Christopher O. Tiemann

Localization of marine mammals near Hawaii using an acoustic propagation model

Humpback whale songs were recorded on six widely spaced receivers of the Pacific Missile Range Facility (PMRF) hydrophone network near Hawaii during March of 2001. These recordings were used to test a new approach to localizing the whales that exploits the time-difference of arrival (time lag) of their calls as measured between receiver pairs in the PMRF network. The usual technique for estimating source position uses the intersection of hyperbolic curves of constant time lag, but a drawback of this approach is its assumption of a constant wave speed and straight-line propagation to associate acoustic travel time with range. In contrast to hyperbolic fixing, the algorithm described here uses an acoustic propagation model to account for waveguide and multipath effects when estimating travel time from hypothesized source positions. A comparison between predicted and measured time lags forms an ambiguity surface, or visual representation of the most probable whale position in a horizontal plane around the array. This is an important benefit because it allows for automated peak extraction to provide a location estimate. Examples of whale localizations using real and simulated data in algorithms of increasing complexity are provided.

Observations of potential acoustic cues that attract sperm whales to longline fishing in the Gulf of Alaska

Sperm whales (Physeter macrocephalus) have learned to remove fish from demersal longline gear deployments off the eastern Gulf of Alaska, and are often observed to arrive at a site after a haul begins, suggesting a response to potential acoustic cues like fishing-gear strum, hydraulic winch tones, and propeller cavitation. Passive acoustic recorders attached to anchorlines have permitted continuous monitoring of the ambient noise environment before and during fishing hauls. Timing and tracking analyses of sperm whale acoustic activity during three encounters indicate that cavitation arising from changes in ship propeller speeds is associated with interruptions in nearby sperm whale dive cycles and changes in acoustically derived positions. This conclusion has been tested by cycling a vessel engine and noting the arrival of whales by the vessel, even when the vessel is not next to fishing gear. No evidence of response from activation of ship hydraulics or fishing gear strum has been found to date.

Integral measurements of mass transport and heat content in the Strait of Gibraltar from acoustic transmissions

Three View the MathML sourceacoustic transceivers were deployed on either side of the eastern entrance of the Strait of Gibraltar during April–May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, horizontally integrated measurements of flow and temperature in straits. Reciprocal transmissions between the transceivers were used to test the feasibility of using traditional ray differential travel times to monitor the component of flow along the acoustic paths. Transmissions directly across the Strait were used to test the feasibility of using horizontal arrival angle fluctuations and acoustic intensity scintillations to monitor the flow perpendicular to the acoustic path. The geometry was selected to provide ray paths that only sample the lower-layer Mediterranean water, so that the feasibility of monitoring the Mediterranean outflow using the various methods could be evaluated. The acoustic scintillation method did not yield useful current estimates, but the experimental parameters were not optimized for this approach. Since the low-frequency variability in log-amplitude was found to be highly correlated at receivers View the MathML source apart, it is possible that acoustic scintillation measurements using different receiver spacings and more rapid sampling might yield better results. The horizontal deflection method gave encouraging results at the time of neap tides, but less so during spring tides. For this approach, both theoretical estimates and measured phase differences between the horizontally separated receivers suggest that internal-wave-induced horizontal arrival angle fluctuations may fundamentally limit the precision with which arrival angles can be measured. Further work is needed to determine if a smaller horizontal spacing and higher signal-to-noise ratios would yield better results. Reciprocal travel time measurements diagonally across the Strait performed the best of the three methods, giving absolute flow estimates consistent with those derived from current-meter data. The fractional uncertainty variance for the lower layer tidal transport from a single tomographic path was estimated to be 0.017 (i.e. 98% of the a priori tidal transport variance was resolved). The spatial scales of the sub-tidal flow are thought to be significantly shorter than those of the tidal flow, however, which means that a more elaborate monitoring network is required to achieve the same performance for sub-tidal variability. Finally, sum travel times from the reciprocal transmissions were found to provide good measurements of the temperature and heatcontent in the lower layer.

Assessing the Deepwater Horizon oil spill impact on marine mammal population through acoustics: Endangered sperm whales

Long-term monitoring of endangered species abundance based on acoustic recordings has not yet been pursued. This paper reports the first attempt to use multi-year passive acoustic data to study the impact of the Deepwater Horizon oil spill on the population of endangered sperm whales. Prior to the spill the Littoral Acoustic Demonstration Center (LADC) collected acoustic recordings near the spill site in 2007. These baseline data now provide a unique opportunity to better understand how the oil spill affected marine mammals in the Gulf of Mexico. In September 2010, LADC redeployed recording buoys at previously used locations 9, 25, and 50 miles away from the incident site. A statistical methodology that provides point and interval estimates of the abundance of the sperm whale population at the two nearest sites is presented. A comparison of the 2007 and the 2010 recordings shows a decrease in acoustic activity and abundance of sperm whales at the 9-mile site by a factor of 2, whereas acoustic activity and abundance at the 25-mile site has clearly increased. This indicates that some sperm whales may have relocated farther away from the spill. Follow-up experiments will be important for understanding long-term impact.

