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Panzner M.,Norwegian University of Science and Technology | Weibull W.W.,Norwegian University of Science and Technology | Morten J.P.,EMGS ASA
SEG Technical Program Expanded Abstracts | Year: 2014

In this study we demonstrate how incorporating controlled source electromagnetic (CSEM) and magnetotelluric (MT) data in the velocity model building improves seismic depth imaging in sub-basalt settings. We consider data acquired in the Faroe-Shetland basin. The resistivity model obtained from inversion of the electromagnetic (EM) data provides information about sub-basalt structure and resistivity. The EM data can image both the thickness and the extent of basalt layers, as well as the sediments underneath. We update the velocity model using a correlation between resistivity and seismic velocity derived from nearby well data. Our reverse-time migration (RTM) imaging improvements demonstrate the advantage of this integrated approach. © 2014 SEG.

Senger K.,EMGS ASA | Fanavoll S.,EMGS ASA | Nerland E.,EMGS ASA | Gabrielsen P.,EMGS ASA | And 3 more authors.
3rd EAGE Workshop on Rock Physics: From Rocks to Basin - Applying Rock Physics in Prospect Evaluation and Reservoir Characterization | Year: 2015

In frontier exploration, explorationists often struggle with lack of information, particularly due to lack of well data. Exploration thus relies primarily on elastic (e.g., 2D/3D seismic) and electric (e.g., 3D controlled-source electromagnetic surveys) data sets between the few (if any) wildcat boreholes. Traditional exploration involving rock physics relies almost exclusively on elastic parameters derived from seismic inversion, in itself a challenging task without adequate well control. Inversion of CSEM data, on the other hand, is much more robust in the absence of well control and provides a geologically meaningful representation of sub-surface resistivity. Having two geophysical data sets sensitive to the same subsurface, but relying on different physics, reduces uncertainty in frontier exploration if these are integrated at the interpretation stage. Furthermore, universal rock physical relationships linking elastic and electric properties to various geological parameters (e.g., Hashin-Shtrikman bounds, Archie's Law) can be used to relate the two measurement types to geological drivers, such as porosity and fluid trends. Here we present a simple workflow incorporating both elastic and electric data sets in frontier exploration, combining them using cross-plotting. We illustrate how the workflow can be used on a regional-scale case study where resistivity and velocity maps can be used together to provide a qualitative background porosity map.

Amaya M.,Norwegian University of Science and Technology | Hansen K.R.,EMGS ASA | Morten J.P.,EMGS ASA
Inverse Problems | Year: 2016

For the first time in 3D controlled source electromagnetic data inversion, we explore the use of the Newton and the Halley optimization methods, which may show their potential when the cost function has a complex topology. The inversion is formulated as a constrained nonlinear least-squares problem which is solved by iterative optimization. These methods require the derivatives up to second order of the residuals with respect to model parameters. We show how Green's functions determine the high-order derivatives, and develop a diagrammatical representation of the residual derivatives. The Green's functions are efficiently calculated on-the-fly, making use of a finite-difference frequency-domain forward modelling code based on a multi-frontal sparse direct solver. This allow us to build the second-order derivatives of the residuals keeping the memory cost in the same order as in a Gauss-Newton (GN) scheme. Model updates are computed with a trust-region based conjugate-gradient solver which does not require the computation of a stabilizer. We present inversion results for a synthetic survey and compare the GN, Newton, and super-Halley optimization schemes, and consider two different approaches to set the initial trust-region radius. Our analysis shows that the Newton and super-Halley schemes, using the same regularization configuration, add significant information to the inversion so that the convergence is reached by different paths. In our simple resistivity model examples, the convergence speed of the Newton and the super-Halley schemes are either similar or slightly superior with respect to the convergence speed of the GN scheme, close to the minimum of the cost function. Due to the current noise levels and other measurement inaccuracies in geophysical investigations, this advantageous behaviour is at present of low consequence, but may, with the further improvement of geophysical data acquisition, be an argument for more accurate higher-order methods like those applied in this paper. © 2016 IOP Publishing Ltd.

