1. Background. Lessons Learned in the OSNPE

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1 1. Background The development of broadband ( Hz), high dynamic range (24 bit) seismic data acquisition systems has revolutionized whole Earth seismology. However our ability to image the interior of the Earth has been restricted by the limited geometric coverage of the globe provided by continental and island seismic stations [Wysession, 1996]. The Global Seismographic Network (GSN) has the objective of evenly distributing about 128 stations over the surface of the globe. This will require around 20 stations in the deep ocean where there is no suitable land site within ~2000 km. In early 1998, the first Ocean Seismic Network Pilot Experiment (OSN-1 or OSNPE) was carried out at ODP Hole 843B about 225 km southwest of Oahu [Stephen et al., 1999] (see Figure 1). The experiment demonstrated the technical capability to deploy borehole systems using wireline re-entry technology. The scientific experience from the Ocean Seismic Network Pilot Experiment [Stephen et al., 1999; Collins et al., 2001] indicates that broadband borehole sensors deployed in oceanic basement are: 1) among the best seismic sensors in the world in terms of ambient noise floor from 10 mhz to 100 mhz (vertical component) [Rayleigh and Love waves used in the analysis of crustal and upper mantle structure occur in this band], 2) among the best oceanic sensors Figure 1. Hawaii station locations and site OSN1. (including island and coastal sensors) in terms of ambient noise floor from 100 mhz to 5 Hz (vertical and horizontal components) [Teleseismic P and S body waves are used for tomographic imaging of the mantle, for studies of the core-mantle boundary, and for studies of inner core anisotropy], and 3) the best oceanic sensors for observing short period teleseismic body waves (from ranges greater than 30 degrees) [These waves are particularly useful for high resolution mantle tomography]. Below 100 mhz, the OSNPE borehole sensor had high noise levels that were possibly associated with fluid flow in the well and that had also been observed during pre-cruise tests at the Pinon Flat Observatory [Useful signals at these frequencies include long period surface waves and normal modes, which can be used to determine whole Earth structure including observations of anisotropy in the inner core]. Figure 2 shows spectra comparisons as well as the frequency bands used for the subsequent time domain seismic comparisons. The goal of this project was to install the first permanent broadband borehole GSN station in the seafloor at the Hawaii-2 Observatory (H2O) site half way between Hawaii and California. Power from and telemetry to/from the H2O borehole system will be provided using a retired transoceanic telephone cable combined with a seafloor junction box, which was installed in September 1998 [Butler et al., 1999; Chave et al., 2000]. Leg 200 of the Ocean Drilling Project drilled a borehole at the H2O site specifically for an OSN/GSN seismic station [Shipboard Scientific Party, 2003; Stephen et al., 2006]. This project covers the engineering of a borehole seismic sensor in this hole and its planned connection to H2O located about 2 km away. We moved all the data telemetry components to WHOI for a system test in Fall 2003 during which we were able to send data through the JBOX and back onto the Internet to the data collection center at UCSD. A failure of the junction box prevented the installation in the planned hole. The NSF eventually terminated the H2O project. This led to subsequent plans for installation in a planned IODP hole at the Monterey Accelerated Research System (MARS) and a planned Ocean Observatories Initiative (OOI) installation in the Mid-Atlantic. These two efforts will be described briefly later in the report. Lessons Learned in the OSNPE During this project we calculated threshold-detection magnitudes (Table 1) for P-, S-, Rayleigh-, and Love-wave arrivals Earthquake data from three ocean seismic network Figure 2. (a) Vertical- and (b) horizontal component acceleration spectra for the three OSN instruments, three Island stations, and a short-period (1-Hz) seismometer (WHOI1) (from Collins et al., 2001). The frequency bands used to filter P waves, S waves, and Rayleigh waves are indicated.