Acoustic scattering by internal solitary waves in the Strait of Gibraltar

High-frequency underwater acoustic transmissions across the Strait of Gibraltar were used to examine acoustic scattering caused by the unique internal wave field in the Strait. Internal solitary waves of 100 m in amplitude propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water. The interface is also strongly modulated by internal tides of comparable amplitude. As internal solitary waves cross the acoustic path, they cause sharp soundspeed gradients which intermittently refract acoustic rays away from normal sound channels. Internal tides vertically shift soundspeed profiles for additional travel time variability. Although the acoustic scattering is quite complicated, it is also surprisingly robust, making it a good candidate for modeling. Key features of the acoustic arrival pattern can be accounted for in some detail by a model description of the complex hydraulics in the Strait.

Acoustic remote sensing of internal solitary waves and internal tides in the Strait of Gibraltar

High-frequency underwater acoustic transmissions across the Strait of Gibraltar are used to examine the feasibility of acoustically measuring several physical processes in the Strait, a difficult area to sample with conventional instruments. Internal undular bores propagate along the interface between an upper layer of Atlantic water and a lower layer of Mediterranean water. As they cross the acoustic path they are recognized by their scattering effects in the acoustic record. The time between internal bore crossings is influenced more by the tidal phase of the bore release at the Camarinal Sill than by variability in the bore’s propagation time to the acoustic path. When internal bores were present, the acoustic arrival patterns could be classified as one of three types with different internal bore and internal tide amplitudes. The arrival types alternate during spring to neap tide transitions, suggesting that internal bore amplitude is not linearly related to tidal height. The sensitivity of acoustic observables to several physical parameters is investigated using a forward model, and a demonstration of inverse techniques provides estimates of several physical parameters from spring tidal cycles.

Automated model-based localization of sperm whale clicks

In previous work we described an algorithm based on an acoustic propagation model that was used for passive acoustic localization of vocalizing marine mammals. Differences between measured and modeled pair-wise time-differences of arrival were used to define an ambiguity surface that identifies the most probable whale location in a horizontal plane around an array. The algorithm has been successfully applied to humpback whale calls in deep water near Hawaii and blue whale calls in shallow water near California. In this work, we return to Hawaii to track another species of whale, the sperm whale, whose calls are brief, broadband clicks. Acoustic localizations are in close proximity in space and time to aerial observations of sperm whales. We were also able to verify the algorithms accuracy in a controlled source experiment at the Navys AUTEC range in the Bahamas. Recordings of sperm whale clicks were broadcast and successfully tracked. The localization accuracy of the model-based technique and traditional hyperbolic techniques is compared.

Acoustic monitoring of flow through the Strait of Gibraltar

The Strait of Gibraltar Acoustic Monitoring Experiment was conducted during April–May 1996 to determine the feasibility of using acoustic methods to make routine, rapidly repeated, transport measurements in the Strait of Gibraltar, as well as to explore the acoustic scattering caused by the internal wave bores generated in the Strait. Three different approaches to monitoring the flow were explored: (i) high‐frequency (2‐kHz) reciprocal transmissions; (ii) high‐frequency (2‐kHz) horizontal arrival angle measurements of ray bending due to currents; and (iii) one‐way transmissions from a low‐frequency (250‐Hz) source to a vertical receiving array on the opposite side of the Strait. Extensive independent measurements of the temperature, salinity, and velocity fields were also made. The deep‐turning rays are stable and yield differential travel times that give reasonable values for the current components along the acoustic paths. Horizontal arrival angles (phase differences) are strongly correlated with rough ...

Automated model-based localization of marine mammals near California

In a previous work, we developed an algorithm for acoustically tracking singing humpback whales near Hawaii. Pair-wise time-differences in arrival of whale calls as measured by a phase-only correlation process are compared to time-lags predicted by an acoustic propagation model. Differences between measured and modeled time-lags defined an ambiguity surface that identifies the most probable whale location in a horizontal plane around an array. In this work, we describe the application of this technique to a very different environmental scenario involving blue whales off the coast of California. The whale calls are much lower in frequency and the receivers are ocean bottom seismometers. Again the algorithm performs extremely well, providing the capability for real-time, automated monitoring and alert.

A comparison of model‐based and hyperbolic localization techniques as applied to marine mammal calls

A common technique for the passive acoustic localization of singing marine mammals is that of hyperbolic fixing. This technique assumes straight‐line, constant wave speed acoustic propagation to associate travel time with range, but in some geometries, these assumptions can lead to localization errors. A new localization algorithm based on acoustic propagation models can account for waveguide and multipath effects, and it has successfully been tested against real acoustic data from three different environments (Hawaii, California, and Bahamas) and three different species (humpback, blue, and sperm whales). Accuracy of the model‐based approach has been difficult to verify given the absence of concurrent visual and acoustic observations of the same animal. However, the model‐based algorithm was recently exercised against a controlled source of known position broadcasting recorded whale sounds, and location estimates were then compared to hyperbolic techniques and true source position. In geometries where di...