Björn Lund

Stress tensor inversion using detailed microearthquake information and stability constraints: Application to Ölfus in southwest Iceland

Using detailed microearthquake data, we present a stress tensor inversion scheme with new methods for selecting the fault planes and allowing for errors in the focal mechanisms. The nonuniqueness of earthquake focal mechanisms is accounted for in our inversion scheme through the introduction into the inversion of a range of well-fitting focal mechanisms for each event. The range of focal mechanisms significantly improves the quality of the estimated stress tensor. Relative localization of clusters of microearthquakes is used to obtain information about which nodal plane could be correct fault plane. The clusters frequently fall on a common fault plane, and if there are acceptable focal mechanisms where one nodal plane has orientation similar to the common plane, we assume this is the correct fault plane for the event. If there is no predefined fault plane, we utilize a simple Mohr-Coloumb failure criterion to obtain a physical choice of fault plane between the two nodal planes in the focal mechanism. The nodal plane with highest relative instability is chosen as the fault plane. Differences between the instability and the standard slip angle criterion are investigated. The new inversion scheme has been applied to microearthquake data from the Olfus area in the vicinity of the southwest Iceland triple junction. We estimate an oblique strike-slip state of stress, maximum horizontal stress at N30°E, and minimum horizontal stress at N60°W, with significant normal faulting influence. The instability fault selection criterion predicts very well the orientation of faults mapped by relative localization.

Wind Retrieval From Shipborne Nautical X-Band Radar Data

In this paper, we study the retrieval of wind information from nautical X-band radar data. In contrast to previous studies, where data from stationary research platforms were used, this study focuses on data from a moving platform, encountering a larger variety of conditions than a platform at a fixed location. Compared to traditional in situ sensors, wind data derived from nautical radar images are much less susceptible to air flow distortion by the platform, since the images cover a large area around the ship. Images collected with a standard nautical HH-polarized X-band radar operating at grazing incidence exhibit a single intensity peak in upwind direction. The wind retrieval method developed here uses a harmonic function that is least-squares fitted to the radar backscatter intensity as a function of antenna look direction. The upwind direction is given by the direction that corresponds to the peak of the fitted function. An empirical model function is derived to retrieve the wind speed from the average radar backscatter intensity. Contrary to wind retrieval methods that have been proposed before, this approach is well suited for data acquired from a moving platform, as it functions well even if the radar field of view is partially shadowed and does not require ship motion correction. Here, we focus on data that were collected during two storms, using the first storm to derive and the second to test the empirical model functions. The method is validated using measurements from two ship-based anemometers.

Marine radar ocean wave retrieval’s dependency on range and azimuth

The strength of the surface wave signal in marine X-band radar (MR) images strongly depends on range and azimuth (i.e., the angle between antenna look and peak wave direction). Traditionally, MR wave analysis is carried out in a set of rectangular windows covering the radar field of view (FOV). The FOV is typically partially obstructed, e.g., due to the coastline or ship superstructures. Especially for ships that are subject to regular course changes, this results in an increased variability or error associated with wave parameters. Using MR measurements from R/P FLIP, acquired off California during the 2010 US Office of Naval Research (ONR) high resolution air–sea interaction (Hi-Res) experiment, this study quantifies the dependency of the radar-based 2D wave spectrum and parameters on range and azimuth. With the help of reference data from a nearby Datawell Waverider buoy, we propose empirical methods to remove the dependency and we illustrate their efficacy.

Wave Measurement Intercomparison and Platform Evaluation during the ITOP (2010) Experiment

AbstractSpectral wave parameters from 11 platforms, measured during the recent Impact of Typhoons on the Ocean in the Pacific (ITOP) experiment, are intercompared. Two moorings, separated by ~180 km, were deployed in a section of “typhoon alley” off the coast of Taiwan for 4 months. Each mooring consisted of an Air–Sea Interaction Spar (ASIS) buoy that was tethered to a moored Extreme Air–Sea Interaction (EASI) buoy. EASI, the design of which is based on the hull of a 6-m Navy Oceanographic Meteorological Automatic Device (NOMAD) buoy, is validated as a 1D wave sensor against the established ASIS. Also, during this time three drifting miniature wave buoys, a wave-measuring marine radar on the Research Vessel Roger Revelle, and several overpasses of Jason-1 (C and Ku bands) and Jason-2 (Ku band) satellite altimeters were within 100 km of either the northern or southern mooring site. These additional measurements were compared against both EASI buoys. Findings are in-line with previous wave parameter interc...

Analysis of Internal Wave Signatures in Marine Radar Data

Satellite images have long been used to study surface manifestations of internal waves (IWs). More recently, marine X-band radar data have been employed to retrieve IW packet parameters. Marine radars have the advantage over satellite systems that their high temporal resolution enables the study of the IW evolution. Until today, no method to automatically detect IW surface signatures in marine radar data has been suggested. In this paper, we present a new fully automated tool to retrieve IW signatures from marine radar image sequences. First, after various preprocessing steps, the IW packet velocity is determined using a combination of localized Radon transform and cross-correlation techniques. Temporal averaging of the marine radar data significantly enhances the IW signatures. The knowledge of the IW packet velocity is used to correct for the IW motion, enabling us to extend the averaging period, which further enhances the IW signal. An IW-motion correction is necessary because, otherwise, the IW signal would become smeared if the averaging period were much longer than the time it takes the IW to propagate between radar resolution cells. The IW-enhanced images are then utilized for the IW signature analysis. Here, we identify local backscatter peaks and exploit the marine radars high temporal resolution to distinguish signal from noise. The resulting series of IW soliton maps provides information on changes in soliton wavelength, velocity, and backscatter intensity. Our marine radar IW signature analysis tool therefore offers a great opportunity of studying the spatiotemporal evolution of IWs as they grow and decay.

