Åge Kristensen

Density- and sound speed contrasts in sub-Arctic zooplankton

SummaryThe sound speed was determined for Meganyctiphanes norvegica, for a mixture of Thysanoessa raschii and Thysanoessa inermis and for a mixture of Calanus finmarchicus and Calanus hyperboreus. The sound speed contrasts ranged from 1.014 to 1.044. Seasonal variations in specific density were measured for Thysanoessa inermis, Thysanoessa raschii, Meganyctiphanes norvegica, Calanus finmarchicus and Calanus hyperboreus. The density of 20 mm T. inermis was lowest in November (1.052 g/cm3) and highest in February–March (1.065 g/cm3). For a 20 mm T. raschii the minimal density was determined in December (1.059 g/cm3) and the maximum in February–March (1.074 g/cm3). M. norvegica individuals of 35 mm also had their lowest density in December (1.060 g/cm3), but reached their maximum density in July (1.076 g/cm3).The density of the euphausiids was found to be size dependent. The density increases as the size decreases. C. finmarchicus and C. hyperboreus had densities less than seawater (1.026 g/cm3) during most of the year. Just before spawning the density increased to 1.028 g/cm3 and 1.036 g/cm3 for C. finmarchicus and C. hyperboreus respectively. The seasonal variations of the density were closely related to the lipid content of the animals.

The Statfjord 3-D, 4-C OBC survey

This paper presents initial processing and interpretation of 3-D multicomponent seismic data acquired over parts of Statfjord Field in the last quarter of 1997. Statfjord Field, in the Tampen Spur area straddling the British/Norwegian border in the northern portion of the Viking Graben, covers about 500 km2 and has estimated oil reserves of 674×106 m3. Production started in 1979. The reservoir units are Jurassic sandstones of the Brent Group, Dunlin Group, and Statfjord Formation.

Modified Thomson–Haskell Matrix Methods for Surface-Wave Dispersion-Curve Calculation and Their Accelerated Root-Searching Schemes

In this paper we introduce two modified Thomson–Haskell matrix (TH) methods for dispersion-curve calculation of the Rayleigh and Love waves: the doubled Thomson–Haskell matrix method (DTH) for the Rayleigh wave and the normalized Thomson–Haskell matrix method (NTH) for the Love wave. Both DTH and NTH keep the brevity of the original TH method. They remedy the high-frequency numerical instability of the dispersion-curve calculation of surface waves by compulsorily setting some positive exponential terms to be zeros and doing some proper normalization. The validity of these methods is tested. The capability of these methods to calculate the dispersion curves in high frequencies is illustrated. We also introduce an auxiliary function to accelerate the calculation of the dispersion curves of the surface waves. Compared with the conventional root-searching scheme, our scheme apparently speeds up the root search. Both of the modified TH methods are deduced in vertical transversely isotropic (VTI) media. The impact of surface-wave dispersion due to VTI anisotropy is investigated. The modified TH methods and their root-searching schemes are expected to be a useful tool for investigating the anisotropy and simulating complete seismograms of VTI media.

Acoustic assessment of krill stocks in Ullsfjorden, north Norway

Abstract An acoustic assessment of the krill stock based on estimates from a back-scattering model is presented. The model is empirical and based on measurements of the back-scattering intensity, the physical properties, and orientational characteristics of krill. Two main swarms were recorded in the fjord. The bigger swarm was c. 7 km long, with concentrations of 2000 krill/m2 while the other was c. 6 km long, with up to 1000 krill/m2. Within these swarms the krill formed distinct layers with up to 75 krill/m3.

Multicomponent Seabed Seismic Data: A Tool for Improved Imaging and Lithology Fluid Prediction

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s).

Sparse CSEM inversion driven by seismic coherence

Marine controlled source electromagnetic (CSEM) data inversion for hydrocarbon exploration is often challenging due to high computational cost, physical memory requirement and low resolution of the obtained resistivity map. This paper aims to enhance both the speed and resolution of CSEM inversion by introducing structural geological information in the inversion algorithm. A coarse mesh is generated for Occams inversion, where the parameters are fewer than in the fine regular mesh. This sparse mesh is defined as a coherence-based irregular (IC) sparse mesh, which is based on vertices extracted from available geological information. Inversion results on synthetic data illustrate that the IC sparse mesh has a smaller inversion computational cost compared to the regular dense (RD) mesh. It also has a higher resolution than with a regular sparse (RS) mesh for the same number of estimated parameters. In order to study how the IC sparse mesh reduces the computational time, four different meshes are generated for Occams inversion. As a result, an IC sparse mesh can reduce the computational cost while it keeps the resolution as good as a fine regular mesh. The IC sparse mesh reduces the computational cost of the matrix operation for model updates. When the number of estimated parameters reduces to a limited value, the computational cost is independent of the number of parameters. For a testing model with two resistive layers, the inversion result using an IC sparse mesh has higher resolution in both horizontal and vertical directions. Overall, the model representing significant geological information in the IC mesh can improve the resolution of the resistivity models obtained from inversion of CSEM data.

Image-guided Regularized Marine Controlled Source Electromagnetic Inversion

We present an image-guided regularization inversion method for marine controlled-source electromagnetic data. The regularization method is that the electrical parameters have a structure similar to the geological features or the structure observed from seismic image. A classic regularization example is the roughness penalty applied by Occam’s inversion. The image-guided regularization consists of three major steps. First, the inversion mesh is conformed the geometry derived from geological image. Second, metric tensors fields are calculated from the geological image. Third, non-Euclidean distance of neighbor elements is computed to replace the spatial distance for roughness penalty. This regularization in the Occam’s inversion encourage the smoothing direction following the geological features. The neighbor cells with the similar metric tensor provide a strong model weight to smooth the resistive image. In this paper, a synthetic example proves the CSEM inversion can be improved by the image-guided regularization. In this way, an approach for incorporating seismic constraints into EM inversion is successful applied by using a non-Euclidean distance.

A New Stabilizer for ERT Inversion

Smoothness inversion under L2 norm regularization does normally not provide enough resolution for certain inversion problems, especially for models with interface reconstruction and edge preserving. In this paper we propose a new stabilizer based on the same mechanism as the minimum gradient support (MGS) stabilizer. Combined with nonlinear conjugate gradient method, the new stabilizer gives excellent results for electric resistivity tomographyic (ERT) inversion problem. The comparison of the new stabilizer and MGS stabilizer is also discussed.

Comparison of finite‐difference and fast‐field modeling

The finite‐difference technique is used to model Scholte wave propagation excited by a P‐wave source in the water layer. The finite‐difference approach encompasses models in which the parameters are allowed to vary with position both vertically and laterally, and is well suited for studies of the acoustic field at and below the seafloor. The finite‐difference algorithm implemented is a 2‐D acceleration‐stress staggered grid scheme, of second order in time and order 2L in space. The formulation results in a simple numerical implementation of irregular liquid–solid interfaces. To investigate numerical issues which require special attention when implementing finite‐difference schemes, the finite‐different seismograms are compared with seismograms modeled by a fast‐field technique valid for range‐independent models. In particular, the methods are compared for models containing sharp liquid–solid interfaces representative of the seafloor. Numerical analysis indicates that the liquid–solid interface is located ...