Leonard J. Srnka

An introduction to marine controlled-source electromagnetic methods for hydrocarbon exploration

Early development of marine electromagnetic methods, dating back about 80 years, was driven largely by defense/military applications, and use for these purposes continues to this day. Deepwater, frequency-domain, electric dipole-dipole, controlled-sourceelectromagneticCSEMmethodsarosefromacademic studies of the oceanic lithosphere in the 1980s, and although the hydrocarbon exploration industry was aware of this work, the shallow-water environments being explored at that time were not ideally suited for its use. Low oil prices and increasingly successful results from 3D seismic methods further discouraged investment in costly alternative geophysical data streams. These circumstances changed in the late 1990s, when both Statoil and ExxonMobil began modeling studies and field trialsofCSEMsurveyingindeepwateraround1000 mordeeper, specifically for characterizing the resistivity of previously identified drilling targets. Trials offshore Angola in 2000‐2002 byboththesecompaniesshowedthatCSEMdatacansuccessfully be used to evaluate reservoir resistivity for targets as deep as several thousand meters. Both companies leveraged instrumentationandexpertisefromtheacademiccommunitytomakeswift progress. The resulting rapid growth in the use of marine EM methodsforexplorationhascreatedademandfortrainedpersonnelthatisdifficulttomeet;nevertheless,atthistime,CSEMdata represent a commercial commodity within the exploration business, and acquisition services are offered by three companies. The ability to determine the resistivity of deep drilling targets from the seafloor may well make marine CSEM the most important geophysical technique to emerge since 3D reflection seismology.

EXPLORATION IN THE BASALT-COVERED AREAS OF THE COLUMBIA RIVER BASIN, WASHINGTON, USING ELECTROMAGNETIC ARRAY PROFILING (EMAP)

The electromagnetic array profiling (EMAP) exploration method was tested at three basalt-covered locations on the Columbia Plateau in central Washington state. Three 16 km survey lines were located to intersect test wells drilled by Shell. The data were EMAP filtered, and inverted using a nonlinear 1-D Riccati approach developed for conventional magnetotelluric (MT) processing. Large and small features on the resulting resistivity cross-sections produced by EMAP filtering near the drill hole closely approximated the electric induction logs. Sedimentary units within the survey area appear to be thin, with some thickening indicated toward the center of the basin. Structuring within the sediments is mild within the limits of the survey, although there is appreciable structure within the Miocene flood basalts on one test line.

Marine controlled-source electromagnetic methods — Introduction

We are very pleased to introduce this special section containing papers related to marine controlled-source electromagnetic (CSEM) methods. As you will read in the first paper, by Constable and Srnka, marine CSEM methods have been used in academia since the early 1980s. However, their application to resource exploration has developed only recently.

Random scattering in magnetotellurics

Typical well logs show substantial variations of formation electrical resistivity over small spatial scales, down to the resolution of the logging tool. Using a plane stratified earth model, we examine the effects of this fine‐scale microstructure on scattering of the naturally occurring electromagnetic (EM) waves used in magnetotellurics. We show how 1-D magnetotelluric (MT) data may be viewed as arising statistically from a smoothed effective medium version of the resistivity‐depth profile. The difference between the data produced by the true medium and the effective medium is attributable to random scattering noise. This noise is fundamental to magnetotellurics and other diffusive‐wave EM exploration methods since it arises from the very small spatial scales that are usually ignored. The noise has unique statistical properties, which we characterize. We show that if scattering is the dominant noise source, a thin layer of increased resistivity at depth can be reliably detected only if the noise statist...

VORTEX: A quasi-steady supermagnetosonic rotating plasma experiment

A rotating plasma experiment has been built which produces a stable, reproducible, super-magnetosonic flow in highly ionized argon. The rotation is produced by a radial electrical discharge across a vertical magnetic field. A preheat discharge gives uniform starting conditions for the flow. The flow velocity exceeds the limit predicted for argon by the Alfven critical velocity theory. Magnetosonic mach numbers up to 2.0 and flow times of 0.5 msec are obtained. The VORTEX plasma is used to model some aspects of the solar wind flow past bodies in the solar system.

