The geophysical very low frequency electromagnetic (VLF-EM) method: effects of topography and surface water investigated with simulations and field measurement
Date
2015-04-17
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Masters
Abstract
The Very Low Frequency Electromagnetic (VLF-EM) method, which enables surveying without contact with the ground, is suitable for ground surveys in a wide area and has been used in mapping geology for decades. The technique makes use of signal radiation from military navigation radio transmitters operating in the frequency range of 15-30 kHz. When the electromagnetic wave impinges on the surface it is both reflected back into the air and refracted into the earth. By measuring the shifted reflected magnetic field relative to the primary field, subsurface structures can be constrained.
Although the VLF method has been widely used to map geology in the last several decades, few modeling studies have been published. Particularly the effect of topography on VLF measurements is poorly characterized. The objective of my research is to study and simulate the VLF topographic responses, and therefore to distinguish between such responses and actual subsurface resistivity anomalies in VLF-EM data.
A few basic models (homogenous half-space, horizontal contact and vertical contact) were first created using the finite-elements modeling software Comsol Multiphysics and verified with theoretical solutions. Subsequently, features such as hills and lakes were incorporated into these basic models and further analyzed. When modeling topographic effects, two relationships between max inphase / max slope and max Quad / max slope versus skin depth / hill width are found, which can be used to predict topographic effects when the slope of a hill and resistivity of the ground are known. Two different sets of field data acquired at Saskatoon’s Diefenbaker Hill and at Cameco’s Cree Extension are compared with the modeling results.
Description
Keywords
electromagnetic, VLF-EM, topography, surface water, finite-element modeling.
Citation
Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geology