Lisbeth Engell‐Sørensen

University of Bergen

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Seismotectonic significance of the 29 January 1989 Etne earthquake, SW Norway

Terra Nova (1991)

M. Ridvan Karpuz Roy H. Gabrielsen Lisbeth Engell‐Sørensen Karl Anundsen

ABSTRACT The integrated analysis of geological, seismological and field observations with lineament data derived from satellite images allows the identification of a possible seismogenic fault zone for an earthquake which occurred near Etne in southwestern Norway, on 29 February 1989. The hypocentre of the earthquake was located at the mid‐crust at a depth of 13.8±0.9 km which is typical of small intraplate earthquakes. The Etne earthquake occurred as a result of normal faulting with a dextral strike‐slip component on a NW–SE trending fault. Available geological and lineament data indicate correlation of the inferred seismogenic fault with the NW–SE trending Etne fault zone. An aeromagnetic anomaly related to the Etne fault zone forms a regional feature intersecting both Precambrian basement and allochthonous Caledonian rocks. Based on these associations the occurrence of the Etne event is ascribed to the reactivation of a zone of weakness along the Etne fault zone. Slope‐instabilities developed in the superficial deposits during the Etne event demonstrate the existence of potentially hazardous secondary‐effects of such earthquakes even in low seismicity areas such as southwestern Norway.

Lineament

10.1111/j.1365-3121.1991.tb00189.x

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Citations (12)

Estimating background velocity from reflection seismic data — A feasibility study

Springer eBooks (2005)

Lisbeth Engell‐Sørensen T. J. Moser J. Pajchel

Reflection

10.1007/bfb0011771

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Citations (0)

Inversion of arrival times of microearthquake sources in the North Sea using a 3-D velocity structure and prior information. Part I. Method

Bulletin of the Seismological Society of America (1991)

Lisbeth Engell‐Sørensen

Abstract A new method for earthquake source location and velocity structure determination is formulated based on the Bayes theory. The medium is represented by homogeneous layers with plane-dipping interfaces. Inversion of the arrival-time data yields the velocity parameters, the hypocenters, and the origin times of the earthquakes. A linear combination of model parameters is included in the method, and the determination of the standard deviations of the estimated parameters is elaborated. The possibility of constraining a linear combination of two model parameters are used for the P-to-S velocity ratio. Because the travel time cannot be found analytically, the forward problem is solved numerically, namely, by two-point raytracing.

Microearthquake

Arrival time

10.1785/bssa0810041183

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Citations (8)

Linear Inversion as a First Step in Entropy of Image Contrast Optimization

Proceedings (1995)

T.J. Moser Lisbeth Engell‐Sørensen J. Pajchel

Inversion of seismic data usually exhibits a strong nonuniqueness. Constraints are necessary to reduce the large number of images (solutions of inversion) that all satisfy the requirements of minimal data misfit.

10.3997/2214-4609.201409387

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Citations (0)

Inversion for the moment tensor and source location using Love- and Rayleigh-type waves

Springer eBooks (2005)

Lisbeth Engell‐Sørensen G. F. Panza

Earth structure

Rayleigh Wave

Seismic moment

10.1007/bfb0010286

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Refined inversion of potential field data using a relief model

Geoexploration (1983)

Lisbeth Engell‐Sørensen

Potential field

10.1016/0016-7142(83)90023-6

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Inversion of arrival times of microearthquake sources in the North Sea using a 3-D velocity structure and prior information. Part II. Stability, uncertainty analyses, and applications

