Warren W. Denner

Higher frequency ambient noise in the Arctic Ocean

Higher frequency (1000 Hz) arctic ambient noise episodes during nonsummer months were used to study generating mechanisms. In most cases in which noise amplitudes were relatively high (>103 μPa), thermal fracturing of sea ice was responsible. Numerical simulations with a daily heating cycle and no snow cover implied that maximum noise occurred at 1900 h local. Radiational heat balances were more important than sensible heat flux in producing fracturing of sea ice. With snow cover, a daily heating cycle produces maximum fracturing at 0300–0800 h local, a common feature seen in noise data. Fracturing estimated from numerical simulations using observed arctic heat flux parameters was in good agreement with the observed 1000‐Hz noise signal. However, results indicate that blowing snow and ice fog may be additional factors in the heat flux balance of sea ice and the generation of arctic ambient noise. The short space scales of higher frequency noise are likely a result of spatial variations in snow cover. The ...

Arctic ambient noise in the Beaufort Sea: Seasonal relationships to sea ice kinematics

Various modes of sea ice motion in the Beaufort Sea are correlated with under‐ice noise at 10, 32, and 1000 Hz. Seasonal variations are considered, and a parameterization of ice microfracturing caused by sensible heat flux is included in the correlations. During the summer, the correlations indicate that all frequencies are primarily a response to the ice rushing through the water. During the fall, 10‐ and 32‐Hz noise correlate best with a linear combination of the speed of the ice parcel plus the total rate of change in the shape of the ice parcel. This latter factor indicates noise generation due to the individual ice floes moving past one another as they rearrange into new shapes. The correlations indicate differential motions of other forms (primarily ice convergence) become important in generating lower frequency noise only during winter. As for 1000 Hz during fall and winter, the correlations are low and the results are ambiguous. Several factors are discussed that emphasize our lack of knowledge ab...

Arctic ambient noise in the Beaufort Sea: seasonal space and time scales

Data from the Beaufort Sea are used to quantify some of the seasonal characteristics of under‐ice ambient noise. Noise at 10, 32, and 1000 Hz was studied and seasonal space and time scales were calculated. An interesting result is that summer 10‐Hz noise apparently can often be contaminated by cable strum. The summer period is also the only time during which 1000‐Hz noise is almost totally a product of ice motion. Within the central Beaufort Sea, lower frequency noise tends to have e‐folding space scales of the order of 1000 km. This is true for 1000‐Hz noise only during summer, with space scales for the rest of the year being ∼300 km. The time scale analysis also shows this trend. During summer, all frequencies have e‐folding time scales of the order of 10–30 h. During fall and winter, the time scales for lower frequencies increase (up to 72 h or longer), but the 1000‐Hz time scales drop to as low as 3 h.

Forward Modeling of the Barents Sea Tomography Vertical Line Array Data and Inversion Highlights

A coupled normal-mode approach for the prediction of pulse propagation in a complex coastal environment was used to model the modal arrival structure in the 1992 Barents Sea Coastal Acoustic Tomography Test. Model results as well as the comparison to the data measured by a vertical hydrophone array are discussed. The predicted arrival structure was used for beamforming, identification and tracking of normal modes. The modal travel time series has been inverted in conjunction with the beamformed ray travel time data to produce high-resolution sound speed maps of the Barents Sea Polar Front at five-minute intervals over a tidal period. The inversion results are highlighted.

A Three-Dimensional, Broadband, Coupled Normal-Mode Sound Propagation Modeling Approach

A three-dimensional, coupled normal-mode approach for the prediction of pulse propagation in a complex coastal environment is presented. In this broadband approach, the received time-domain sound pressure signal is generated by Fourier synthesis using the product of the source signal spectrum and the source-to-receiver ocean transfer function. For a given frequency, the basic formulation involves decomposing the acoustic pressure into slowly varying complex envelopes that modulate (mode by mode) analytic, rapidly-varying, adiabatic-mode solutions. A coupled set of differential equations governing these complex modal envelopes are derived. Since the formulation allows for sound speed and density to vary three-dimensionally and bathymetry to vary two-dimensionally, it is particularly useful for shallow-water, low-frequency applications including coastal acoustic tomography.

Monterey Bay acoustic environmental monitoring system.

