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.

Repeated ocean ambient noise measurements at a shallow water (110 m) site near San Clemente Island reveal little increase in noise levels in the absence of local ships. Navy reports document ambient noise levels at this site in 1958–1959 and 1963–1964 and a seafloor recorder documents noise during 2005–2006. When noise from local ships was excluded from the 2005–2006 recordings, median sound levels were essentially the same as were observed in 1958 and 1963. Local ship noise, however, was present in 31% of the recordings in 1963 but was present in 89% of the recordings in 2005–2006. Median levels including local ships are 6–9 dB higher than median levels chosen from times when local ship noise was absent. Biological sounds and the sound of wind driven waves controlled ambient noise levels in the absence of local ships. The median noise levels at this site are low for an open water site due to the poor acoustic propagation and low average wind speeds. The quiet nature of this site in the absence of local ships allows correlation of wind speed to wave noise across the 10–220 Hz spectral band of this study.