2 (OSN) sensors, located (1) on the seafloor, (2) buried in seafloor sediments and (3) in a borehole, together with those from Hawaiian Island stations (Sutherland et al., 2004). Sensor Location Observed Events Table 1 P-Wave Data and Results Lowest Observed m b Min TM* Mean TM Mean SNR Mean TM Deep Mean TM Shallow OSN1 Borehole OSN1B Buried OSN1S Seafloor HAUH Kauai MAUH Maui MOLH Molokai BIG2 Hawaii KIP Oahu *Threshold magnitude Signal-to-noise ratio Data were available only for part of the experimentperiod for htis station, andnumbers of observed events and results are not directly comparable with the other stations. KIP has beenleft inthe reults, as many people are familiar with this station. Our results show that the borehole seismometer had noise levels similar to those of the Island stations and produced highquality high- and low-frequency body- and surface-wave data. Shallow burial of the seismometer in the sediments had no effect on higher frequencies but significantly reduced low-frequency noise levels so that data for S and Rayleigh waves were of high quality. In fact, the lowest noise levels at very low frequencies (<20 mhz; Collins et al., 2001) were observed on the buried seismometer. The ocean-floor seismometer was consistently noisy, and the data produced were of lower quality. Figure 3. Example P-wave arrivals from an mb 5.1 earthquake in Japan at 36.89N, E, km depth, 55 from the Hawaiian Islands. At this azimuth the array aperture is approximately 5. Bandpass filtered Hz. The signal-to-noise ratio (SNR) for OSN1, the borehole sensor, is 2.3, just above the SNR 2.0 assigned detection level. Noise levels at OSN1B and OSN1S are too high to pick a P-wave arrival. SNRs for the Island stations are: KAUH 3.3; MAUH 3.3; MOLH 2.7; BIG (noticeably lower noise levels); KIP 3.5. Both observed magnitudes and calculated threshold magnitudes were lower by more than an order of magnitude than those observed in previous studies. Results for short-period body waves at the borehole instrument in particular were much better than those that were previously found for any ocean-bottom recording. Table 1 lists the numbers of picked arrivals and the lowest recorded magnitude events. All the lowest recorded magnitude events for P arrivals were from events in the deep subduction zones around the Pacific Rim. Both the buried and seafloor instruments (OSN1B and OSN1S) observed less than half the number of P waves in comparison with the borehole seismometer, OSN1, and the Island stations. The noise levels in this frequency band on the vertical component for OSN1S and OSN1B were highest (Figure 2a), which is also clear in the time domain as shown on the seismogram (Figure 3) from an m b 5.1 event in Japan. The noise levels at the other stations were similar, with the borehole instrument having almost the same noise levels as MOLH on Molokai, and station KAUH on Kauai being particularly quiet. For larger magnitude earthquakes, high quality data were obtainable from all stations, as shown in Figure 4 from an m b 5.9 deep-trench earthquake in Tonga, where even the seafloor OBS, OSN1S, recorded a very high signal-to-noise ratio (SNR) in contrast to the results from the MELT site (Wilcock et al., 1998), where no teleseismic compressional waves were observed in this frequency range. It is clear that the surface sensor, OSN1S, produced low-quality data for all arrivals when compared to the other sensors in the experiment because of the high noise levels directly at the seafloor. Burial of the seismometer to a shallow depth

3 had no effect on high-frequency seismic noise (Collins et al., 2001). At frequencies above 0.2 Hz, seismic noise levels observed in the buried instrument were the same as for the seafloor sensor, but the buried instrument exhibited dramatically improved noise levels at lower frequencies. This reduction in noise causes the teleseismic S-wave detection threshold to improve by 0.5 magnitude units simply by burying the sensor. Placing a broadband seismometer in a borehole reduced noise levels relative to the surface deployed OBSs throughout the spectrum. When compared to the borehole sensor, the vertical component of the buried OBS had similar performance in the frequency band 0.01 to 0.2 Hz, accounting for the similarity of the Rayleigh-wave detection threshold between the two instruments. Nevertheless, the borehole horizontal components were significantly noisier than the buried OBS at frequencies below 0.05 Hz, resulting in the buried sensor having better S-wave- and Love-wave-detection thresholds. For the Hawaiian Island stations, KAUH on the island of Kauai produced the best results and had especially low noise levels at higher frequencies. The