An evolution of near surface geophysical imaging : directionality, physical properties and challenging conventional wisdom (2014)
Type of ContentTheses / Dissertations
Degree NameDoctor of Science
PublisherUniversity of Canterbury
AuthorsNobes, David C.show all
The fundamental changes in applied geophysics in the last few decades have to a great extent been in the development of near-surface geophysics (NSG) – what used to be called environmental and engineering geophysics. In some locations and for some purposes, it still is. The developments in my publications have, to some extent, paralleled and sometimes foreshadowed some significant developments. My earliest papers were on marine electromagnetic (EM) sounding, and some of those papers are still cited. The work from my PhD and my post-doctoral fellowship laid the groundwork for what was to become controlled source EM (CSEM), a technique of growing importance in marine oil and gas exploration. Because the depths involved were less than 1 km, it can still perhaps be called “near surface”, but that was not the original intention. However, a theme central to that early work has carried on, explicitly or implicitly, through much of my research – anisotropy and the directionality of the geophysical response. One of my early theoretical papers was on the inclusion of anisotropy in Maxwell’s equations, and recent papers have used the directionality of the EM response as a tool in archaeological imaging. Another pair of linked themes that have recurred almost from the beginning are the influence of physical properties on the geophysical response, and the inter-relationships of physical properties. It allowed me to determine the physical property variations at depth in Middle Valley, on the northern Juan de Fuca Ridge. Those predictions were confirmed by the results from Ocean Drilling Program (ODP) Leg 139, which drilled those Middle Valley sites. While the marine research was interesting and rewarding, I was also moving more and more onshore, and began doing archaeological imaging in the late 1980’s. Much of that work was focussed around student projects, but then expanded into forensic geoscience, and ultimately to the non-invasive imaging of burial sites. That work continues today. The onshore research also allowed me to move from EM induction methods into ground penetrating radar (GPR), which involved the propagation of high-frequency EM waves. There are many hypotheses and approaches to GPR that were based on incorrect assumptions. For example, it was often assumed that rocky debris in debris-covered and debris-laden glaciers would not prevent the propagation of significant GPR energy at depth, an assumption that we proved wrong. The publication from 1994 on GPR imaging of the debris-covered lower Tasman Glacier was not followed by a paper by other researchers on GPR imaging of debris-laden glaciers until 1997, and GPR is now a common technique for imaging of all types of glaciers. Thus glacier imaging has been an ongoing application, and has expanded to include imaging of permafrost, including 4-dimensional (4D) imaging, i.e. time lapse 3-dimensional (3D) imaging, of permafrost polygonal patterned ground (PPG) in the Dry Valleys of Antarctica. The utility of near-surface geophysics in Antarctica has expanded greatly over the years. Similarly, surface water was assumed to degrade GPR signal penetration. Again, this was based on an incorrect assumption – that water was inherently conductive. While the presence of water does increase the electrical conductivity, if the water is fresh then the conductivity still remains quite low, and the attenuation of the GPR signal is minimal. 6 Thus the applications for EM and GPR have expanded, and the principles and applications are better understood now, ranging from archaeological and forensic geoscience, through non-destructive testing (NDT) and other geotechnical projects, to neotectonics and the imaging of active faults. Recently, I and my students have combined GPR more and more with electrical imaging. The two complement each other nicely. Finally, I have included two review papers, each in the section of greatest relevance. I recognise that this is not standard practice, but one from 1996 was used as a benchmark and a starting point for the later reviews of the environmental applications of EM, and the other from 2011 provides what I hope will be a paper used to help glacier imaging surveys to be better designed and completed. Both also include recent research results that had yet to be published, and thus represented the state of the art. I would note that I have included a number of papers from conference proceedings. In applied geophysics, the conference papers are normally peer reviewed, just as in engineering. Sometimes those papers are then expanded and augmented and subsequently published in peer-reviewed journals. If the peer-reviewed conference papers were later published as peerreviewed journal articles, then the journal article is included here. There are papers I decided not to include because they did not fit into the overall theme of this collection of papers – the evolution of my work in near-surface geophysics, which I took very broadly to embrace my work in marine geophysics as well. The papers not included here were two papers on paleoclimatology, for which I did the crucial spectral analysis, and three papers on social science and philosophy of science. I also excluded a few papers that were superseded by later work. There appears to be no set configuration to the form of a DSc, beyond collecting the papers together into some sort of coherent form that reflects the themes the work represents. In principle, a collection of papers submitted for the DSc represents the best of a lifetime of work. However, I hope that my best work is still to come. Only time will tell if that is true. For the papers submitted here, I have done a significant amount of the work, if not the majority of the work. In the case of papers based on student projects that I supervised, if the student wrote the first draft, then I made them first author, regardless of how much additional work was required to get the paper to its published form.