6. Magnetic Methods in Near-Surface Geophysics

Hansen, R. O.; Racic, L.; Grauch, V.J.S.

doi:10.1190/1.9781560801719.ch6

Cited by 28 publications

(23 citation statements)

References 123 publications

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“…The general principles behind the technique are identical to those set out in the aeromagnetic section (or see Hansen et al, 2005). For ground based surveys the magnetometer, along with the associated electronics, data logger and display package, can be carried by an operator or mounted on an all terrain vehicle.…”

Section: Ground Based Magnetic Measurementsmentioning

confidence: 99%

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

Robinson

Binley

Crook

et al. 2008

Hydrological Processes

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Abstract:We want to develop a dialogue between geophysicists and hydrologists interested in synergistically advancing process based watershed research. We identify recent advances in geophysical instrumentation, and provide a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology. The focus of the paper is to identify instrumentation that could significantly advance this vision for geophysics and hydrology during the next 3-5 years. We acknowledge that this is one of a number of possible ways forward and seek only to offer a relatively narrow and achievable vision. The vision focuses on the measurement of geological structure and identification of flow paths using electrical and magnetic methods. The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved. Of specific interest are the airborne systems that can cover large areas and are appropriate for watershed studies. Although airborne geophysics has been around for some time, only in the last few years have systems designed exclusively for hydrological applications begun to emerge. These systems, such as airborne electromagnetic (EM) and transient electromagnetic (TEM), could revolutionize hydrogeological interpretations. Our vision centers on developing nested and cross scale electrical and magnetic measurements that can be used to construct a three-dimensional (3D) electrical or magnetic model of the subsurface in watersheds. The methodological framework assumes a 'top down' approach using airborne methods to identify the large scale, dominant architecture of the subsurface. We recognize that the integration of geophysical measurement methods, and data, into watershed process characterization and modelling can only be achieved through dialogue. Especially, through the development of partnerships between geophysicists and hydrologists, partnerships that explore how the application of geophysics can answer critical hydrological science questions, and conversely provide an understanding of the limitations of geophysical measurements and interpretation.

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Section: Ground Based Magnetic Measurementsmentioning

confidence: 99%

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

Robinson

Binley

Crook

et al. 2008

Hydrological Processes

Self Cite

243

169

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“…On a small scale (1 m and less), granitic inclusions in the glacial drift are expected to have a stronger magnetic response than the surrounding glacial materials (e.g. Hansen et al ., ; Nabighian et al ., ; Sanborn‐Barrie et al ., ).…”

Section: Southern Baffin Island and The Ldfa‐1 Area 5 Survey Sitementioning

confidence: 99%

“…Magnetometers map spatial variations in the Earth's magnetic field produced through both natural and anthropogenic sources, while vertical gradiometer magnetometers measure the response of the magnetic field at a higher spatial resolution (e.g. Hansen et al ., ). Electromagnetic surveying is designed to map the electrical conductivity of the near surface with the additional and complementary capability of mapping magnetic susceptibility (e.g.…”

Section: Introductionmentioning

confidence: 97%

Combined Geophysical Approach in a Complex Arctic Archaeological Environment: A Case Study from the LdFa‐1 Site, Southern Baffin Island, Nunavut