thresholds for detecting body Figure 4. P-wave arrivals from an m b 5.9 deep (530-km) Tonga earthquake: N, E, 42 from the Hawaiian Islands; array aperture approximately 3. Bandpass filtered, Hz. Noise levels are average for the experiment, but the large magnitude and deep source produce high-quality data for every station. Even the noisiest station, OSN1S, has a SNR of 22. waves at the seafloor are much lower than those reported in any past studies. Using a borehole instrument, P waves for mb 4.4 earthquakes have been detected with decent-quality results, and we estimate that data at a 2.0 SNR level should be obtainable for magnitude 4.2 events, an orderof-magnitude improvement over published observations. The SNRs are even better for S waves, with observed events at mb 4.0 and calculated thresholds as low as mb 3.5, an even greater improvement over previous studies. We have also shown that high-quality surface-wave data at teleseismic distances can be obtained at levels lower than previously thought, and that Rayleigh waves should be readily detectable with either a borehole or buried seismometer at magnitude 4.5. Figure 5. This depicts the substantial improvement in very low frequency seismic noise when packing small glass beads around the sonde in a water-filled borehole. The top figure shows the installation in the ODP-scale, flooded hole at PFO prior to placing glass beads (centering clamps used alone). The bottom figure depicts the impact of the beads - the horizontal and vertical noise leves are essentially identical and small in the latter case. Based on the observations made in this paper and the results from Collins et al. (2001), the best system to record high-quality teleseismic P waves at the OSN1 site was the borehole seismometer, which was coupled to the basalt at the base of the sediments. The preferred system to record teleseismic S and Love waves was the buried broadband OBS, which had the best horizontal noise performance at frequencies below 0.04 Hz. For observing Rayleigh waves, both the buried and the borehole seismometers performed comparably. The seafloor broadband sensor had the highest detection threshold for each of the four types of measurements made. When compared to nearby Island stations, the borehole system provided similar results for the magnitude detection thresholds for teleseismic P, S, Rayleigh, and Love waves, whereas the buried broadband sensor gave improved detection thresholds for teleseismic S, Rayleigh, and, in particular, Love waves, but gave significantly higher detection thresholds for teleseismic P waves. In all cases the use of buried or borehole broadband seismometers on the seafloor

4 yielded remarkable decreases in detection thresholds over previous observations and theoretical estimates so that in the future, large data sets will be obtainable in shorter amounts of time, saving money and greatly improving research. Reduction of Low Frequency Noise The presence of substantial low frequency noise on the horizontal components of the KS54000, both in the ODP test borehole at Cecil & Ida Green IGPP Piñon Flat Observatory (PFO) and in the OSNPE led us consider several possibilities for the origin of the noise. The presence of even low-power electronics in the sonde makes it possible to create convection in fluids (air or water) in the instrument package itself or in the surrounding fluid. The internal package is evacuated a refilled with helium at low pressure (low density and low conductivity) to reduce internal convection and the bulk of the heat-producing electronics are place well above the seismometers themselves. In addition, we elected to pour very small diameter glass beads in the annulus between the borehole casing and sonde itself. Figure 5 shows the dramatic impact of using these glass beads around Figure 6: Laboratory mockup of IODP borehole the sonde. The horizontal noise with periods of hours is dominant on the being cut open after a 2-year test with glass beads horizontals when the annulus was filled with ground water. However, when around dummy sonde. the glass beads were inserted in the annulus, the noise levels on the vertical and horizontal components are essentially equal (and small - see Figure 5). Generally, packing the bottom of the borehole with beads had a substantial impact on the noise levels, but also presented other longer term problems. Borehole Sensor Upgrades for GSN Design Goals The important lesson learned at OSN1 was that, for observing teleseismic and regional body waves, a broadband borehole seismometer coupled to the basalt layer eliminates signalgenerated noise observed by sensors emplaced in sediments. In addition, a borehole deployment provides an improvement of 