
Alfred HanssenUniversity of Tromsø, Tromsø, Norway · Dept. of Geosciences
Alfred Hanssen
PhD
About
99
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Introduction
Additional affiliations
May 2015 - March 2020
May 2015 - March 2020
July 2001 - June 2002
Education
January 1990 - December 1992
August 1983 - June 1988
Publications
Publications (99)
Source-to-sink dynamics are subjected to complex interactions between erosion , sediment transfer and deposition, particularly in an evolving tectonic and climatic setting. Here we use stratigraphic forward modelling (SFM) to predict the basin-fill architecture of a multi-source-to-sink system based on a state-of-the-art numerical approach. The mod...
Shear-wave (S-wave) resonances are typically observed when the surficial marine sediments over a rock substrate have relatively low S-wave velocities. We observe these phenomena using ocean-bottom fiber-optic distributed acoustic sensing (DAS) in two subsea fiber-optic telecommunication cables in Svalbard, Norway. Strong seismic energy from suffici...
Unravelling past, large-scale ocean circulation patterns is crucial for deciphering the long-term global paleoclimate. Here we apply numerical modelling to reconstruct the detailed paleobathymetry-topography of the southwestern inlet of the Barents Seaway that presently connects the Atlantic and Arctic oceans. Subaerial topography was likely enough...
In this study we took an intentionally low-tech approach, aiming to estimate key physical parameters of lake ice using a single, inexpensive microphone. We consider this approach highly relevant to the issue of transport safety and the relatively high number of accidents involving breakthrough failure of thin ice underlines the importance of this t...
The small-aperture Spitsbergen seismic array (SPITS) has been in continuous operation at Janssonhaugen for decades. The high-Arctic location in the Svalbard archipelago makes SPITS an ideal laboratory for the study of cryoseisms, a nontectonic class of seismic events caused by freeze processes in ice, ice–soil, and ice–rock materials. We extracted...
The small-aperture Spitsbergen seismic array (SPITS) has been in continuous operation at Janssonhaugen for decades. The high Artic location in the Svalbard archipelago makes SPITS an ideal laboratory for the study of cryoseisms, a nontectonic class of seismic events caused by freeze processes in ice, ice-soil and ice-rock materials. We extracted a...
Air-coupled flexural waves (ACFWs) appear as wave trains of constant frequency that arrive in advance of the direct air wave from an impulsive source travelling over a floating ice sheet. The frequency of these waves varies with the flexural stiffness of the ice sheet, which is controlled by a combination of thickness and elastic properties. We dev...
Uplift and erosion are complex phenomena in terms of their governing processes, precise timing and exact magnitude. The intricate relationship between different geodynamic processes leading to uplift may increase uncertainties in estimating spatial and temporal patterns. Sediment distribution from uplifted (and eroded) topography and the correspond...
Air-coupled flexural waves appear as wave trains of constant frequency that arrive in advance of the direct air-wave from an impulsive source travelling over a floating ice sheet. The frequency of these waves varies with the flexural stiffness of the ice sheet, which is controlled by a combination of thickness and elastic properties. We develop a t...
A series of transient seismic events were discovered in passive seismic recordings from 2-D geophone arrays deployed at a frost polygon site in Adventdalen, Svalbard. These events contain a high proportion of surface wave energy and produce high-quality dispersion images using an apparent offset re-sorting and inter-trace delay minimisation techniq...
A series of transient seismic events were discovered in passive seismic recordings from 2D geophone arrays deployed at a frost polygon site in Adventdalen, Svalbard. These events contain a high proportion of surface wave energy and produce high-quality dispersion images through an innovative source localisation approach, based on apparent offset re...
Underwater vapor cavities can be generated by acoustic stimulation. When the acoustic signals from several air guns are reflected from the sea surface, the pressure drop at some locations is sufficient for cavity growth and subsequent collapse. In this paper the generation of multiple water vapor cavities and their collapses are numerically modeled...
This paper will investigate how new developments in remote sensing and sensor technologies can be applied to image the structure of the sea ice surface. Both segmentation of multi-polarimetric synthetic aperture radar images and strategies for the analyses of polarimetric SAR data of sea ice are addressed. The analysis is based on a Radarsat 2 PolS...
Traditionally, subsea services such as intervention, inspection, maintenance, and repair, are performed by divers or Remotely Operated Vehicles (ROVs) tethered to, and operated from, a surface vessel. Some of the challenges with this approach are that such surface vessels are associated with high day rates and the availability is typically constrai...
