Allan W. Sauter

A Study of Sea Floor Structure Using Ocean Bottom Shots and Receivers

Much of the ocean floor is covered with layers of soft sediment that cannot withstand large shearing forces without tearing. As a consequence, shear-wave slowness is an order of magnitude larger in ocean sediments than in crustal rock. The soft sea floor is a low velocity zone that focuses seismic energy, causing interface waves (or Stoneley waves) to travel along the ocean bottom in the same fashion that Rayleigh waves travel along the surface of the earth. In this paper we present Stoneley wave data from sea-floor sources recorded on ocean bottom seismometers (OBS’s) at deep water sites off the California coast. To interpret the data, we generate synthetic seismograms based on thinly layered, low shear velocity models.

Observing the historic eruption of northern Mariana Islands volcano

Anatahan volcano erupted for the first time in recorded history at about 7:30 GMT on 10 May 2003, covering the island of Anatahan, in the Commonwealth of the Northern Mariana Islands (CNMI), with ash, and providing scientists with important opportunities to study this volcano. The eruption was first reported by the National Oceanic and Atmospheric Administrations Volcanic Ash Advisory Center at 12:32 GMT, based on satellite images of the ash cloud. At about the same time, unusual light flares were observed from an approaching small ship, the Super Emerald, which was carrying a group of seismologists from Washington University in St. Louis, Scripps Institution of Oceanography, and the CNMI Emergency Management Office. As morning broke, the ship was approximately 10 km from the island, and those on board witnessed billowing ash and gas rise from the volcanos caldera to form a great cloud exceeding 6 km in altitude (Figure 1). The scientists were in the region installing land seismographs for the Mariana Subduction Factory Imaging Experiment, a joint U.S. Japanese deployment of 20 land broadband seismographs and 58 ocean bottom seismographs funded (on the U.S. side) by the Margins program of the National Science Foundation. The experiment has the goal of imaging the magma production regions and mantle flow patterns within the upper mantle beneath the Mariana arc and backarc (see http://epsc.wustl.edu/seismology/MARIANA).

Short‐range seismoacoustic propagation on and off the beach

Cultural seismic noise (noise caused by human activities) can be used for tracking and surveillance. During June and July 1995, a field experiment, dubbed ‘‘Adaptive Beach Monitoring’’ was conducted on both sides of the shoreline at U.S. Marine Corps Base Camp Pendleton, near Oceanside, California. Various sources were observed by seafloor seismic and acoustic sensors (four ocean‐bottom seismometers), and by a 24‐element seismometer array ashore. Preliminary results on the propagation of surface (and interface) waves shows that the surficial shear velocity is similar on both sides of the shoreline (nominally 250 m/s). The dispersion of the waves is, however, markedly different. The dispersion of Scholte waves observed on the OBSs was strong, with group velocity varying by a factor of 2 in the 2‐ to 10‐Hz range. Ashore, however, Rayleigh waves in the 5‐ to 20‐Hz frequency range showed little or no dispersion. [Work sponsored by the Office of Naval Research, Code 32.]

High‐frequency acoustic tracking of odontocetes at the Southern California Offshore Range (SCORE)

The SCORE site, located near San Clemente Island, is known to be abundant in marine mammals, and is the site of frequent naval operations, making it an ideal site to study what impact man‐made activities may have upon these creatures. In August 2004, an experiment was performed to collect both visual and acoustic observations of odontocetes at the SCORE site. Odontocete vocalizations consist of clicks and whistles. The clicks are very wideband (often 100 kHz), so times of arrival can be measured with extremely high resolution, which makes clicks attractive waveforms for multipath ranging and depth estimation. Although the whistles are narrowband, their fundamental frequency often ranges over tens of kilohertz, so these too can be processed as if they were wideband. We will present results of processing excerpts from several weeks of data recorded on a high‐frequency horizontal line array deployed from the FLIP research vessel. Bearing tracks of individuals and groups from a number of different species wil...

Techniques for separating interfering blue and fin whale calls

Recordings of blue and fin whales were made at the Southern California Offshore Range (SCORE) in August 2003. In analyzing this data, previously successful tracking algorithms, based on time differences of arrival (TDOA) measured on geographically distributed hydrophones, were frustrated by the mutual interference between the shorter, broadband, but higher amplitude fin whale calls and the longer duration, narrow‐band, blue whale calls. We present a series of progressively more powerful detectors designed to isolate these two types of calls, by zeroing in on specific features of these calls. Because the limited bandwidth of both types of calls presents a challenge to time delay estimation, we also present a comparison of a number of TDOA estimators, including correlation of suitably isolated time‐series and spectral features. We apply these techniques to data from different years and geographic sites to assess previous anecdotal evidence of strikingly similar calls within groups of blue whales, despite th...