Landry

Ferguson

Milne

et al. 2015

Archaeological Prospection

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In 2014, we mapped the complex landscape of component Area 5 at LdFa-1, a 3000-year-old Palaeoeskimo site located in the deep interior of southern Baffin Island, using a combined magnetic and electromagnetic approach to define the physical characteristics of any large-or small-scale anthropogenic anomalies. Measurements were made using a GEM Systems Overhauser magnetometer-gradiometer and Geonics EM31 instrument, and a survey configuration designed to map in high resolution the total magnetic field, magnetic susceptibility and electrical conductivity responses of the underlying soils. Data-reduction methods were used for each survey, including, for example, removal of temporal drift, to produce final responses related closely to the subsurface physical properties. Six geophysical responses are presented in the results: total magnetic field, vertical magnetic gradient, horizontal-and verticaldipole-mode apparent susceptibility, and horizontal-and vertical-dipole-mode apparent conductivity. Spatial assemblages of small-scale (<2 m) anomalies, correlated between the magnetic and magnetic susceptibility results and interpreted to be associated with groupings of igneous boulders in the shallow subsurface, define three areas of archaeological interest. One grouping is a roughly circular arrangement of anomalies with a diameter of 4 m. Its spatial correlation with a productive excavation unit at the site suggests an anthropogenic origin. The complementarity of data acquired at this site reduces the chances of misidentifying anomalies, for example, the apparent conductivity results support interpretation of larger-scale anomalies observed at the site as being of lithogenic origin. This archaeogeophyiscal case study demonstrates the value of geophysical investigation in a complex Arctic archaeological environment and its role in providing information on subsurface archaeological features.

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“…The magnetic field, due to the subsurface susceptibility, is calculated by a magnetic dipole equation (e.g. Hansen et al, 2005). As a result of the toroidal shape of the electric primary field and dipolar shape of the magnetic primary field the sensitivity is at maximum near the coil axis for susceptibility and around the transmitter coil for quadrature conductivity (Figure 1-5).…”

Section: Fundamentals Of the Csem Methodsmentioning

confidence: 99%

Characterization of marine near-surface sediments by electromagnetic profiling

Müller

2010

GEOPHYSICS

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II PrefaceThe research, development and implementation of an electromagnetic seafloor profiler GEM Shark has been made possible through the joint funding of the Center for Marine Environ- The main part of this cumulative thesis (Chapters 2-4) consists of three journal manuscripts (one published, two ready for submission), which are for the most part based on my own theoretical and technical developments, programming, field work and data interpretation. AbstractThis thesis investigates the theory, develops the technology and demonstrates the applicabilities for synchronous in-situ mapping of the magnetic and electric signatures of marine near-surface sediments. A marine electromagnetic profiler has been developed for coastal and shelf operations in 5 to 500 m water depths. The system was designed to resolve subtle and gradual variations in silt and clay content, porosity and diagenesis state of marine sediments at a sub-meter scale. An electromagnetic (EM) induction sensor (Geophex GEM-3)was modified for submarine operations and mounted into a special bottom-towed sled.Seafloor magnetic susceptibility and electric conductivity were calculated from the in-phase and quadrature component of the secondary electromagnetic field using a half-space inversion algorithm. Seawater conductivity was measured synchronously by an integrated conductivitytemperature-depth (CTD) probe. The electric conductivity depends on porosity, the grain-size distribution and conductivity of the pore fluid. Magnetic susceptibility was used in this study as a proxy for fine-grained terrigenous clay/silt content, diagenetically forced magnetite depletion, as well as anthropogenic contaminants.The three attached manuscripts address different applications of the modified EM profiler.The first manuscript develops the physical and technical principles of the method and presents first results regarding the facies, early diagenesis, transport, and deposition of near-surface sediments on the NW Iberian shelf. The second manuscript combines high-resolution acoustic, magnetometric, and electromagnetic profiling with sample-based rock magnetic and geochemical analyses related to submarine groundwater discharge in a Baltic Sea estuary. The third manuscript is an application of the EM profiler to study the contamination of coastal areas by ferrous objects, organic matter and heavy metal pollution for coastal zone management of the Ría de Vigo, a NW Iberian estuary. Chapter 1

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6. Magnetic Methods in Near-Surface Geophysics

Cited by 28 publications

References 123 publications

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

Advancing process‐based watershed hydrological research using near‐surface geophysics: a vision for, and review of, electrical and magnetic geophysical methods

Combined Geophysical Approach in a Complex Arctic Archaeological Environment: A Case Study from the LdFa‐1 Site, Southern Baffin Island, Nunavut

Characterization of marine near-surface sediments by electromagnetic profiling

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