10-20dB in signal-to-noise, which is equivalent to one full earthquake magnitude unit. The borehole sonde used in the OSNPE comprises the Teledyne KS54000 broadband, 3-component seismometer and digitizing system, along with the camera and lights necessary for the borehole reentry operation. The digitizer Figure 7: Laboratory mockup of IODP borehole cut open to was based on reveal lithified glass beads, the inner dummy sonde and (blue the 24-bit Reftek arrow) the tube used previously to inject fluid at the bottom of 72A data logger the sonde to float the system free of the beads. used by the IRIS PASSCAL program, chosen for its small form factor. A titanium housing, which can withstand the pressure at full ocean depth, surrounds the KS54000 and all associated electronics. This B3S2 system has two clamps at either end that were triggered electrically and that are operated by ambient ocean Figure 8: Laboratory mockup of IODP borehole pressure (7000 psi at OSN-1). These clamps were designed to expand cut open to reveal lithified glass beads. The blue symmetrically in order to center the B3S2 sonde in the borehole casing. arrow shows clearly the fluid injection tube for dislodging the sonde. Packing the sonde in glass beads did substantially reduce low frequency noise. We undertook a series of experiments for deploying the sonde practically surrounded by the beads using a test, water-filled Ocean Drilling Program borehole at the IGPP Piñon

5 Flat Observatory near Palm Springs. The data in Figure 5 were collected there. Because of the travel time to Piñon, however, we constructed both an ODP-diameter casing and dummy sonde for longer-term tests (Figure 6). We found, as deployment times increased, that pulling the sonde out of the borehole became increasingly difficult. To solve this problem, we added a flushing tube along the sonde that could force water into the volume below the seismometer. For times up to a week, the pressure required to break the sonde free was around 750psi. For more than a week, the pressure went up to 1000psi. After a year s deployment, the sonde required 1800psi. When the sonde was left in place for two years, no pressure could break the sonde free (see Figures 6-8). After two years, the sonde becomes essentially a permanent installation! Based on our experience from the OSNPE and the existing facilities already deployed at H2O, we are made significant improvements to the B3S2 sonde to match the capability of a standard land-based GSN station tempered by the constraint that all equipment must be installed significantly below the sediment-basalt interface in a 9.8 inch ID casing. A standard configuration for a GSN station includes: 1) A three component broadband seismometer with a flat response to velocity between 360 s and 3 Hz (either a Steckeisen STS-1 or a Teledyne KS-54000) sampled at 20 sps and 1 sps with 24-bit resolution. 2) A three component broadband seismometer with a flat response to velocity between 120 s and 50 Hz (either a Steckeisen STS-2 or a Guralp CMG3T) sampled at 200 sps and 40 sps with 24-bit resolution. The basic B3S2 sonde (Figure 9) used at OSN1 satisfies the first specification. However, since the goal was to install a borehole system using the GSN design standards at H20, the sonde needed the following modifications: 1) Adding a Guralp CMG3T borehole seismometer inside the sonde 2) Replacement of the Reftek 72A datalogger with the current state-of-the-art Quanterra Q330 datalogger and a Marmot Field Processor 3) Extending the titanium pressure case to accommodate the CMG3T. In order to enhance reliability of the sonde, we included two Q330 digitizers and two Marmot Field Processors. Either of the Q330s and/or either of the Marmots can be used to support all on-board digitizing, computing and communications. The Marmot runs a modified Antelope data management system. Data Handling Figure 9: Layout of current Inside the B3S2 sonde, the data from each sensor and sample rate are packetized and broadband seismic sonde with compressed before being telemetered with an aggregate data rate of about 12 kbps ( 50GB/yr). electronics at the top and two The data packets are transmitted from the B3S2 sonde to a seafloor junction box using a fiber seismic systems at the bottom. optic extension cable, retransmitted to the surface, and relayed via satellite to the HiSeasNet A camera assists in the reentry of the seafloor borehole real-time with latencies of a few seconds and also pushed to the IDA/IRIS GSN facility for QC ground station at the San Diego Supercomputer Center (SDSC). The data are available in near- and submission to the IRIS Data Management System (DMS). The real time data system called Real-time Observatory, