Kenneth R. Peal

Broadband seismology in the oceans: Lessons from the Ocean Seismic Network Pilot Experiment

The fundamental objective of the Ocean Seismic Network Pilot Experiment (OSNPE)—which was carried out over a period of about 4 months at a site 225 km southwest of Oahu, Hawaii—was to learn how to make high-quality, broadband seismic measurements in the deep oceans. The OSNPE results demonstrate that broadband data of quality similar to that of quiet land stations can be acquired with seafloor seismographs, but that the location of the seismometer—whether it be on the seafloor, surficially buried within the seabed, or in a deep borehole—has a profound effect on data quality. At long-periods ( 0.1 Hz), data quality was best for a seismometer deployed 242 m below the seafloor in a borehole.

Seafloor seismic stations perform well in study

Broadband seafloor seismic installations can yield quality data comparable to land stations in terms of ambient noise levels and time series events, a study has shown. This is good news for plans to fill in the gaps in the Global Seismic Network (GSN) around the world. About 25 GSN stations are needed on the seafloor. The study the Ocean Seismic Network (OSN) Pilot Experiment, was carried out to test different systems for broadband seafloor seismology. So far GSN has about 115 high quality, broadband, three-component, seismic stations on continents and islands around the world.

The seafloor borehole array seismic system (SEABASS) and VLF ambient noise

The Seafloor Borehole Array Seismic System (SEABASS) has been developed to measure the pressure and threedimensional particle velocity of the VLF sound field (2–50 Hz) below the seafloor in the deep ocean. The system consists of four three-component borehole seismometers (with an optional hydrophone). a borehole digitizing unit, and a seafloor control and recording package. The system can be deployed using a wireline re-entry capability from a conventional research vessel in Deep Sea Drilling Project (DSDP) and Ocean Drilling Project (ODP) boreholes. Data from below the seafloor are acquired either onboard the research vessel via coaxial tether or remotely on the seafloor in a self-contained package. If necessary the data module from the seafloor package can be released independently and recovered on the surface. This paper describes the engineering specifications of SEABASS, the tests that were carried out, and preliminary results from an actual deep sea deployment. VLF ambient noise levels beneath the seafloor acquired on the Low Frequency Acoustic-Seismic Experiment (LFASE) are within 20 dB of levels from previous seafloor borehole seismic experiments and from land borehole measurements. The ambient noise observed on LFASE decreases by up to 12 dB in the upper 100 m of the seafloor in a sedimentary environment.

A new undersea geological survey tool: ANGUS

Abstract A towed, near-bottom survey system termed ANGUS (Acoustically-Navigated Geological Undersea Surveyor) has been developed for geological and geophysical studies in the median rift valley of the Mid-Atlantic Ridge. System communication is solely acoustic and incorporates a relay transponder attached to either a camera fish, rock dredge, corer, or heat probe device with a shipboard transducer and a seafloor transponder array. The devices can be located with a precision of 5 to 10 m along a simultaneously collected high-resolution bathymtric profile. Automatic digital computer processing of the acoustic navigational information permits real-time monitoring of the path of the device on shipboard graphic display units. Approximately 60,000 high-quality photographs, 70 rock dredges, and 100 heat-flow observations have been collected and related to seafloor features with relief as small as 20 to 30 m. The basic camera fish allows for the addition of other geophysical instruments. Also, expendable sonobuoys have been positioned precisely to locate microearthquakes.

The Woods Hole Oceanographic Institution digital ocean bottom hydrophone instrument : technical report

Abstract : This report describes the design and capabilities of a new ocean bottom hydrophone instrument. The instrument is microprocessor controlled and records digitally on a commercially available cartridge tape recorder with a formatted capacity of 16.7 megabytes. It can operate at sampling intervals between 80 and 8500 Hz and has a dynamic range of 120dB. Both the hardware and software are designed to provide the maximum flexibility in operation allowing either preprogrammed or event detect operation for either short deployment high sampling rate experiments or extended deployment low data rate applications. The microprocessor and recording electronics are capable of handling four data channels and thus the existing recording package is suitable for the ocean bottom seismometer application (or similar) with little or no modification. (Author)

A leveling system for an ocean-bottom seismometer

As part of a new broad-band, ocean-bottom seismometer (OBS) system that has been developed at the Woods Hole Oceanographic Institution, a gimbaled leveling system was designed and built. The goal of the broad-band system is to measure ocean-bottom vibrations from a period of 120 seconds up to 20 Hertz. During system deployment, a sphere containing the seismic sensor is dropped into sediment on the ocean bottom. Seismic sensors need to be leveled before use, and it is not practical to accurately control the attitude of the sphere as it settles on the ocean bottom. The sphere holds the seismic sensor in gimbals whose axes have brakes. The gimbal axes have brakes to prevent tilting in response to slow horizontal accelerations that would complicate long-period seismometer response. To level the seismic sensor, the brakes are released, the righting moment of the seismic sensor in the gimbals levels the seismometer attitude, and the brakes are reasserted. The brake systems were designed to have zero play and had to be modified to raise the lowest system natural frequency above 20 Hertz. This paper describes the mechanical aspects of the system and the modifications needed to push up the mechanical resonances. Twenty-five of the OBS systems have been built and deployed off Hawaii as part of the Plume-Lithosphere Undersea Melt Experiment (PLUME)

ORB-a new ocean bottom seismic data logger

The authors describe a new, state-of-the-art data logger for ocean-bottom seismology. This instrument, which they call ORB, is a versatile system that can be deployed in a variety of configurations ranging from a stand-alone ocean-bottom hydrophone with a two-week recording capability to an ocean-bottom seismometer with a one-year recording capability. The data logger has a dynamic range of more than 130 dB, a clock with a stability of a few parts in 10/sup 9/, and a data storage capacity of many gigabytes. ORB is based on a commercially available system (RefTek/sup TM/ 72A-07) that is also used in large numbers by the U.S. IRIS (Incorporated Research Institutions for Seismology) consortium.

A simple ocean bottom hydrophone with 200 megabyte data capacity

Funding provided by the National Science Foundation under Grant Nos. OCE-9019918 and OCE-8917628.

Multi-season acoustic tomography experiment in the Arctic

Acoustic properties of sea ice are important parameters which affect the propagation of sound both within the ice and in the water column under the ice. These properties are functions of space and time. The authors present details of a crosshole tomography experiment conducted in the Arctic to estimate two important acoustic parameters (compressional and shear wave speeds) of multi-year sea ice in three dimensions and to monitor their evolution with season. Some preliminary results of the data analysis are also presented.<<ETX>>

Seafloor borehole array seismic system (SEABASS)

This work was carried out under JHU Contract # 602809-0 and under ONR contracts #N00014-89-C-0018, #N00014-89-J-1012, and #N00014-90-C-0098.