Methods for obtaining rapid, generally unconstrained, depths to the sources of magnetic and gravity anomalies have been in use since before the computer era in interpretation. Over the years, a wide range of depth estimation algorithms have been developed, but most share a few common underlying themes. The lecture will survey a selection of the more popular methods, emphasizing these common features. Remarkably, the development and use of depth estimation techniques continues to be a vigorous area of study. Recent developments include a strong movement to three-dimensional methods and the use of techniques which can resolve anomaly interference. Some of these recent algorithms, and the outlook for the future, will be discussed.

Richard O. Hansen received a BSc in mathematics and physics from Carleton University in 1968, and MS (1969) and PhD (1973) degrees in physics from the University of Chicago. He held postdoctoral positions at the University of Pittsburgh from 1973 to 1975, at the University of Oxford from 1975 to 1976, and was a lecturer at the University of California at Berkeley from 1976 to 1978. From 1979 to 1985 he worked at EG&G Geometrics, where he was finally Staff Scientist. From 1985 to 1995 he was a member of the Geophysics Department at the Colorado School of Mines, finally holding the positions of Research Professor and Director of the Gravity and Magnetics Project. In 1995, he joined Pearson, deRidder and Johnson, Inc. as Chief Geophysicist and now holds the position of Principal Geophysicist. His interests are in processing and interpretation of potential field data.

Catastrophic earthquakes account for 50% of worldwide casualties associated with natural disasters. Economic damage from earthquakes is increasing, even in technologically advanced countries with some level of seismic zonation. Reliable estimates of seismic hazard is essential to facilitate improved building design and construction, emergency response preparedness plans, economic forecasts, and housing and employment decisions. The Global Seismic Hazard Map, the first ever quantitative global seismic map, depicts peak ground acceleration (pga) with a 10% chance of exceedance in 50 years for the Western Hemisphere. Pga, a short-period ground motion parameter that is proportional to force, is the most commonly mapped ground motion parameter because current building codes that include seismic provisions specify the horizontal force a building should be able to withstand during an earthquake. The seismic hazard map of the Americas depicts the likely level of short-period ground motion from earthquakes in a fifty-year window. Short-period ground motions affect short period structures (e.g. one to three story buildings). The largest seismic hazard values in the world generally occur in areas that have been, or are likely to be, the sites of the largest plat boundary earthquakes: the subduction plate boundary regions of the Kuriles-Kamchatka, the Aleutians, southern Alaska, Iceland, the Pamir-Hindu Kush-Karakorum and China/Myanmar border regions of the India-Asia collision zone, Taiwan, the transform plate boundary of the western U.S., and the southeast cost of Hawaii. Areas with very high hazard values include the subduction plate boundary regions along the Pacific coasts of southern Mexico, Central and South America, many of the island nations of the southwest Pacific Ocean, and the transform fault and subduction boundary regions of the eastern Mediterranean.

Kaye M. Shedlock, who earned her Ph.D. from MIT, is a Research Geophysicist with the USGS Geologic Hazards Team. Her professional interests include seismic hazard assessments of tectonically active regions, investigations of seismic source zones, global, and investigations of the structure of the crust and lithosphere. Professional activities include Secretary-General, International Lithosphere Program; a 5+ year term as Chief of the USGS Branch of Earthquake and Landslide Hazards and NEHRP Program coordinator for two regions and two disciplines, member Southern California Earthquake Center Advisory Panel, and United States National Member of the PAIGH Commission on Geophysics. She has also served as a member of the National Science Foundation Geophysics Review Panel, a National Science Foundation Presidential Young Investigators Panel; member of the Board of Directors of the Seismological Society of American; Scientific Advisor for Hidden Fury ( a film); Co-editor of several special professional journal issues and Co-editor, Investigations of the New Madrid Seismic Zone. She served as lead author on Earthquakes, a popular USGS special publication and has appeared in several television shows discussing earthquakes. Dr. Shedlock has also made invited presentations to professional societies worldwide and published over 120 technical papers, maps and abstracts.

Archean cratons (~2.5-3.6 Ga) contain none of the plate-tectonic indicators of rifting and convergence such as typify terrains <2.0 Ga. Instead, there are very thick regional successions of submarine mafic and ultramafic lavas and, mostly younger, felsic volcanic rocks and clastic strata. Concurrent with later volcan-ism, domiform granites, mixtures of remobilized basement and partial melts from volcanic rocks, rose, while upper-crust stratified rocks sank as synforms. The middle and deep crust consists of gneisses.

Voluminous ultramafic liquidus lavas require ~50 percent partial melting of mantle rocks, so at least regional magma oceans still existed in Archean upper mantle. Most of the Earth's heat-shedding volcanism must have occurred in the non-preserved regions, where thin crust was quickly returned to hot mantle.The trendy concept of deep-mantle "plumes" is easily disproved for the modern Earth and is unnecessary for the Archean one. The early Earth was hot and fractionated, as in the models of cosmo-logists, not warm and primitive, as in the models of terrestrial geochemists. The Archean Earth had water oceans, but the pre-Archean Earth may have had a very hot 200-bar atmosphere. Perhaps the tonalitic gneisses, to 4.1 Ga, discontinuous basement rocks to Archean volcanic rocks, formed by reaction of mafic magma and this atmosphere.

Warren Hamilton's research career with the U.S. Geological Survey emphasized plate tectonics and crustal evolution. He moved to CSM after retirement and continues active research. He is a member of the National Academy of Sciences, and a holder of the Penrose Medal of the Geological Society of America.