On Shipboard Marine X-Band Radar Near-Surface Current ''Calibration''

AbstractThe ocean wave signatures within conventional noncoherent marine X-band radar (MR) image sequences can be used to derive near-surface current information. On ships, an accurate near-real-time record of the near-surface current could improve navigational safety. It could also advance understanding of air–sea interaction processes. The standard shipboard MR near-surface current estimates were found to have large errors (of the same order of magnitude as the signal) that are associated with ship speed and heading. For acoustic Doppler current profilers (ADCPs), ship heading errors are known to induce a spurious cross-track current that is proportional to the ship speed and the sine of the error angle. Conventional mechanical gyrocompasses are very reliable heading sensors, but they are too inaccurate for shipboard ADCPs. Within the ADCP community, it is common practice to correct the gyrocompass measurements with the help of multiantenna carrier-phase differential GPS systems. This study shows how a ...

On recording sea surface elevation with accelerometer buoys: lessons from ITOP (2010)

Measurements of significant wave height are made routinely throughout the world’s oceans, but a record of the sea surface elevation (η) is rarely kept. This is mostly due to memory limitations on data, but also, it is thought that buoy measurements of sea surface elevation are not as accurate as wave gauges mounted on stationary platforms. Accurate records of η which contain rogue waves (defined here as an individual wave at least twice the significant wave height) are of great interest to scientists and engineers. Using field data, procedures for tilt correcting and double integrating accelerometer data to produce a consistent record of η are given in this study. The data in this study are from experimental buoys deployed in the recent Impact of Typhoons on the Ocean in the Pacific (ITOP) field experiment which occurred in 2010. The statistics from the ITOP buoys is under that predicted by Rayleigh theory, but matches the distributions of Boccotti and others (Tayfun and Fedele) (Ocean Eng 34:1631-1649, 2007). Rogue waves were recorded throughout the experiment under various sea state conditions. Recommendations, as a result of lessons learned during ITOP, are made for the routine recording of η which may not add significantly to the existing data burden. The hope is that we might one day collect a worldwide database of rogue waves from the existing buoy network, which would progress our understanding of the rogue wave phenomenon and make work at sea safer.

An investigation into the dispersion of ocean surface waves in sea ice

This investigation considers theoretical models and empirical studies related to the dispersion of ocean surface gravity waves propagating in ice covered seas. In theory, wave dispersion is related to the mechanical nature of the ice. The change of normalized wavenumber is shown for four different dispersion models: the mass-loading model, an elastic plate model, an elastic plate model extended to include dissipation, and a viscous-layer model. For each dispersion model, model parameters are varied showing the dependence of deviation from open water dispersion on ice thickness, elasticity, and viscosity. In all cases, the deviation of wavenumber from the open water relation is more pronounced for higher frequencies. The effect of mass loading, a component of all dispersion models, tends to shorten the wavelength. The Voigt model of dissipation in an elastic plate model does not change the wavelength. Elasticity in the elastic plate model and viscosity in the viscous-layer model tend to increase the wavelength. The net effect, lengthening or shortening, is a function of the particular combination of ice parameters and wave frequency. Empirical results were compiled and interpreted in the context of these theoretical models of dispersion. A synopsis of previous measurements is as follows: observations in a loose pancake ice in the marginal ice zone, often, though not always, showed shortened wavelengths. Both lengthening and shortening have been observed in compact pancakes and pancakes in brash ice. Quantitative matches to the flexural-gravity model have been found in Arctic interior pack ice and sheets of fast ice.

Shipboard wave measurements in the Southern Ocean

AbstractSurface wave measurements from ships pose difficulties because of motion contamination. Cifuentes-Lorenzen et al. analyzed laser altimeter and marine X-band radar (MR) wave measurements from the Southern Ocean Gas Exchange Experiment (SOGasEx). They found that wave measurements from both sensors deteriorate precipitously at ship speeds 3 m s−1. This study demonstrates that MR can yield accurate wave frequency–direction spectra independent of ship motion. It is based on the same shipborne SOGasEx wave data but uses the MR wave retrieval method proposed by Lund et al. and a novel empirical transfer function (ETF). The ETF eliminates biases in the MR wave spectra by redistributing energy from low to high frequencies. The resulting MR wave frequency–direction spectra are shown to agree well with laser altimeter wave frequency spectra from times when the ship was near stationary and with WAVEWATCH III (WW3) model wave parameters over the full study period.

Multi-directional wave spectra from marine X-band radar

The signal measured by heave–pitch–roll directional wave buoys yields the first four coefficients of a Fourier series. Data adaptive methods must be employed to estimate a directional wave spectrum. Marine X-band radars (MRs) have the advantage over buoys that they can measure “model-free” two-dimensional (2D) wave spectra. This study presents the first comprehensive validation of MR-derived multi-directional wave characteristics. It is based on wave data from the 2010 Impact of Typhoons on the Ocean in the Pacific (ITOP) experiment in the Philippine Sea, namely MR measurements from R/V Roger Revelle, Extreme Air–Sea Interaction (EASI) buoy measurements, as well as WAVEWATCH-III (WW3) modeling results. Buoy measurements of mean direction and spreading as function of frequency, which do not require data adaptive methods, are used to validate the WW3 wave spectra. An advanced MR wave retrieval technique is introduced that addresses various shortcomings of existing methods. Spectral partitioning techniques, applied to MR and WW3 results, reveal that multimodal seas are frequently present. Both data sets are in excellent agreement, tracking the evolution of up to 4 simultaneous wave systems over extended time periods. This study demonstrates MR’s and WW3’s strength at measuring and predicting 2D wave spectra in swell-dominated seas.