Rasool J.,Norwegian University of Science and Technology | Hassel V.,Telenor Norway | de la Kethulle de Ryhove S.,EMGS ASA | Oien G.E.,Norwegian University of Science and Technology
Eurasip Journal on Wireless Communications and Networking | Year: 2011

Offering throughput guarantees for cellular wireless networks, carrying real-time traffic, is of interest to both the network operators and the customers. In this article, we formulate an optimization problem which aims at maximizing the throughput that can be guaranteed to the mobile users. By building on results obtained by Borst and Whiting and by assuming that the distributions of the users' carrier-to-noise ratios are known, we find the solution to this problem for users with different channel quality distributions, for both the scenario where all the users have the same throughput guarantees, and the scenario where all the users have different throughput guarantees. Based on these solutions, we also propose two simple and low complexity adaptive scheduling algorithms that perform significantly better than other well-known scheduling algorithms. We further develop an expression for the approximate throughput guarantee violation probability for users in time-slotted networks with the given cumulants of the distribution of bit-rate in a time-slot, and a given distribution for the number of time-slots allocated within a time-window. © 2011, Rasool et al; licensee Springer.

Concepts such as reflections, refractions, diffractions, and transmissions are very useful for the interpretation of seismic data. Moreover, these concepts play a key role in the design of processing algorithms for seismic data. Currently, however, the same concepts are not widely used for the analysis and interpretation of marine controlled-source electromagnetic (CSEM) data. Connections between seismic and marine CSEM data are established by analytically transforming the diffusive Maxwell equations to wave-domain Maxwell equations. Seismic data and wave-domain electromagnetic data are simulated with 3D finite-difference schemes. The two data types are similar; however, the wave-domain electromagnetic data must be transformed back to the diffusive domain to properly describe realistic field propagation in the earth. We analyzed the inverse transform from the wave domain to the diffusive domain. Concepts like reflections, refractions, diffractions and transmissions were found to be valid also for marine CSEM data but the properties of the inverse transform favored refracted and guided events over reflected and diffracted events. In this sense, marine CSEMdata were found to be similar to refraction seismic data. © The Authors.

Danielsen B.E.,EMGS ASA | Madsen H.B.,Eni Norge AS
Near Surface Geophysics | Year: 2013

In rock engineering construction projects it is common to make core drillings to quantify rock quality using different classification systems such as weathering and rock quality designation. The classifications are done on a metre scale by a geologist on site. However this is a subjective assessment where the human factor comes into play. Important observations might be overlooked or interpreted incorrectly. Therefore a more objective method is needed. The scope of this paper is to show how even low degrees of weathering of rocks lowers the resistivity, an effect that otherwise might be overlooked by the geologist because the core appears unaltered and is thus not reflected in the standard rock quality measures. This was done by means of thin-section microscopy, point counting and resistivity logging on gneisses and amphibolites from two drill cores done in connection with the construction of the Hallandsås Tunnel, Southern Sweden. The study showed that the resistivity logs can detect even low grades of weathering of amphibolites, which can be important in determining the mechanical properties of the rock. The result suggests that focusing on the transitions between different lithologies is important because the rocks at the lithological contacts are more susceptible to fracturing, water flooding and weathering. This might not be obvious from the regular geological core description but this is indicated by the resistivity logs. © 2013 European Association of Geoscientists & Engineers.

Mittet R.,EMGS ASA | Morten J.P.,EMGS ASA
Geophysics | Year: 2012

We have developed a formalism to systematically study the sensitivity of the marine controlled source electromagnetic method for hydrocarbon exploration, taking into account measurement errors. We utilize error propagation to estimate the data uncertainty, and find that contributions that scale by the transmitter dipole moment give qualitatively different behavior than noise contributions that are independent of the dipole moment. The uncertainty is used in quantitative criteria to determine if a hydrocarbon reservoir can be detected and whether it can be successfully imaged. Our formalism can be used to make detailed feasibility studies incorporating equipment accuracy limitations, and to identify which experimental component that must be improved to ensure overall improvement in sensitivity. The criteria can be applied to identify the limiting factors for detection and imaging of targets with increasing burial depth. We have defined experimental accuracy conditions for next generation CSEM equipment to successfully image a large hydrocarbon reservoir at 5 km burial depth. © 2012 Society of Exploration Geophysicists.