Attenuation of coherent noise, typically weather generated noise and more specifically swell noise, is a major concern in seismic. Such noise, usually characterized by its low-frequency content and high amplitudes, is a common problem in seismic acquisition and, in particular, for marine data. We propose a multiple-input adaptive noise canceller as...
Sparse code shrinkage is a method that is commonly used for image denoising and that has recently found some applications in seismic for random noise attenuation and multiple removal in a simplified form. Sparse coding finds a representation of the data in which each component is only rarely significantly active. Such a representation is closely re...
Stacking is a common technique to improve the signal-to-noise ratio (S/N) and the imaging quality of seismic data. Conventional stacking that averages equally a collection of normal moveout corrected or migrated shot gathers with a common reflection point is not always satisfactory. Instead, we propose a novel time-dependent weighted average stacki...
Attenuation of random noise has been extensively covered by f-x predictive deconvolution, median filtering and decomposition into principal components. We suggest instead using sparse code shrinkage and we propose a method to obtain the necessary noise-free realization of the data. This realization, that is in fact noise-attenuated and that aims at...
IntroductionHarmonizable Stochastic ProcessesStochastic WavefieldsWave DispersionConclusions
AcknowledgmentsReferences
The classical bispectrum based tests for linearity of time series are based on Gaussian asymptotics and a suboptimal smoothing in the bispectral domain. We show that the resulting classical tests may lead to vastly incorrect significance levels for non-Gaussian time series. This implies that a non-Gaussian linear time series may incorrectly be clas...
We present and discuss the properties of a time-domain CSEM (controlled source electromagnetic) technology that utilizes vertically oriented transmitters and receiver antennas. The data are recorded in transient mode, wherein voltage time-series are recorded after transmitter switch-off. A square pulse with an alternating polarity is followed by a...
In this paper we propose a new method for estimating the ambiguity function (AF) of a random process with limited spreading support. The observed process is modelled as the aggregation of a non-stationary signal of interest and noise. As the AF has limited spreading, thresholding is a suitable estimation procedure. Some key stochastic properties of...
Towed arrays of hydrophones are commonly used in marine seismic to infer properties about the sea bot-tom. Normally, air guns are used as impulsive sources of pressure waves, and the hydrophone recordings are employed to estimate three dimensional maps of the ge-ological layer structure of the subsurface sea bottom. In this paper, we present a stat...
The relative height of the seafloor can be estimated by using two vertically displaced receivers. In this paper, we propose techniques to improve the accuracy of the estimated height. Our results are based on the use of synthetic aperture sonar (SAS) imaging, which implies coherent addition of complex images acquired from a moving platform. The SAS...
Complex-valued nonstationary random processes have nonvanishing complementary second-order moment functions. In this paper, we propose generalized dual-frequency and time-frequency coherence functions for harmonizable processes. The proposed generalized spectral coherences are based on widely linear estimators, and they result in coherence measures...
We propose and design two classes of robust subspace classifiers for classification of multidimensional signals. Our classifiers are based on robust M-estimators and the least-median-of-squares principle, and we show that they may be unified as iterated reweighted oblique subspace classifiers. The performance of the proposed classifiers are demonst...
We briefly review how the thermionic valve works, and how it can be used in amplifier circuits. The multitude of non-linearities are identified and briefly discussed in the context of instrument amplifiers. We present a spectral analysis of the output signal from a single-ended class A valve ampilfier for guitar, and demonstrate how very different...
Fractional Brownian motion (fBm) is a ubiquitous nonstationary model for many physical processes with power-law time-averaged spectra. In this paper, we exploit the nonstationarity to derive the full spectral correlation structure of fBm. Starting from the time-varying correlation function, we derive two different time-frequency spectral correlatio...
A complex random vector is called improper if it is correlated with its complex conjugate. We present a hypothesis test for impropriety based on a generalized likelihood ratio (GLR). This GLR is invariant to linear transformations on the data, including rotation and scaling, because propriety is preserved by linear transformations. More specificall...
A complex random vector is called improper if it is correlated with its complex conjugate. In this paper, we present a generalized likelihood ratio test (GLRT) for impropriety. This test is compelling because it displays the right invariances: The proposed GLR is invariant to linear transformations on the data, including rotation and scaling, just...
In this paper, we present an analysis of recordings of a transversal flute made from the dried stem of the Heracleum laciniatum (Tromsoe Palm or Hogweed). While the lower octave exhibits conventional harmonic spectra, the upper octave surprisingly includes subharmonic components. We believe that the subharmonic contributions are due to nonlinear os...
