3. Borehole Seismological Observatory¹

Shipboard Scientific Party²

SYSTEM OVERVIEW

A major limitation of our understanding of active tectonic processes largely comes from the fact that we lack in situ long-term observations in the oceans where many areas of major tectonic activity are found. Since the Deep Sea Drilling Project era, there have been many attempts to utilize boreholes for such purposes. For example, recent circulation obviation reentry kit (CORK) deployments to measure pressure and temperature changes in sealed boreholes are beginning to produce interesting results. The Ocean Drilling Program (ODP) continues to recognize the importance of long-term observatory objectives (ODP Long Range Plan, 1996).

Tomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. Perhaps the greatest problem facing seismologists who wish to improve such tomographic models is the uneven distribution of seismic stations, especially the lack of stations in large expanses of ocean such as the Pacific. The International Ocean Network (ION) project, an international consortium of seismologists, has identified gaps in the global seismic net and is attempting to install digital seismometers in those locations. One of the highest priorities of ION is to install a station beneath the deep seafloor of the northwest Pacific. A primary objective of Leg 191 was to install a permanent observatory at Site 1179, situated in the Northwestern Pacific basin (Fig. F1), which would become a long-term borehole seismological observatory. This section is surrounded by stations at Petropavlosk to the north, many Japanese stations to the west, Minami-Torishima Island station to the south, and the proposed Midway Island station to the east.

A global seismographic network was envisaged by the Federation of Digital Seismographic Networks to achieve homogeneous coverage of the Earth's surface with at least one station per 2000 km in the northwestern Pacific area. Thus, the Site 1179 seismic observatory will provide invaluable data, obtainable in no other fashion, for global seismology. Data from this observatory will help revolutionize studies of global earth structure and upper mantle dynamics by providing higher resolution of mantle and lithosphere structures in areas that are now poorly imaged. In addition, this observatory will provide data from the seaward side of the northwest Pacific trenches, giving greater accuracy and resolution of earthquake locations and source mechanisms.

There are many bathymetric highs in the northwestern Pacific (e.g., Shatsky Rise and Hess Rise) whose roots are poorly known. Body-wave studies have not been able to determine the thickness of the plate; although large-scale anisotropy and lateral heterogeneity have been detected. Accumulation of broadband seismic data from within the basin part of the Pacific plate is needed to obtain detailed lithosphere and asthenosphere structure.

The northwestern Pacific borehole broadband seismic observatory WP-2 aims to provide seismic data to increase the resolution of tomographic studies.

We outline the NEREID-191 system of the borehole broadband seismic observatory WP-2 in this section (Fig. F2). The details of each component are described in separate sections. The observatory is designed to last for many years as a stand-alone system. Unlike other existing (Sites 1150 and 1151) and planned (proposed Site WP-1) oceanic borehole observatories, there are no nearby coaxial transoceanic telephone cables to utilize for data recovery and power. Therefore, the NEREID-191 installation is designed as a self-contained system with its own batteries and recorder. The two seismometers are designed to be placed near the bottom of the hole, each housed in a separate pressure vessel. Both sensors are feedback-type broadband seismometers (Guralp Systems Ltd., CMG-1T). Two separate cables are required to connect the sensors uphole. The signals are digitized in the sensor packages and sent in digital form to the seafloor packages.

The seafloor package (MEG-191) (see Table T1 for abbreviations) serves to combine the digital data from the two seismometers to a single serial data stream. It also distributes power to the individual seismometers. The data are stored in digital format in a separate module (SAM-191) after being sent via an RS232C link using Guralp Compressed Format (GCF) protocol. The SAM-191 has four 18-GB SCSI hard disks capable of storing more than six channels of 1.5-yr-long continuous data of 24-bit dynamic range at 100-Hz sampling rate. In this case, there are three channels for each of the seismometers. The seismometers are emplaced in the borehole permanently; they are cemented into the hole as required to assure good coupling. The MEG-191, on the other hand, may be serviced by a remotely operated vehicle (ROV) or submersible. The MEG-191 can be physically replaced and accepts commands and software upgrades through the SAM-191. The SAM-191 must be replaced by an ROV or submersible before the hard disks become full, which is ~1.5 yr with the present design.

