The Hess Deep rift valley, at approximately 2° 14' N, 101° 33' W, displays exposures of young, lower crustal and upper mantle rocks formed at the nearby, fast­ spreading East Pacific Rise. A seismic refraction experiment was conducted across the Hess Deep rift valley to provide p-wave travel times between sea floor explosives and Ocean Bottom Seismometers. These travel time data were processed with an iterative, damped least-squares, inverse method to produce a velocity model of the subsurface structure. The resulting velocity contrasts were interpreted as lithologies originating at different depths and/or alteration of the preexisting rock units. Petrologic and bathymet­ric data from previous studies were used, along with the seismic interpretation, to pro­duce a geologic model. The model supports low-angle detachment faulting with serpentinization of peridotite as the preferred mechanism for creating the distribution and exposure of lower crustal and upper mantle rocks within the Hess Deep.

In addition to the geologic information gained from this study, linearity limita­tions of the tomographic inversion have been shown to be dependent on topography. Topography on the scale of the ray paths has been shown to effectively increase or decrease the velocity gradient, as does the Earth-flattening approximation. Valleys decrease the apparent velocity gradient; whereas, the converse is true for hills. If the velocity gradient is already weak (ie. at depths > 500 m in oceanic crust), then further decrease in gradient beneath a valley produces an environment where ray paths are highly sensitive to model change. Consequently, to avoid violating the linearity assump­tion, model changes beneath a valley must be smaller than for a hill or flat topography.

Advancements in low-power and high-data-capacity consumer computer technology during the past decade have been adapted to autonomously record sounds from marine mammals over long periods. Acoustic monitoring has advantages over traditional visual surveys including greater detection ranges, continuous long-term monitoring in remote locations under various weather conditions and independent of daylight, and lower cost. However, until recently, the technology required to autonomously record whale sounds over long durations has been limited to low-frequency (< 1000 Hz) baleen whales. The need for a broader-band, higher-data capacity system capable of autonomously recording toothed whales and other marine mammals for long periods has prompted the development of a High-frequency Acoustic Recording Package (HARP) capable of sample rates up to 200 kHz. Currently, HARPs accumulate data at a rate of almost 2 TB per instrument deployment which creates challenges for processing these large data sets. One method we employ to address some of these challenges is a spectral averaging algorithm in which the data are compressed and viewed as long duration spectrograms. These spectrograms provide the ability to view large amounts of data quickly for events of interest, and they provide a link for quickly accessing the short time-scale data for more detailed analysis. HARPs are currently in use worldwide to acoustically monitor marine mammals for behavioral and ecological long-term studies. The HARP design is described and data analysis strategies along with software tools are discussed using examples of broad-band recorded data.

Blue (Balaenoptera musculus) and fin whales (B. physalus) produce high-intensity, low-frequency calls, which probably function for communication during mating and feeding. The source levels of blue and fin whale calls off the Western Antarctic Peninsula were calculated using recordings made with calibrated, bottom-moored hydrophones. Blue whales were located up to a range of 200 km using hyperbolic localization and time difference of arrival. The distance to fin whales, estimated using multipath arrivals of their calls, was up to 56 km. The error in range measurements was 3.8 km using hyperbolic localization, and 3.4 km using multipath arrivals. Both species produced high-intensity calls; the average blue whale call source level was 189±3 dB re:1 µPa-1 m over 25–29 Hz, and the average fin whale call source level was 189±4 dB re:1 µPa-1 m over 15–28 Hz. Blue and fin whale populations in the Southern Ocean have remained at low numbers for decades since they became protected; using source level and detection range from passive acoustic recordings can help in calculating the relative density of calling whales.