Maao F.A.,EMGS ASA | Nguyen A.K.,EMGS ASA
Geophysics | Year: 2010

One of the main challenges of using marine controlled-source electromagnetic (CSEM) sounding for hydrocarbon detection has been a relatively low-resolution and restricted depth penetration. In addition, the use of CSEM in shallow waters has been perceived as particularly difficult. A new, robust method for enhancing marine CSEM subsurface response is particularly useful in shallow waters. The method is designed to reduce acquisition imprints and attenuate the dominant primary airwave contribution. Synthetic examples show that it is possible to detect thin, deeply buried 3D resistive bodies in a shallow-water environment with a complex resistivity structure. The results highlight the importance of a sufficient signal-to-noise ratio. © 2010 Society of Exploration Geophysicists.

The computer time required to solve a typical 3D marine controlled-source electromagnetic surveying (CSEM) simulation can be reduced by more than one order of magnitude by transforming low-frequency Maxwell equations in the quasi-static or diffusive limit to a hyperbolic set of partial differential equations that give a representation of electromagnetic fields in a fictitious wave domain. The dispersion and stability analysis can be made equivalent to that of other types of wave simulation problems such as seismic acoustic and elastic modeling. Second-order to eighth-order spatial derivative operators are implemented for flexibility. Fourth-order and sixth-order methods are the most numerically efficient implementations for this particular scheme. An implementation with high-order operators requires that both electric and magnetic fields are extrapolated simultaneously into the air layer. The stability condition given for high-order staggered-derivative operators here should be equally valid for seismic-wave simulation. The bandwidth of recovered fields in the diffusive domain is independent of the bandwidth of the fields in the fictitious wave domain. The fields in the fictitious wave domain do not represent observable fields. Propagation paths and interaction/reflection amplitudes are not altered by the transform from the fictitious wave domain to the diffusive frequency domain; however, the transform contains an exponential decay factor that damps down late arrivals in the fictitious wave domain. The propagation paths that contribute most to the diffusive domain fields are airwave (shallow water) plus typically postcritical events such as refracted and guided waves. The transform from the diffusive frequency domain to the fictitious wave domain is an ill-posed problem. The transform is nonunique. This gives a large degree of freedom in postulating temporal waveforms for boundary conditions in the fictitious wave domain that reproduce correct diffusive frequency-domain fields. © 2010 Society of Exploration Geophysicists.

Mittet R.,EMGS ASA | Gabrielsen P.T.,EMGS ASA
Geophysics | Year: 2013

We describe a data-driven method to estimate the topformation resistivity using marine controlled-source electromagnetic data. The procedure exploits the fact that the airwave contributions are traveling up or down with a Poynting vector close to the vertical axis. The proposed method is based on forming an impedance in the same way as it is done in magnetotelluric processing. The top-formation resistivity is used to perform up-down decomposition of the electric field below the seabed. This procedure suppresses the contribution from the airwave in the resulting upgoing electric field and increases the data sensitivity to the subsurface resistivity distribution. The upgoing electric field is used in the misfit kernel for an inversion scheme. This is a method that is intended for use in shallow water. Inversion using the upgoing electric field means that the observed electric and magnetic fields contribute to the data misfit. The optimization procedure seeks to reduce the misfit between the observed and predicted data for both fields. We compared this procedure with the conventional procedure of predicting electric fields only.The addition of magnetic data improved the resolution for the synthetic and the real data examples. We performed postinversion modeling on the final resistivity models. The weighted misfit of the electric data was added to the weighted misfit of the magnetic data to form the total error. We found that this total error was smallest for the case in which we used the upgoing electric field in the misfit kernel. © 2012 Society of Exploration Geophysicists.

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