In order to gather information about the physics of the complex plasma crystals from the experimental data, particles have to be tracked through a sequence of images. An application of the Kalman filter for that purpose is presented, using a one-dimensional approximation of the particle dynamics as a model for the filter. It is shown that Kalman fi...
In this brief, we introduce a subspace theory for differential chaos-shift keying (DCSK) systems. We show that DCSK systems operate by transmission of chaotic signals residing in a low-dimensional subspace. The subspace formalism of DCSK schemes leads to the derivation of useful subspace detectors that can be applied to decode the DCSK signal for v...
In this paper, we examine kernel-based estimators for the Kirkwood-Rihaczek time-frequency spectrum of harmoniz-able, nonstationary processes. Based on an inner product consideration, we propose and implement an estimator for the Kirkwood-Rihaczek spectrum. The estimator is con-structed from a combination of the complex demodulate with a short-time...
Fractional Brownian motions (fBm) provide important mod-els for a wide range of physical phenomena whose empiri-cal spectra obey power laws of fractional order. Extensions of fBm to higher dimension has become increasingly impor-tant. In this paper we study isotropic d-dimensional fBm in the framework of inhomogeneous random fields, and we de-rive...
We have analyzed the sound of an electric guitar that has been amplified by a high-quality all-tube amplifier, and emitted by means of a speaker cabinet. We re-amplified a recording of a clean guitar through a state-of-the-art all-tube amplifier at three different preamplifier gain settings: one clean, one half-distorted, and one massively distorte...
Based on the Cramer-Loeve spectral representation for a harmonizable random process, the Rihaczek distribution is a time- and frequency-shift covariant, bilinear time-frequency distribution. It can be expressed as a complex Hilbert space inner product between the time series and its infinitesimal stochastic Fourier generator. We show that we may at...
Fractional Brownian motion (fBm) is a useful non-stationary model for certain fractal and long-range dependent processes of interest in telecommunications, physics, biology, and finance. Conventionally, the power spectrum of fBm is claimed to be a fractional power-law. However, fBm is not a wide-sense stationary process, so the precise meaning of t...
The heavy-tailed multivariate normal inverse Gaussian (MNIG) distribution is a recent variance-mean mixture of a multivariate Gaussian with a univariate inverse Gaussian distribution. Due to the complexity of the likelihood function, parameter estimation by direct maximization is exceedingly difficult. To overcome this problem, we propose a fast an...
The Rihaczek distribution for stochastic signals is a time- and frequency-shift covariant bilinear time-frequency distribution (TFD) based on the Crame´r-Loe`ve spectral representation for a harmonizable process. It is a complex Hilbert space inner product (or cross correlation) between the time series and its infinitesimal stochastic Fourier...
We present a definition of a dual-frequency dual-wavenumber cross-spectrum of two nonstationary and inhomogeneous harmonizable random fields, which is in fact a generalization of the Loeve (dual-frequency) cross-spectrum of two random processes. Furthermore, a geometric argument shows that a proper normalization yields a natural measure of cross-co...
Harmonizable processes constitute an important class of non-stationary stochastic processes. In this paper we study the important extension to multivariate harmonizable random fields. We derive the multivariate-multidimensional Rihaczek spectrum and show that it determines a complex time-frequency varying Wiener filter for approximating a multivari...
In this paper, we propose and design robust subspace detectors for classification of multidimensional subspace signals. Using the principle of M-estimators and least-median-of-squares (LMedS), we formulate the robust subspace detectors as weighted subspace detectors, where we weigh the rows of the measurement matrix prior to the signal matching. Th...
We present a definition of a dual-frequency dual-wavenumber cross-spectrum of two nonstationary and inhomogeneous harmonizable random fields, which is in fact a generalization of the Loeve (dual-frequency) cross-spectrum of two random processes. Furthermore, a geometric argument shows that a proper normalization yields a natural measure of cross-co...
In this paper we consider shape classification of partially occluded objects. We model the occlusion as non-Gaussian noise, and apply robust subspace detectors in the classification module. We show that the robust subspace detectors can be formulated as a weighted subspace detector, and the elements in the boundary vector will be weighted before th...
We have put Hinich's asymptotic tests for Gaussianity and linear-ity under scrutiny, and we show that these tests suffer from severe statistical problems. We propose the use of carefully designed sur-rogate data to ensure correct false alarm rate. Using theoretical considerations about estimation of higher order spectra, we propose new high-precisi...
