Supplementary material to “Electromagnetic Induction in the Earth”
Ian Fergurson, Department of Geological Sciences, University of Manitoba, Winnipeg, Canada; Lee Slater, Department of Earth and Environmental Sciences, Rutgers University, Newark, N. J.; Pilar Queralt and Juanjo Ledo, Geodinàmica i Geofísica, Facultat de Geologia, University of Barcelona, Barcelona, Spain
Citation:
Fergurson, I., L. Slater, P. Queralt, and J. Ledo (2007),
Electromagnetic induction in the Earth,
Eos Trans. AGU, 88(36), 351.
[Full Article (pdf)]
Abstract
Measurements of electrical properties of the Earth using electromagnetic induction (EM) can elucidate geological structures and processes ranging from metre to mantle scale. The 18th International Workshop on Electromagnetic Induction in the Earth (EMIW) was held in El Vendrell, Catalonia, Spain in September 2006 to advance EM methods and data interpretation. The one-week meeting was attended by 264 scientists and students. Highlights of the meeting described here include reports on advances in the application of EM to lithospheric studies, near-surface environmental studies, seismology and volcanology, marine studies, and developments in instrumentation and modeling/inversion strategies. This workshop included a series of discussion forums in which critical areas of future research were identified: here the community noted needs for (1) improved methodologies for the calculation and display of resolution of the models derived from highly non-linear inversions of EM data, and (2) petrophysical studies that can advance interpretation of electrical conductivity models in terms of physicochemical properties and processes of the deep earth.
Article
International Workshop on Electromagnetic Induction in the Earth (EMIW) are organized semi-annually by Working Group 1-2 of the International Association of Geophysics and Aeronomy (IAGA) and, since the first workshop in Edinburgh in 1972, have played an important role in advancing EM investigations of the Earth via the workshops, and special journal volumes of review and submitted papers (see http://www.dias.ie/mtnet/ for listing). The 18th International Workshop on Electromagnetic Induction in the Earth (EMIW) was held in El Vendrell, Catalonia, Spain in September 2006 to advance EM methods and data interpretation. At El Vendrell 319 papers were presented by authors from 44 countries, attesting to the global growth of the EM community. EMIW workshops have traditionally focused on studies of the Earth’s crust and mantle. However, near surface geophysics has increasingly been represented and in El Vendrell, the second largest session was on EM Applications for Environmental Studies. The EMIW also considered EM studies of energy and mineral resources including the developing field of marine EM hydrocarbon exploration.
EMIW also allow identification of critical subjects for future study and at El Vendrell this was emphasized in four discussion sessions. Areas identified as requiring attention included more petrophysical studies to improve interpretation of EM survey data from the deep earth. There was also discussion on the use of the phase tensor in magnetotelluric (MT) interpretations and socio-political aspects impacting the EM induction community such as the decreasing mathematical expertise of graduate students.
Advancement of EM Studies
The El Vendrell EMIW highlighted how recent theoretical and instrumental advances are now being applied in high-quality studies from around the world, at lithospheric, crustal, and near-surface scales. A number of crustal studies highlighted the resolution provided by dense 2-D MT profiles and 3-D grids. Use of the recently improved impedance tensor decomposition to correct regional MT responses and to quantify local EM effects has now become common. Consideration of implicitly anisotropic materials within multi-dimensional MT models has become a widespread feature of EM studies and, in the numerical modelling field use of unstructured meshes has increased greatly.
EMIW papers illustrated the contribution of MT in resolving structural features of crust and mantle ranging from Archean to Recent in age. A review paper by T. Korja (University of Oulu) on EM investigations of the lithosphere in Europe provided an overview of the large-scale EM surveys and the spatial variations in the regional lithosphere. The workshop also addressed the role of EM in monitoring crustal processes. A review paper by M. Uyeshima (University of Tokyo) described EM monitoring of seismic and volcanic processes and other papers examined correlation of electrical resistivity models with crustal melting, seismicity, and fault zones. In the near surface, EM studies of advective and diffusive flow fluid processes provided insights into coastal aquifer vulnerability and an appraisal of possible environmental consequences of underground atom bomb tests.
Environmental EM
The increasing need to characterize the near-surface of the earth, defined here as the upper 100 m impacted by human activities, has motivated members of the EM community to apply their skills at this smaller scale. The Environmental EM session at the 18th EMIW included papers on the investigation of multiple processes including (1) saline intrusion due to groundwater resource depletion, (2) contaminant flow and transport from industrial sites threatening soil and groundwater resources, (3) carbon gas releases to the atmosphere. EM instrumentation is evolving fast to cater for this new measurement scale. A number of papers describing recent methodological developments in Radio MagnetoTellurics (RMT), in which the plane waves produced by radio stations are used, in conjunction with available 2D MT source codes, to image the upper tens of meters of the earth. One advantage of the method in imaging of the near surface environment is the high speed of data acquisition relative to direct contact DC electrical resistivity. The environmental EM review paper by L. Slater (Rutgers University) summarized theoretical and empirical relationships between electrical properties and hydraulic properties governing groundwater flow and illustrated how hydrogeophysicists are developing non-invasive approaches to imaging the distribution of hydraulic conductivity.
Marine EM Exploration
The rapid expansion of marine EM methods in petroleum (and gas hydrate) exploration was reflected at the El Vendrell workshop by a number of papers from this field and it is clear that the subject has evolved into the commercial arena. Papers presented show that many of the instrumental challenges of marine EM work have been overcome and that marine EM data processing methods are becoming increasingly sophisticated. Other papers, including one examining resolution of resistive layers by different sources, and another parameterizing the effects of the air layer above the ocean, remind us of the basic science involved. In fact, partly in recognition of his contribution to fundamental advances in marine EM, Nigel Edwards (University of Toronto) was honored with 2005 Hohman Award presented at the gala dinner of the EMIW. It is anticipated that the increasing application of, and funding devoted to, marine EM surveys, as well as leading to hydrocarbon discoveries, will lead to new advances in EM induction methodology and interpretations.
Joint Inversion of EM and other Geoscience Data
EM researchers sometimes grapple with the issue of how to interpret the significance of geophysical models. One way of improving confidence in these models is through application of joint inversion strategies whereby multiple geophysical datasets are used to constrain estimated earth models. This was the subject of a review paper by P. Bedrosian (US Geological Survey) on the integration of larger-scale MT EM responses with other datasets. Examples of reductions in model non-uniqueness, as well as reduced sensitivity bias, through the inversion of MT data with other geophysical data were presented and showed how joint inversion interpretations enhance the definition of geological structures and geometry, allow inferences to be made on the thermal state and composition of the crust and mantle, and, because of the sensitivity of electrical resistivity to aqueous fluids and melt, permit quantitative definition of Earth processes e.g. crustal melting. The spectrum of such approaches and applications is illustrated by the diversity of papers presented in the session: e.g., one study considered joint structural 3-D inversion of cross-hole electrical resistance tomography and ground penetrating radar travel-times for distributions of electrical conductivity and relative permittivity; another considered joint inversion of MT and seismic receiver function data using a genetic algorithm approach.
Future Directions
What are the future directions of EM induction studies, what will be hot topics at the next EMIW to be held in Beijing, China in 2008? An area in which the EM community requires improved methodology is the calculation and display of resolution of the models derived from highly non-linear inversions of EM data. A paper at the El Vendrell EWIW investigating a formal approach to the evaluation of resolution may stimulate new work in this field. A second critical area is petrophysical studies. Almost all EM researchers regard this work as a necessity for improving the interpretations of field data yet the call for papers for the Petrophysics and Laboratory Studies Session at the 18th EMIW yielded only four abstracts. What can be done to stimulate new work in this area? The inaccessibility of the deep Earth severely limits petrophysical studies in this arena. In contrast, petrophysical relations linking electrical conductivity to the pore volume and pore surface properties of near-surface porous media have developed far beyond the seminal empirical relations of Archie. It thus seems reasonable to assume that the deep earth EM community will benefit from the increasing participation of the near surface community in the EMIW. The gathering of EM scientists working at a broad range of scales will improve the understanding of how the petrophysical relations for near surface materials, linking electrical conductivity to pore fluids, grains texture and mineralogy, can be transferred to the high temperature and pressure environment of the deeper earth.
Acknowledgments
The El Vendrell EM Induction workshop was organized by an Organizing Committee from the University of Barcelona and an international Program Committee under the general auspices of IAGA Working Group 1-2. Funding was provided by the US National Science Foundation, the Spanish Ministerio de Educación y Ciencia and Catalonian Departament d’Educació i Universitats, IAGA and the International Union of Geodesy and Geophysics, and industry and local sponsors. It enabled support to be provided to 50 participants, including 34 students, from 21 different countries. We acknowledge the ideas provided by many colleagues for this review.
