3 Lecture Topics

Fall 2016 SEG/AAPG Distinguished Lecturer

Steven Constable

Scripps Institution of Oceanography

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Abstract

Geophysical Inversion: Which Model Do You Want?

With a broad suite of geophysical inversion tools now available, it is seductively easy to submit your data, turn the crank, and obtain a model. But is the model meaningful? Are the data properly fit? How much does the model depend on the data, and how much on parameters in the inversion code, such as model discretization and regularization penalty? The inversion process depends as much, if not more, on the error structure of the data and inversion parameters as the data themselves. We all know that geophysical inversion is nonunique, but many people are surprised just how different models can be that fit the data equally well. And what exactly constitutes an adequate fit to the data? Without a rigorous analysis of error structure, choice of misfit can be highly subjective. Some rely on “Lcurves”, but it can be shown that they too are subjective, and depend very much on the choice of plotting parameters. Seeking to drive misfit down as low as possible can also be perilous – the least squares best fitting models for some problems are known to be pathological, and it is likely that this is true in general. In this lecture I shall attempt to provide an understanding of the practical issues associated with geophysical inversion, and provide a roadmap for avoiding common pitfalls.

Mapping Gas Hydrate using Electromagnetic Methods

Gas hydrate is found globally on the continental shelves and is important as an unconventional hydrocarbon source, a hazard to drilling and seafloor infrastructure, a potential source of potent greenhouse gas, and a confounding resistor in the interpretation of conventional marine EM data. Yet, estimates of global hydrate volume vary by three orders of magnitude and identified recoverable reserves are rare, the reason being that it is difficult to image hydrate using seismic methods alone. However, gas hydrate is highly resistive and presents a good EM target at high saturations. Conventional controlledsource EM (CSEM) methods can be used to image hydrate, but is inefficient because seafloor receivers need to be closely spaced to achieve the appropriate resolution in the upper hundreds of meters of the seafloor. Several groups, including Scripps Institution of Oceanography, have developed towed CSEM systems designed to map hydrate in deep water, and such equipment is now being used commercially to image hydrate with a potential for methane production. In this lecture I will describe marine gas hydrate, laboratory studies of its electrical properties, and the equipment that we use to image it, with case studies from offshore California and the Gulf of Mexico.

Marine EM: The Past, The Present, and The Future

The high cost of deepwater exploration motivated the development of commercial marine magnetotelluric (MT) exploration in 1995, but it wasn’t until marine controlledsource electromagnetic (CSEM) methods burst upon the industry scene with the formation of three new contractors in 2002 that things got really exciting. Now the bubble has burst and the excitement has diminished, but marine EM remains an important tool for offshore exploration. Early mistakes were made as a result of poor instrumentation and a lack of good interpretation tools unlike seismics, EM relies heavily on inversion to produce useful results but both equipment and inversion codes have improved significantly. Still, EM images resistivity, not hydrocarbon content, and false positives occasionally occur, but false negatives are rare. That is, without an EM signature there is little chance of discovering economical hydrocarbons. In this lecture I will review the history, discuss the 10 important things you need to know about marine EM, and look to the future of the method.

Biography

Constable

Steven Constable studied geology at the University of Western Australia, graduating with first class honors in 1979. In 1983 he received a Ph.D. in geophysics from the Australian National University for a thesis titled “Deep Resistivity Studies of the Australian Crust” and later that year took a postdoc position at the Scripps Institution of Oceanography, University of California San Diego, where he is currently Professor of Geophysics. Steven is interested in all aspects of electrical conductivity, and has made contributions to inverse theory, electrical properties of rocks, mantle conductivity, magnetic satellite induction studies, global lightning, and instrumentation. However, his main focus is marine electromagnetism; he played a significant role in the commercialization of marine EM for hydrocarbon exploration, work that was recognized by the G.W. Hohmann Award in 2003, the 2007 SEG Distinguished Achievement Award, and now the SEG 2016 Reginald Fessenden Award. He also received the R&D 100 Award in 2010, the AGU Bullard Lecture in 2015, followed in 2016 by being named Fellow of the AGU. More recent efforts have involved the development of equipment to map gas hydrate and permafrost. Steven has served as an associate editor for the journal Geophysics, as a section secretary and corresponding editor for the American Geophysical Union, and on the MARELEC steering committee.

Date Location Host
5 Aug 2016 Tokyo, Japan

SEG Japan

7 Aug 2016 Akita, Japan

Akita University Geophysical Society

10 Aug 2016 Kyoto, Japan

Kyoto University Geophysical Society

12 Aug 2016 Hanoi, Vietnam

Vietnam Association of Geophysicists

20 Sep 2016 Sydney, NSW, Australia

University of Sydney Geophysical Society

21 Sep 2016 Canberra, ACT, Australia

ASEG ACT

22 Sep 2016 Brisbane, QLD, Australia

AASEG QLD

26 Sep 2016 Melbourne, SA, Australia

ASEG VIC

27 Sep 2016 Adelaide, SA, Australia

ASEG SA/NT

28 Sep 2016 Adelaide, SA, Australia

University of Adelaide Geophysical Society

29 Sep 2016 Perth, WA, Australia

ASEG WA

30 Sep 2016 Perth, WA, Australia

University of Western Australia Geophysical Society

11 October 2016 Santa Cruz, CA, USA University of California Santa Cruz, Whole Earth Seminar
8 November 2016 Paris, France Institut de Physique du Globe de Paris
9 November 2016 Oslo, Norway Oslo Society of Exploration  Geophysicists
11 November 2016 Berlin, Germany Deutsche Geophysikalische Gesellschaft (DGG) and Freie University Student Geoscientific Society
14 November 2016 Milan, Italy Sezione Italian EAGE-SEG
15 November 2016 Trieste, Italy University of Trieste - Licio Cernobori Student Chapter & OGES
12 January 2017 Golden. CO. USA Colorado School of Mines Heiland Lecture Series

SEG Distinguished Lecturers
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