Lisbeth Engell‐Sørensen

Abstract In Part I of this article, a new method of earthquake source location and velocity structure determination was formulated based on the Bayes theory. In Part II, stability and uncertainty analyses on synthetic data are carried out. The synthetic tests show that, with the type of arrival-time data found in Western Norway, the following can be resolved: the P and S velocities of the second and fourth layers (Moho is the third interface), the dip of the third interface, and the source parameters for the earthquakes. For these parameters initial estimates can be in error of 10 per cent of velocities, 100 per cent of dips, 10 km for hypocenters, and 10 sec for origin times. The remaining parameters had to be fixed within 0.3 per cent. Using arrival times from 23 earthquakes in the Northern North sea, an inversion was made with constraints as found necessary from the synthetic test. The results were as follows: the Mohorovicoc discontinuity has an eastward dip of 3.3 ± 0.6 per cent in the region between the latitude of 59° and 61° and 1.1 ± 0.5 per cent in the region between 59° and 62° north. The Moho dips northward about 1 per cent in the southern region. The upper mantle velocities are 7.94 ± 0.05 km/sec in the southern and 8.05 ± 0.03 km/sec in the northern region. In the source layer (layer 2), the velocity is 6.62 ± 0.03 km/sec and 6.55 ± 0.02 km/sec in the southern and northern regions, respectively. The epicenters moved (in average) 2 km and 5 km to the west and north, respectively, and 2 km toward shallower depths, as a result of the inversion. The average change of origin time was 0.5 sec toward earlier occurrence.

Arrival time

Microearthquake

Discontinuity (linguistics)

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Citations (3)

CSEM Using Ocean Bottom Cables and Streamer

72nd EAGE Conference and Exhibition incorporating SPE EUROPEC 2010 (2010)

Lisbeth Engell‐Sørensen B. A. Farrelly R. Henman S.E. Johnstad K. Kristiansen

Abstract The use of OBC for CSEM is proven to work, and in fact to work better than nodes. Work is now in progress on using a towed streamer or equivalent platform.

10.3997/2214-4609.201400741

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Citations (0)

Optimized 3D Finite Difference Modelling of Basaltic Region

Lisbeth Engell‐Sørensen

P211 OPTIMIZED 3D FINITE DIFFERENCE MODELLING OF BASALTIC REGION Background 1 The paper describes the work carried out in a project between Norsk Hydro GEUS and Parallab. The project is based on the three-dimensional finite-difference code (FD) TIGER made by SINTEF Norway. The FD code was optimized (parallelized) by Parallab to run on parallel super computers (Engell-Sørensen and Koster 2002). The aim of the collaboration was to generate realistic synthetic data to be applied for testing of seismic processing and migration tools for basaltic regions. Forward Method The parallel code enables us to model large-scale realistic geological models and to

Code (set theory)

Finite difference

10.3997/2214-4609-pdb.6.p211

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Low-frequency dispersion and attenuation in anisotropic partially saturated rocks

Geophysical Journal International (2017)

F. Cavallini José M. Carcione Daniel Vidal de Ventós Lisbeth Engell‐Sørensen

The mesoscopic-loss mechanism is believed to be the most important attenuation mechanism in porous media at seismic frequencies. It is caused by P-wave conversion to slow diffusion (Biot) modes at material inhomogeneity on length scales of the order of centimetres. It is very effective in partially saturated media, particularly in the presence of gas. We explicitly extend the theory of wave propagation at normal incidence to three periodic thin layers and using this result we obtain the five complex and frequency-dependent stiffness components of the corresponding periodic finely layered medium, where the equivalent medium is anisotropic, specifically transversely isotropic. The relaxation behaviour can be described by a single complex and frequency-dependent stiffness component, since the medium consists of plane homogeneous layers. The media can be dissimilar in any property, but a relevant example in hydrocarbon exploration is the case of partial saturation and the same frame skeleton, where the fluid can be brine, oil and gas. The numerical examples illustrate the implementation of the theory to compute the wave velocities (phase and energy) and quality factors. We consider two main cases, namely, the same frame (or skeleton) and different fluids, and the same fluid and different frame properties. Unlike the two-phase case (two fluids), the results show two relaxation peaks. This scenario is more realistic since usually reservoirs rocks contain oil, brine and gas. The theory is quite general since it is not only restricted to partial saturation, but also applies to important properties such as porosity and permeability heterogeneities.

Poromechanics

Saturation (graph theory)

Transverse isotropy

Biot number

Mesoscopic physics

10.1093/gji/ggx107

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Citations (9)