The Lofoten Maelstrom has been known for centuries as one of the strongest open-ocean tidal currents in the world, estimated to reach 3 m s −1 , and by some estimates as much as 5 m s −1 . The strong current gives rise to choppy seas when waves enter the Moskenes Sound, making the area extremely difficult to navigate. Despite its reputation, few studies of its strength exist and no stationary in situ measurements for longer time periods have been made due to the challenging conditions. By deploying for the first time in situ wave and current instruments, we confirm some previous estimates of the strength of the current. We also show that its strength is strongly connected with wave breaking. From a consideration of specific forcing terms in the dynamical energy balance equation for waves on a variable current, we assess the impact of the underlying current using a convenient metric formulated as a function of the horizontal current gradients. We find that the horizontal gradients are a likely explanation for the observed enhanced wave breaking during strong currents at a rising tide.

Nortek has developed a new Doppler velocity log (DVL) based on a novel bottom detection principle. This allows for the ability to estimate individual beam and ping Doppler measurement noise in real time. Another capability that has been embedded in the new DVL is an accurate timing reference of the velocity estimate. Using the Kongsberg Maritime (KM) HUGIN autonomous underwater vehicle (AUV) as testbed, we present in this work sea trial results for the new Nortek DVL.

ABSTRACT A typical oil spill recovery vessel has been historically outfitted with an oil spill detection (OSD) radar. During an oil spill recovery operation, there is a dedicated operator who is responsible for interpreting information from the radar image. Industry developments over the last several years now require that an OSD radar automatically detect and track an oil spill. There are two primary needs driving this development. The first is that OSD systems and operations are becoming more sophisticated; automatic OSD aids for a more efficient oil spill operation where an operator's attention may be directed to a potential spill. The automatic OSD also aids a multi-sensor system; one such example is where an OSD radar is used to steer an IR camera to a candidate spill for more detailed evaluation or validation. The other primary driver for automatic OSD is for monitoring systems, which serve for early warning. Monitoring systems may be found along coastal installations or oil platforms. The automatic spill detection functionality of an OSD system may be implemented in different levels of sophistication. Perhaps the simplest configuration is one that uses fixed thresholds relative to the image for alarming whether a region in a radar image is a spill or not. The benefit of simple threshold detector is that it is easy to implement in software. The weakness is that it is prone to both lower overall detection rate and high false alarm rate. A more robust automatic spill detection method is one that treats it as an image-processing problem. The paper here presents a model based OSD. Generation of confidence maps is central to the method and provides an indication of the likelihood of oil. Inputs to the confidence maps come from multiple sources, several of which are based on uniquely constructed models. Among these is a histogram comparator, which scans a radar image and compares the data to reference models from real oil spills. A discussion of the methods used focuses on (a) the necessary steps prior to the confidence map construction, (b) how the confidence maps are layered with inputs, (c) how the information in the confidence maps is transitioned into the detection of oil, (d) and finally alarming.

Nortek provides a combined wave and current profiling instrument in the form of the AWAC. This variant of the traditional ADCP has managed to circumvent the classic limitations of measuring short waves in deep waters by introducing a vertical beam that directly measures the height of the water-air interface (waves) above the instrument. This same vertical beam has also demonstrated that it is capable of measuring the distance to the water-ice interface, and as a result can be used as means to estimate ice draft or ice thickness. Measurement campaigns that have taken place in relatively shallow coastal waters (less than 50 meters) are finding themselves more often in deeper waters. These deployments are also occurring at more extreme latitudes where the presence of ice is more common. This means that common measurement requirements now include measuring ice thickness in addition to current profiles and directional wave observations. Nortek has addressed this need of measurements in deeper waters by building upon the success of the AW AC. A 400 kHz AWAC has been developed and intended for deployment depths of 100 meters. The 400 kHz AWAC is outfitted with a temperature compensated pressure sensor and firmware with a dual functioning surface tracking measurements for ice and waves. The wave burst measurement contains detection methods for both water-air interface and water-ice interface; this means the AWAC can transition seamlessly from wave measurements in the summer to ice measurements in the winter. Ice thickness data is presented for two AWACs deployed in the Beaufort Sea, Alaska. These data are compared to an ASL ice profiler. AWAC data was collected in a special diagnostic mode and allowed for the user to select the best water-ice interface detection method in post-processing. The results show that there is favorable agreement between the AWAC and the ASL ice profiler. This data set was useful in developing the now existing firmware used to detect the water-ice interface. These shallow water data did not illustrate errors associated with an unknown speed of sound profile which is identified as the primary source of error for the AST distance measurement. Additionally, wave results are presented for the 400 kHz AWAC and compared to a 600 kHz AWAC, which was deployed in the vicinity. The data show that non-directional estimates agree well, however directional estimates are complicated by the fact that the two AWACs were deployed at different locations and exposed to different wave directions due to local refraction. It is clear that another comparison test needs to be conducted with the reference instrument collocated with the 400 kHz AWAC.

