol 52 (12) ◽  
pp. 1697-1707 ◽  
Author(s):  
Victor F. Labson ◽  
Alex Becker
Keyword(s):  
Numerical Models ◽  
Low Frequency ◽  
Washington State ◽  
Trial And Error ◽  
North Central ◽  
Natural Field ◽  
Very Low Frequency ◽  
Basement Faults ◽  
Vertical Contact ◽  
Metasedimentary Sequence

Anomalous vertical magnetic field (tipper) profiles acquired using natural or very low‐frequency (VLF) radio transmitter sources can be interpreted simply and rapidly for a number of geologic settings. The relations between computed numerical models, and outcropping dipping and buried vertical contacts are presented here in a series of interpretation charts. Use of the tipper phase in the analysis minimizes the effect of transmitter azimuth in the VLF case. Two examples illustrate the application to field data. An audiofrequency natural‐field tipper profile over a conductive bed in a north‐central Washington State metasedimentary sequence demonstrates the interpretation procedure for a dipping contact. VLF profiles over covered basement faults in Ontario demonstrate the application for a buried vertical contact. In both cases the quick results are in agreement with the much more laborious trial‐and‐error matching to two‐dimensional models.

The numerical calculation of wave-fields, reflexion coefficients and polarizations for long radio waves in the lower ionosphere. II

1965 ◽  
Vol 257 (1079) ◽  
pp. 243-271 ◽  
Keyword(s):  
Collision Frequency ◽  
Numerical Models ◽  
Low Frequency ◽  
Lower Ionosphere ◽  
Preceding Paper ◽  
Radio Waves ◽  
Very Low Frequency ◽  
Cross Modulation ◽  
Long Wave ◽  
Ionization Structure

The characteristics of radio waves of low and very low frequency reflected from numerical models of electron density and collision frequency are calculated by the methods described in the preceding paper (Pitteway 1964). The models used are based on those found by cross-modulation experiments and approximate to those likely to be found in the ionosphere. The effects of changing the models are investigated with the object of developing methods of solving the inverse problem of deducing the ionization structure from the experimental data on long and very long wave propagation. The relations between the computed and observed characteristics are examined in a preliminary way.

scholarly journals Magnetic and Very Low Frequency Electromagnetic (VLF-EM) investigations for gold exploration around Ihale in Bunnu-Kabba Area of Kogi, north-central Nigeria

2018 ◽  
Vol 48 (3) ◽  
pp. 191-205
Author(s):  
Olawale Olakunle Osinowo ◽  
Michael Oluseyi Falufosi
Keyword(s):  
Current Density ◽  
Low Frequency ◽  
Magnetic Anomalies ◽  
Magnetic Data ◽  
Pegmatite Vein ◽  
Geophysical Investigation ◽  
North Central ◽  
Very Low Frequency ◽  
Reference Profile ◽  
Ground Magnetic

Abstract This study integrates ground magnetic and Very Low Frequency Electromagnetic (VLF-EM) methods of geophysical investigation to probe the subsurface in terms of rock magnetic susceptibility and ground conductivity for the purpose of identifying mineralized pegmatite veins that could serve as host for gold and associated metallic deposits. Twenty five (25) carefully planned ground magnetic and VLF-EM profiles, each, were occupied east and west of a reference profile which was purposively established on an identified mineralized pegmatite vein around Ihale in Bunnu-Kabba area of Kogi, north-central Nigeria. The acquired ground magnetic data were filtered and Reduced To Pole at Low Latitude (RTPLL) to remove regional field effect, cultural noise as well as focus the peaks of magnetic anomalies over corresponding sources. The measured raw real and raw imaginary components of the EM fields were subjected to Fraser and Karous Hjelt filtering to remove harmonic noise, focus and make anomaly amplitudes relate directly to the causative conductor as well as generate current density employed to characterise the subsurface. Seven (7) zones of relatively high VLF-EM derived current density with matching high residual positive magnetic anomalies present closely correlate-able signatures with subsurface response obtained around the reference profile established where local mining activities indicate evidence of gold and associated metallic mineralization of the pegmatite vein. The delineated zones constitute targets for gold and associated base metal prospects which could be further developed for economic benefit of the community.

The Attenuation of Very Low Frequency Brain Oscillations in Transitions from a Rest State to Active Attention

2009 ◽  
Vol 23 (4) ◽  
pp. 191-198 ◽  
Author(s):  
Suzannah K. Helps ◽  
Samantha J. Broyd ◽  
Christopher J. James ◽  
Anke Karl ◽  
Edmund J. S. Sonuga-Barke
Keyword(s):  
Brain Activity ◽  
Sampling Rate ◽  
Low Frequency ◽  
Future Research ◽  
Adhd Symptoms ◽  
Default Mode ◽  
Very Low Frequency ◽  
Wide Range ◽  
Interference Hypothesis ◽  
Mode Interference

Background: The default mode interference hypothesis ( Sonuga-Barke & Castellanos, 2007 ) predicts (1) the attenuation of very low frequency oscillations (VLFO; e.g., .05 Hz) in brain activity within the default mode network during the transition from rest to task, and (2) that failures to attenuate in this way will lead to an increased likelihood of periodic attention lapses that are synchronized to the VLFO pattern. Here, we tested these predictions using DC-EEG recordings within and outside of a previously identified network of electrode locations hypothesized to reflect DMN activity (i.e., S3 network; Helps et al., 2008 ). Method: 24 young adults (mean age 22.3 years; 8 male), sampled to include a wide range of ADHD symptoms, took part in a study of rest to task transitions. Two conditions were compared: 5 min of rest (eyes open) and a 10-min simple 2-choice RT task with a relatively high sampling rate (ISI 1 s). DC-EEG was recorded during both conditions, and the low-frequency spectrum was decomposed and measures of the power within specific bands extracted. Results: Shift from rest to task led to an attenuation of VLFO activity within the S3 network which was inversely associated with ADHD symptoms. RT during task also showed a VLFO signature. During task there was a small but significant degree of synchronization between EEG and RT in the VLFO band. Attenuators showed a lower degree of synchrony than nonattenuators. Discussion: The results provide some initial EEG-based support for the default mode interference hypothesis and suggest that failure to attenuate VLFO in the S3 network is associated with higher synchrony between low-frequency brain activity and RT fluctuations during a simple RT task. Although significant, the effects were small and future research should employ tasks with a higher sampling rate to increase the possibility of extracting robust and stable signals.

VLF/LF (Very Low Frequency/Low Frequency) Reflection Properties of the Low Latitude Ionosphere

1988 ◽  
Author(s):  
Wayne I. Klemetti ◽  
Paul A. Kossey ◽  
John E. Rasmussen ◽  
Maria Sueli Da Silveira Macedo Moura
Keyword(s):  
Low Frequency ◽  
Low Latitude ◽  
Very Low Frequency ◽  
Low Latitude Ionosphere
Sign in / Sign up

Export Citation Format

Share Document