Concurrent visual and acoustic tracking of fin whales offshore Southern California

Ocean acoustic techniques can be used to monitor whale presence within a region over long time periods. However, using the number of recorded calls to provide an estimate of the number of total whales present has not yet been realized. To help provide a transfer function from the number of acoustic calls to the number of whales present, we conducted a 10‐day, concurrent visual and acoustic experiment aboard the FLoating Instrument Platform (FLIP) focusing primarily on fin whale monitoring. FLIP is a stable platform with a visual observation deck approximately 25 m above the sea surface providing excellent visual range. Suspended beneath FLIP at 90 m below the sea surface was a 105‐m‐long acoustic array consisting of 8 hydrophones, 15 m apart. FLIP was placed in a stationary 3‐point mooring centered above three seafloor‐mounted hydrophone recorders at depths between 250 and 400 m. The FLIP vertical and seafloor horizontal arrays provide excellent geometry for acoustically tracking whales via arrival‐time d...

A swept‐frequency implosive source

Implosive underwater sound sources, such as the 20‐liter single‐shot device we have constructed, generally emit a rarefaction followed by a compression when the implosion is complete. Some purposes, however, are better served by a swept‐frequency, rather than an impulsive, signature, since the lower peak pressures of the swept source are less likely to cause environmental damage. Our current plan for a repeating implosive source operates by venting a bubble (initially at ambient pressure) into a low‐pressure receiver through a control valve. Performing this process in steps with abrupt pauses induces oscillations characteristic of the several sizes of the evolving bubble. From modeling the bubble oscillations using the Gilmore bubble equation, it appears feasible to produce a range of a factor of 2 in frequency, with a peak pressure of a megapascal. The design of the control valve, however, is challenging, as it was for the single‐shot version. The repeater, however, operates on air, whose density and vis...

Model‐based tracking of marine mammals

Data from the August 2003 experiment conducted by the Scripps Institution of Oceanography in the Southern California Offshore Range (SCORE) are used to track marine mammals. SCORE is a naval training area near the island of San Clemente located in relatively shallow water. The water depth where the experiment was conducted is around 360 meters. Data were recorded on a 100‐m, eight‐element vertical line array (VLA) deployed from the floating instrument platform (FLIP) and four bottom‐mounted hydrophones deployed in an area covering approximately 3 square kilometers. During the course of the 7‐day experiment continuous recording of the ocean environment was made. The recordings contain calls from various marine mammals, particularly blue and fin whales. Data recorded on the bottom‐mounted hydrophones and the VLA are used to track singing marine mammals using two entirely different model‐based tracking techniques: The animals are tracked by comparing the predicted (using a propagation model) and measured tim...

Observations of sediment shear velocity in the San Diego Trough

We report observations of shallow seafloor shear velocity structure in the San Diego Trough, whose depth is about 1100 m. Dispersed Scholte waves were generated using a new implosive source, described elsewhere in this meeting, and observed using ocean‐bottom seismographs. Preliminary estimates of the surficial shear velocities are in the 16–25 m/s range, which is very low. The Scholte waves are observed in the 1–8 Hz frequency range and dispersion is well‐developed after 100 m of propagation. Higher modes appear in addition to the fundamental mode, which exhibits a smooth increase in frequency from 2 to an Airy phase at about 3 Hz. The group velocity of the Airy phase approximates the surficial shear velocity. In the work area, the topographic relief is only a few meters. Geologically, the sediments are turbidites distributed by a channel and levee system. High‐resolution multibeam bathymetry is available from NOAA and is being used to aid geological interpretation.

The pressure gun, a wireline implosive source

We report development of an implosive source which is not based on stress failure. It is triggered by an electrical signal carried by armored cable, allowing precise timing. It can be used on the sea floor or in the water column. The trigger releases a small valve, allowing ambient pressure to operate a shuttle which opens a volume to flooding by seawater. It is recocked by returning to the surface. The strength of the source is best described by equivalent earthquake magnitude, which is proportional to volume change (a depth‐independent quantity, in contrast with an explosive source). The 20‐l device we have made has a moment magnitude of about −0.9. The duration of the signal is controlled by the time required to fill the volume, so the radiation of high frequencies is limited. This concentrates the energy radiated in seismically useful frequency band, and reduces effects on marine life. We have used this source to excite Scholte waves at a depth of just over 1100 m. At this depth the peak pressure occu...