The SAM-191 also provides a communication link to the borehole system while the station is being serviced by the ROV or submersible. The SAM-191 can send part of the data to the surface across a serial link to check the health of the system. The SAM-191 measures the time difference between the clocks in the SAM-191 and MEG-191. Before deployment and after retrieval of the SAM-191, the time difference between the SAM-191 and the Global Positioning System (GPS) clocks is measured on board. Because the MEG-191 controls the timing of the whole borehole system, we can adjust the system timing to Universal Time Coordinated using the data from the SAM-191.

All the necessary power is supplied from the battery system, called the seawater battery (SWB). The SWB can supply up to ~24 W with >400 kWh capacity. Its energy comes from electrolytic dissolution of the magnesium anode, which needs to be replaced once it is consumed. The replacement is also designed to be handled by an ROV or submersible. The condition of the SWB system is monitored by the power control system (PCS), and data from the PCS are recorded in the data logger (DL). In addition, the PCS controls the power switch and will turn the switch off for the protection of the system under certain SWB conditions.

Site 1179 is geographically situated in a large gap of the global seismic network (Fig. F1) where no seismic observatory exists within 1000 km. The seismic image of the Earth's structure beneath this area, especially in the upper mantle, is very ambiguous without a seismic station in this area. There are several requirements that must be fulfilled for the permanent installation of a seismic observatory so that the expansion of the global network to ocean is truly effective.

The seismic noise of an observatory should be as small as possible. The number of observed seismic phases depends on the magnitude of the seismic noise in the same frequency band as the seismic phases from earthquakes. Therefore, the reduction of seismic noise at the site directly enhances the value of the observatory. There are many sources of seismic noise. Environmental seismic noise caused by microseisms, infragravity waves, and water currents at the sea bottom are commonly recorded by ocean seafloor observatories. Each environmental seismic noise has a significant characteristic frequency band. In the frequency <0.1 Hz, seafloor seismic observations are significantly degraded by the noise caused by water currents at the seafloor. The magnitude of this type of noise can be higher than that of almost any long-period teleseismic phases. Escaping the flow noise by shallow burial of the seismometer in sediment or borehole installation was suggested and tried by several pilot experiments (e.g., Stephen et al., 1999). Because lower noise level is expected in a borehole rather than at the seafloor or in shallow sediment, especially long term, permanent seismic observatories should be installed in boreholes at the sea bottom.

Installation in a deep borehole seems to eliminate the effect of flow noise, but noises characteristic of borehole installation, such as turbulence in the water column of the borehole, might impair the advantage. For high-sensitivity measurements, pressure fluctuations that are the result of ocean long waves or temperature changes can be noise sources. Any water motion near the sensor is also a potential noise source. The seismometers must be grouted inside the borehole to avoid noise from water motion and to be optimally coupled to the surrounding rocks.

From the experience of Leg 186, during which borehole geophysical observatories were installed on the inner slope of the Japan Trench, it was determined that the infragravity wave was the dominant noise source with frequencies between 0.004 and 0.02 Hz. Using theoretical estimation, it is found that the acceleration of the ground as a result of tilt by the infragravity wave becomes maximum in a sediment layer; however, it becomes quickly negligible below the top of the basement. This is because the sediment has a large V_P/V_S ratio and horizontal shear stress takes a maximum value at a depth of the sensors. As a result, this factor leads to a large horizontal deformation of the sediment, giving rise to large tilts. Horizontal traction is maximum at the bottom of the sediment layers, whereas the vertical traction is maximum at the seafloor, a consequence of the traction-free boundary condition at the seafloor. The depth of the horizontal traction maximum is mostly determined by the wavelength of the applied pressure signal at the seafloor, although shear strength is also an important parameter. The wavelength of infragravity waves is more than a few kilometers in the frequencies of interest. Consequently, the horizontal stress takes a maximum value at the bottom of the sediment column unless the sediment thickness is extreme. Therefore, a seismometer in deep sediment cannot avoid suffering large horizontal infragravity wave noise, whereas the vertical noise level is improved the deeper the installation depth goes.

It is necessary to install a seismometer in igneous basement, rather than in sediment, to reduce the noise caused by infragravity waves. Because the seismic noise from infragravity waves in the horizontal component is smaller by >40 dB in basement, it is very important to install a borehole seismometer in basement.