Directional wave measurements in deep water locations are intrinsically difficult to measure without the use of a surface wave buoy. Traditional acoustic Doppler current profilers do not have the appropriate data collection and processing technique to be mounted on a subsurface buoy. Nortek developed the SUV wave data collection and processing technique for measuring ocean waves from a subsurface buoy using a Nortek acoustic wave and current profiler (AWAC). In 2006 Nortek initiated a collaborative experiment to validate the SUV method and explore mooring performance by deploying two Nortek AWACs on different shape subsurface buoys offshore of Lunenburg Bay, Nova Scotia, Canada. A surface wave buoy was located nearby as an independent reference. The AWACs were deployed from September to November 2006 and measured waves over 4 m in significant wave height during three storms. The results indicate that the acoustic surface tracking (AST), used to measure non-directional wave properties, was a robust technique and worked very well with the AWACs deployed on a subsurface buoy. Greater than 99% of all AST measurements passed the quality control checks (comparable to results from a bottom mounted AWAC) and measurements of wave height and period were in excellent agreement with the surface wave buoy. The wave directional estimates were in good agreement with the surface wave buoy, but indicated clear frequency bands with increased directional uncertainty. An analysis of buoy motion suggests that the frequencies of poor directional estimates are coincident with the natural frequency of the mooring system. Guidance is offered to design a subsurface buoy which has a natural frequency outside of the wave band such that this technique may be used widely for offshore directional wave measurements.

Introduction A new method for making directional wave measurements using an acoustic Doppler current profiler has been developed. The configuration involves mounting the instrument on a subsurface buoy which allows it to move freely. People familiar with field measurements know that deploying a current profiler from a subsurface buoy is nothing new. However, only current profile measurements have been truly successful with off-the-shelf equipment and processing methods. Until now, routine wave measurements have not been possible with current profilers mounted on subsurface buoys.

Directional wave measurements in deep water locations are intrinsically difficult to measure without the use of a surface wave buoy. Traditional acoustic Doppler current profilers do not have the appropriate data collection and processing technique to be mounted on a subsurface buoy. Nortek developed the SUV wave data collection and processing technique for measuring ocean waves from a subsurface buoy using a Nortek acoustic wave and current profiler (AWAC). In 2006 Nortek initiated a collaborative experiment to validate the SUV method and explore mooring performance by deploying two Nortek AWACs on different shape subsurface buoys offshore of Lunenburg Bay, Nova Scotia, Canada. A surface wave buoy was located nearby as an independent reference. The AWACs were deployed from September to November 2006 and measured waves over 4 m in significant wave height during three storms. The results indicate that the acoustic surface tracking (AST), used to measure non-directional wave properties, was a robust technique and worked very well with the AWACs deployed on a subsurface buoy. Greater than 99% of all AST measurements passed the quality control checks (comparable to results from a bottom mounted AWAC) and measurements of wave height and period were in excellent agreement with the surface wave buoy. The wave directional estimates were in good agreement with the surface wave buoy, but indicated clear frequency bands with increased directional uncertainty. An analysis of buoy motion suggests that the frequencies of poor directional estimates are coincident with the natural frequency of the mooring system. Guidance is offered to design a subsurface buoy which has a natural frequency outside of the wave band such that this technique may be used widely for offshore directional wave measurements.