We present a definition of a dual-frequency dual-wave-number cross-spectrum of two nonstationary and inhomogeneous harmonizable random fields, which is in fact a generalization of the Loeve (dual-frequency) cross-spectrum of two random processes. Furthermore, a geometric argument shows that a proper normalization yields a natural measure of cross-c...
In this paper, we introduce a non-conventional communication technique appropriately named Stochastic Multipulse-PAM (SM-PAM). SM-PAM systems operate by transmission of stochastic processes residing in low-dimensional subspaces. The transmitted signal is thus inherently stochastic, which is beneficial from a low-probability of intercept transmissio...
We have examined certain statistical aspects of seabed images formed by wideband synthetic aperture sonar. In particular, we have formed high-quality data-adaptive non-parametric kernel estimates from the envelope images. In most cases, the estimated tails of the distribution appear to be substantially heavier than those of the Rayleigh-distributio...
Harmonizable processes constitute an important class of nonstationary stochastic processes. In this paper, we present a theory of polyspectra (higher order moment spectra) for the harmonizable class. We define and discuss four basic quantities: the th-order moment function, the th-order time-frequency polyspectrum, the th-order ambiguity function,...
In theory, polyspectra can be applied to solve many important problems in signal processing and data analysis. In practice, however, one has been discouraged by the poor statistical properties of most polyspectral estimators. In this paper, we extend Thomson's original multitaper power (and bispectral) estimator to polyspectra of arbitrary order, a...
Harmonizable processes form an important class of nonstationary stochastic processes. In this paper we present a theory of higher-order moment spectra (polyspectra) for the harmonizable class. We show that there are four basic quantities, the n-th order moment function, the n-th order time-varying polyspectrum, the n-th order ambiguity function, an...
The normal inverse Gaussian (NIG) distribution is a variance-mean mixture of a Gaussian with an inverse Gaussian distribution. The NIG can serve as a model for data that are heavy-tailed (lep-tokurtic), and the model was first introduced in empirical finance by Bamdorrf-Nielsen in 1995. We present the important extension to multivariate NIG (MNIG)...
The Rihaczek distribution was originally proposed as a second-order complex valued time-frequency correlation function for energy signals. We show that the generalization to higher orders and to stochastic processes arises naturally from considerations of the nonstationary moment functions. These so-called higher-order Rihaczek spectra (HORS) are s...
In this paper we introduce the recent normal inverse Gaussian (NIG) probability density as a new model for sparsely coded data. The NIG density is a flexible, four-parameter density, which is highly suitable for modeling unimodal super-Gaussian data.
The spectral properties of low frequency electrostatic waves in the polar cap E region were studied experimentally by instrumented rockets. By comparison of the spectral index for fluctuations in the potential signal and plasma density, evidence is obtained for deviations from Boltzmann distributions in the electron dynamics. Investigations of the...
Low-frequency electrostatic fluctuations detectedby the instruments on the Rocket and Scatter Experiment (ROSE) rockets inthe E region over northern Scandinavia are analyzed.It is demonstrated that the time-resolved statistical properties of the fluctuationsdeviate significantly from those associated with a Gaussian process. The characteristicsof t...
A quadratic Volterra model is used to describe the nonlinear relationship between the sea-waves and a mini-TLP surge drift motion, and the practical problems in estimation of the Volterra model from a limited number of random data is discussed. The main difference between assuming Gaussian and non-Gaussian sea-waves in Volterra estimation are discu...
The normal inverse Gaussian (NIG) distribution is a recent flexible closed form distribution that may be applied as a model of heavy-tailed processes. The NIG distribution is completely specified by four real valued parameters that have natural interpretations in terms of the shape of the resulting probability density function. By choosing the para...
The statistical properties for bicoherence estimation are shown to be strongly connected to the properties of the power- and bispectral estimator used. Data tapering will reduce spectral leakage and frequency smoothing will reduce the variance. It is shown that correct normalization is essential to ensure unbiased results. The multitaper approach i...
We present a definition of wavelet-bicoherence based on waveletpolyspectra. We propose a simple estimator for wavelet-bicoherence, and discuss its statistical properties. In particular it is shown that wavelet-bicoherence estimates has a larger number of effective degrees of freedom than traditional Fourier-based bicoherence estimates. The proposed...
We present a new digital modulation technique that introduces
covertness in digital communications in a simple fashion. The basic
principle is to transmit realizations of a stochastic process in such a
manner that the transmitted waveform appears noiselike. The transmitted
waveform is expressed in a subspace formalism, allowing for an elegant
geome...