Interpretation of geophysical data in mineral exploration has traditionally been based on visual examination of data images. As the data arises from the measurement of physical fields that are either diffusive or static in nature in most mining geophysical methods, each point in a data image is influenced by the subsurface structure in a large region. Consequently, a data image represents a very convoluted image of the earth, and direct interpretation is often qualitative, difficult, and highly ambiguous. Much effort has been expended in the last decade on applying inverse theory to the data interpretation, and on developing practical and efficient numerical algorithms that reconstruct physical properties from geophysical data. Application of these algorithms has been shifting the interpretation away from the visual inspection of data images and towards the quantitative examination of the physical-property images.

In this presentation, I will outline the basic concept of applied geophysical inversion. This class of inversion algorithms focuses on reconstructing the physical property model by minimizing a model objective function subject to fitting the observed data according to the error estimate. Prior conception of geology and other independent information can be easily incorporated into the inversion so that geologically plausible models are produced. The effectiveness of these algorithms will be illustrated by examples drawn from examples of magnetic, DC resistivity, and induced polarization data acquired in different exploration problems.

Yaoguo Li received his B.A.Sc. in geophysics from the Wuhan College of Geology in China in 1983, and a Ph.D. in geophysics from the University of British Columbia in 1992. He worked with the UBC-Geophysical Inversion Facility at UBC from 1992 to 1999, first as a Post-doctoral Fellow and then as a Research Associate. He is currently an Associate Professor of Geophysics at the Colorado School of Mines and leads the Gravity and Magnetics Research Consortium. He is a co-recipient of the 1999 Gerald W. Hohmann Award. He is a member of the Editorial Board of the Journal of Applied Geophysics. His research interests include inverse theory; inversion of gravity, and magnetic, and electromagnetic data arising from applied geophysics; and their application to resource exploration, environmental, and geo-technical problems.

Aeromagnetic methods have traditionally been applied in exploration and geologic studies to map igneous and metamorphic rocks and structures related to them due to the generally high magnetizations of these rock types. In contrast, magnetizations of poorly consolidated sediments were considered negligible for most applications. With advances in technology, aeromagnetic surveys can now be designed with close line spacings and small terrain clearances in order to detect subtler magnetization contrasts than had previously been targeted.

High-resolution surveys were conducted over the Albuquerque basin using line spacings and terrain clearances of 100-150 m to aid in locating faults that laterally bound different hydrostratigraphic units within the basin sediments. These surveys are unique due to the alluvial environment of the study area, the application of aeromagnetic data for hydrologic purposes, and the tight line spacing, which is narrower than commonly used in modern hydrocarbon and mineral exploration.

Data from the high-resolution surveys show expressions of numerous faults that offset basin fill or volcanic rocks, many of which were previously unknown due to widespread cover. The fault expressions are commonly sinuous and linearly extensive, up to 50 km in length. Detailed examination of the fault expressions in profile form reveals a range of signatures, from symmetric curves with one inflection point to asymmetric curves with multiple inflection points. The symmetric curves match the expected response of fault contacts. The asymmetric curves have a surprising, apparent low over the fault zone that is best explained by a model where units of significantly different thicknesses and/or magnetizations are vertically offset from each other at the fault.

Geologically, this model likely represents syntectonic sedimentation associated with growth faulting. The asymmetric curves contradict common expectations of fault anomalies, may lead interpreters to geologically unreasonable conclusions, and complicate traditional methods to objectively delineate faults from aeromagnetic data, such as the horizontal-gradient method.
V. J. S.

("Tien") Grauch received a BA (1975) in geology from Carleton College and a PhD (1986) in geophysics from Colorado School of Mines. She has been employed by the U. S. Geological Survey since 1977, where she is currently a senior research geophysicist. Her research interests include application and interpretation of aeromagnetic and gravity data to hydrogeologic problems, the relation between magnetic sources and geology, interpretation of aeromagnetic data over rugged magnetic terrain, geophysical investigations of the tectonic controls on ore deposits, and development of new interpretation methods. She is a member of SEG, AGU, GSA, and the SEG Gravity and Magnetics Committee.

Borehole logs, surface seismic, core analysis, and well tests all contribute input data and constraints for 3-D subsurface models that are used for exploration, appraisal, and production of hydrocarbon reservoirs. Geoscientists and engineers nowadays share these models, often referred to as "unified common earth models", to ensure that all members of an asset team have access to all field data. This is of paramount importance because new reservoirs tend to become smaller, tighter, and dirtier. As a result, distinguishing sub-seismic features (laminations / shale pinch outs / small faults / subtle stratigraphic traps), and assessing productivity (relative permeability) are even more critical factors for reservoir development than in the past. The higher resolution both in delineation of reservoir boundaries, and for flow properties is essential to develop marginal prospects economically.

This lecture will give a number of examples of recent developments in formation evaluation that provide this higher resolution, and improve estimates of productivity. Techniques which will be reviewed include dielectric logging, borehole imaging, x-dipole sonic logging, single well seismic, permeability estimates from both Stoneley waves and nuclear magnetic resonance logs, and finally determining reservoir properties from electro-microscope images.

Professor Peeters has held the Distinguished Baker Hughes Chair since 1998. Prior to his tenure at CSM, he was professor of Petrophysics at Delft University, and Petrophysical Adviser at Shell Research in The Netherlands. Peeters received a MSc in Physics of Delft University of Technology in 1 968, and a lecturer at the Royal Dutch Navy Academy. In Shell International he worked in Australia, England, Brunei, California, and Russia. Peeters received the Distinguished Technical Achievement Award from the Society of Professional Well Log analysts in 1996, and is one of the editors of the Petroleum Geoscience Journal. His current research interests are pulsed neutron logging, invasion corrections, and acoustic wave propagation in porous media.