Numerous wave measurement stations in our coastal communities measure waves in shallow waters, close to structures, where the “incident” waves are transformed by the interaction with these structures. It would be advantageous to parse out incident from transformed waves. We present a technique to separate the incident and reflected waves through an example of incident waves reflecting from a breakwater. Field data was collected with a 1 MHz AWAC. The starting point for this analysis is the full directional spectra, which is a description of the energy as a function of both direction and frequency.

The last 10 years have seen the development of a new class of acoustic Doppler systems that can measure the wave directional spectrum in addition to their classical current profiler capability. Among these systems is the Nortek AWAC (Acoustic Waves and Current), which uses three acoustic beams angled at 25 degrees from vertical for wave direction estimation and profiling current velocities. A fourth beam pointed towards the sea surface is dedicated to measuring surface displacement. This particular system has been the subject of extensive studies worldwide. As a result, we have been able to quantify the AWAC's performance characteristics compared to conventional wave buoys and other forms of wave measurements. Historically, wave data have typically been represented in the form of a series of parameters that characterize the spectral and directional nature of the sea state (e.g. significant wave height, peak period, mean period, peak direction and mean direction). In many applications, such as structural response models or in studies of coastal sediment or pollutant transport in areas with variable bottom topography, this single-parameter representation is not sufficient. Instead, a more comprehensive parameterization is required where the wave data are separated into frequency bands and the wave energy and direction is provided as a function of each band. In turn, this puts more stringent requirements on the accuracy of the wave sensing system itself, which not only has to work in a bulk sense, but also has to be able to provide a true description of the sea state even if the energy content in a given frequency band is quite low. During a recent study near the Diablo Canyon, California, a Datawell Waverider buoy was located only 20 m away from an AWAC deployed in 25 m of water. In this paper, we will describe the results of the comparison between the two instruments and discuss the implications for the possibility of using the AWAC small scale wave array to characterize long waves.

An alternative to the Maximum Likelihood Method (MLM) for directional wave processing is presented for Doppler current profiling type of instruments. The new solution follows a standard triplet analysis for wave directional analysis. The solution uses elements of classic PUV processing as well as the latest in Acoustic Surface Tracking (AST) technology. This new hybrid solution is called the SUV method. One specific advantage is that the SUV solution circumvents the MLM constraint that the Doppler profiler must be static and not moving during the ensemble measurements. The SUV method allows for measurements from a rotating platform such as a subsurface buoy. Results from a directional Waverider (DWR) are compared for a stationary Doppler profiler (AWAC) using the MLM and SUV methods. INTRODUCTION Wave measurements from bottom mounted acoustic Doppler current profilers have circumvented limitations associated with the traditional PUV approach (pressure and horizontal velocity measurements near the instrument) by remotely measuring wave orbital velocities close to the free surface. Here the depth attenuation in the signal is less of a problem, resulting in measurements covering a larger wave frequency range. Thus, acoustic Doppler systems can be mounted at larger depths than the PUV instruments. In addition, the systems are able to measure the average current profile. This effectively provides two measurements from the same instrument. The Nortek AWAC (Acoustic Wave and Current Profiler) is in this class of Doppler current profilers using the MLM for wave measurements. It performs these measurements using a combination of three slanted acoustic beams, which are symmetrically positioned about the center and angled 25 degrees from the vertical. A vertical fourth beam is dedicated to acoustic surface tracking (AST), which provides direct estimates of the surface elevation. 1 Nortek AS, Vangkroken 2, 1351 Rud, Norway, inquiry@nortek.no 2 Dept. of Mathematics, NTNU, Trondheim, Norway, harald.krogstad@math.ntnu.no

Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to inferring wave estimates from truncated wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combined pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and employ the maximum likelihood method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 20 meters or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The introduction of acoustic surface tracking (AST) with the vertical acoustic beam has permitted the AWAC to measure waves in deeper waters with greater accuracy and extended frequency response. This work provides a closer look at the frequency response of the AST and when it is permissible to use it to determine water level.

Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to interfering wave estimates from wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combine pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 10 metres or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The following paper provides an overview of the process of (1) developing the surface track algorithms, (2) comparing with a Datawell wave buoy off the coast of Carqueirance, France (3) and testing limiting conditions such as breaking waves and greater depths (35 metres).