We propose a new digital modulation technique that yields a natural resistance against hostile-detection. The transmitted waveform is constructed of orthogonal low-rank stochastic processes, allowing for an elegant geometrical interpretation of the waveform. Channel effects are considered,, and we propose a channel-interference eliminating subspace...
Beginning with the Cramer-Loeve spectral representation for a nonstationary discrete-time random process, one may derive the stochastic Rihaczek distribution as a natural time-frequency distribution. This distribution is within one Fourier transform of the time-varying correlation and the frequency-varying correlogram, and within two of the ambigui...
The normal inverse Gaussian (NIG) distribution has previously been shown to be a versatile tool to model heavy-tailed processes. A cumulant-matching estimator of the NIG parameters was introduced in Oigard and Hansen (2001). Here, we analyze the performance of this estimator. Next, we study whether NIG models are appropriate models for multi-user i...
We present a definition of wavelet bicoherence based on wavelet polyspectra. We propose a simple estimator for wavelet bicoherence, and discuss its statistical properties. In particular it is shown that wavelet bicoherence estimation has a larger number of effective degrees of freedom than traditional Fourier-based bicoherence estimation. The propo...
The bispectral density provides crucial information about
non-Gaussian and/or non-linear properties of stochastic processes. In
practice however, bispectral estimators are prone to be inaccurate and
statistically inconsistent. In this paper we have discussed the
statistical properties of non-parametric direct bispectral estimators.
Several multitap...
We present a stringent definition of higher-order evolutionary
spectra. On this basis, we define wavelet-polyspectral densities as a
way of dealing with non-stationarities in higher-order statistics. We
propose a simple wavelet-polyspectral estimator, and we discuss its
statistical properties. The proposed wavelet-polyspectral analysis tool
is demo...
We present a new digital modulation technique that introduces
covertness in digital communications. The basic principle is to transmit
realizations of a stochastic process in such a manner that the
transmitted waveform appears noise-like. In this paper, we have chosen
to express the transmitted waveform in a subspace formalism. This allows
for an e...
We have compared the bias and variance properties of multitaper
based bispectral estimators, with focus on an adaptive approach.
Numerical results for a white Gaussian process and a nonGaussian AR(2)
process are presented We found that the bias and variance properties of
the multitaper bispectral estimators are governed mainly by a quantity
we call...
We have examined the bias and variance properties of a recently suggested class of multiwindow estimators for autocorrelation functions (ACF). The derived exact expression for the bias is valid for any amplitude distribution, while the derived exact result for the variance is valid for zero-mean Gaussian processes. We show that the multiwindow ACF...
We have compared the bias and variance properties of a multitaper
based class of bispectral estimators, with those of frequency smoothed
biperiodograms, and a Hann-tapered/frequency-smoothed biperiodogram. The
test processes are a white Gaussian process, a Poisson-driven MA(1)
process, and a Poisson-driven AR(2) process. We found that the bias and...
A digital communications keying concept that applies stochastic processes rather than deterministic waveforms is introduced and demonstrated. The technique implies a higher degree of security that conventional digital communications systems, reducing the risk of eavesdropping. The resulting receivers are non-coherent, and simpler than those of spre...
An experimental study of low-frequency electrostatic fluctuations for a plasma produced by a steady state discharge in a magnetized toroidal plasma device without a rotational transform (a simple magnetized torus) is presented. The data is analyzed by means of statistical and spectral signal processing methods. The turbulent modes present in the ex...
We have developed a deceptively simple generalization of the
multitaper spectral estimation technique to the multidimensional case.
The higher-dimensional tapers were constructed as outer products of
lower dimensional and one-dimensional tapers. The two dimensional case
was treated in some detail
We propose a very simple generalization of the one-dimensional multitaper spectral estimation technique to the multi-dimensional case. Multidimensional tapers are constructed from products of one-dimensional taper elements, or formally a outer products of lower dimensional tapers. The two-dimensional case is treated in some detail, and several prop...
An experimental investigation of low-frequency fluctuations in a toroidal plasma device. The spatial dependencies of the fluctuation characteristics is presented.
A modeling procedure for macroscopic systems is proposed which is based on the large separation of space and time scales between the macro scales and the microscopic correlation lengths of the induced Langmuir tubulence excited by instabilities such as SRS, TPD and IADI. As a first example we have modeled in 1D the propagation of the pump wave and...
This is the final report of a three-year Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). This research concentrated on the time dependence of the heater, induced-turbulence, and electron-density profiles excited in the ionosphere by a powerful radio-frequency heater wave. The macroscopic den...