Over the past few decades sound has been used with increasing success to measure and monitor a variety of oceanographic features and processes−biological populations, water structure, internal waves, bathymetry, and climatic change to name just a few. Monterey Bay and surrounding ocean regions have recently been proposed as a Marine Sanctuary. Scientists see this region as a natural marine laboratory. In an effort to unobtrusively monitor features and processes in this ocean region a group of local scientists have initiated the design of an Acoustic Environmental Monitoring System (AEMS). A preliminary design meeting was held 28–29 February 1992 by a group of interested scientists and engineers at the Monterey Bay Aquarium. This paper reports on the preliminary design and objectives of the AEMS based on that meeting. The AEMS will consist of three monitoring sites within Monterey Bay, one near the head of Monterey Canyon, and two on the adjacent shelf areas. The sites will be connected to shore by high ba...

An assessment of the variability of low‐frequency active sonar performance in shallow water

The performance of a generic low‐frequency, active sonar was simulated using a broadband, coupled mode, propagation model. The low‐frequency active system consisted of a 1000‐Hz source with source level of 220 dB and a towed horizontal array of hydrophones. The ocean environment used in the simulation was 159 tomographic snapshots of the Barents Sea Polar Front, taken every 5 min. These tomographic images over a range of 35 km provide a virtual ocean in which system performance and environmental data requirements can be assessed. The probability of detection calculated as a function of time for 13 h is compared with that estimated using a range‐ and time‐independent assumption. The importance of coastal acoustic tomography for tactical applications will be discussed.

Aspects of the Mechanics and Modeling of Thermally-Induced Stresses in Arctic Pack Ice as Related to Under-Ice Ambient Noise

Thermally-induced stresses tend to produce distinct noise spikes at frequencies >500 Hz. However, the exact relationship between stress variations and under-ice noise oscillations are unclear. Surface heating and cooling of ice floes produce vertical strain rate variations which would tend to bend the ice. However, buoyancy and gravitational forces keep typical ice floes flat, so bending stresses develop. Thus, the overall problem of thermal stress in sea ice deals with the propagation of heat through the ice and relating the thermally-induced strain rate to stress at a given depth in the ice. This involves understanding the rheology relating stress to strain. Specific relationships for a thermal stress model for freshwater and sea ice are presented and discussed. Numerical model results are presented, and the calculated stress variations are compared to concurrent under-ice noise variations. These comparisons provide some interesting results. One is that thermal heating can often produce fracturing well below the surface of an ice floe, accounting for some anomalous noise spikes in higher frequency noise records after the passage of atmospheric warm fronts. Also, the sporadic nature of under-ice noise during cooling at night is related to the non-linear, rheological characteristics of sea ice.

Arctic ambient noise and climate change

In global warming, sea ice plays a significant role, and sea ice conditions may provide one of the most sensitive parameters to monitor. Modeling predicts that warming will be greatest at high latitude, and sea ice properties are very sensitive to warming and changes in atmospheric forcing. Therefore, sea ice might provide a sensitive marker for climate change. The mechanical properties of sea ice are related to the ice temperature, salinity, thickness, concentration, and flow size distribution. It has been established that the ambient noise field under pack ice is correlated to the mechanical properties, ice conditions, and forcing. Many studies over the last two decades have delineated some of these relationships. The hypothesis of this paper is that monitoring the ambient noise under the ice, for which there is well‐developed and reliable technology, may be an excellent way to detect a climate change signal. This paper will review the relationships between under ice ambient noise and the properties of ...

Possible uses of submarine canyons in acoustic thermometry

High‐power sources and high‐gain receivers are used in long‐range ocean acoustic thermometry. Because of power supply and data telemetry the sources and receivers are often placed near coastal facilities. There may be some significant benefits associated with taking advantage of natural terrain features to protect the sources and receivers from fisheries, and to optimize certain aspects of their performance. Sources and receivers are generally moored in deep water, in the axis of the sound channel, on the order of 100 to 1000 m. The shortest distance from shore to deep water is frequently associated with submarine canyons. For example it is less than 15 km from Point Pinos to water depths in excess of 2000 m in Monterey Submarine Canyon, less than half the distance along continental borders without submarine canyons. Fishermen do not fish inside the canyon walls for fear of rapid depth changes. Therefore, the canyons provide a natural barrier to the threat of fishing activity to moored sources and receive...