Elastic inversion of Gulf of Mexico data

An elastic inversion method baaed upon a wave equation for layered media haa been tested on seismic data from the Gulf of Mexico. Computed P-wave velocity and demity compare favorably with well log measurements at two locations, and show extremely small m&ties in depth. Inverted S-wave velocity was also computed, but no nearby shear velocity logs were available for comparison. traveltime for P-wave reflection at depth z starting from depth zo is

Random Scattering and the Detection Capability of the Magnetotelluric Method

Typical well logs show that formation electrical resistivity varies substantially as a function of depth in the earth. These variations consist typically of slow-scale macroscopic changes or trends, modulated by rapid variations due to fine scale layering. The rapidly varying resistivity fluctuations are significant in amplitude and occur over small spatial scales, down to the resolution of the logging tool. Using a plane stratified earth model, we examine the effects of this fine scale microstructure on the scattering of the naturally occurring electromagnetic waves used in magnetotellurics (MT). We show theoretically how MT data are influenced by the multiscale nature of the formation resistivity. MT data may be viewed as arising largely from a smoothed “effective medium” version of the resistivity vs. depth profile. The difference between the data produced by the actual medium and that produced by the effective medium is due to scattering noise arising from the layering microstructure. We model this fine scale layering as a rapidly varying stochastic process. This scattering noise component of MT data is fundamental since it arises from the very structure of the medium being probed. This noise is substantial at frequencies above ≈0.1Hz and has unique statistical properties, which we characterize. We assess the impact of this noise upon the detectability of a thin layer of increased resistivity at depth. We show that the theory agrees well with Monte Carlo simulations.

Sheath-limited unipolar induction in the solar wind

The collection of charged particles by electrodes in plasmas is controlled by the currentvoltage characteristics of the plasma sheath which forms at the electrode surface. This principle is applied to the steady-state electromagnetic interaction of the solar wind with moon-like bodies, or ‘solid-body’ interactions. In some cases the unipolar dynamo response of an electrically conducting body in the solar wind motional electric field can be controlled by sheath effects. This occurs for highly conducting bodies when the body radiusR is less than a critical valueRc, with the result that no induced bow shock wave can form.For the Moon and MercuryR/Rc≫1, so that sheath effects do not limit their unipolar responses. The asteroids are found to be either too cold or too small to maintain steady-state induced magnetospheres. The Martian satellites, the irregular Jovian satellites, and the outer satellites of Saturn also haveR/Rc≪1. No bow shock waves should be generated by these bodies, unless they are highly magnetized or have large magnetic permeabilities.Unipolar induction heating of meteorite parent bodies in a primordial enhanced solar wind should not be inhibited by sheath effects, providedR≳50 m.

Observation of TM-Mode Induction in a Simulated Solar Wind/Moon Interaction

Studies of the interaction of the solar wind with the Moon show that the lunar magnetic field has both steady and transient components. The transient component represents the electrical response of the Moon to electromagnetic action by the solar wind. This response has two modes: a TE mode, driven by interplanetary magnetic field fluctuations and characterized by electrical currents which circulate in the lunar interior; and a TM mode, driven by the solar wind electric field and its variations, and characterized by currents which flow from the Moon and close in the solar wind. The lunar TE mode is observed, but the TM mode has not been detected. The lack of TM mode response is attributed to a cold, resistive lunar crust, and this absence prevents any study of the influence of a surface sheath on the TM mode currents.

Remote Reservoir Resistivity Mapping - Breakthrough Geophysics For The Upstream

Abstract Marine controlled-source electromagnetic surveying has emerged as a new tool for remotely detecting reservoired hydrocarbons offshore. The technology was pioneered by university and government researchers over the past 25 years, and recently has benefited from development by contractors and the oil industry, including ExxonMobil. When integrated carefully with other geoscience information, primarily seismic, marine CSEM shows promise for adding considerable value in Upstream applications. Remote reservoir resistivity detection and imaging results from a recent ExxonMobil survey demonstrate this technology. Early Marine CSEM Development Offshore CSEM research began in the 1920s with studies of seafloor power cables, and saw the first commercial minerals survey offshore Cornwall, England a decade later. The research that led to todays commercial methods began in earnest much later, primarily at the British Geological Survey and UCSD Scripps Institution of Oceanography in the 1970s. Many other university and government groups started research on the technology about that time, and many continue to the present. Exxon began investigating marine CSEM in early 1981 using theory and computer modeling, and scoped the feasibility of field tests. Results looked promising for very deep water applications. However, the lack of suitable acquisition equipment, limited deep water opportunities, and the emphasis on the (then) new 3D marine seismic technology resulted in deferring further marine CSEM research.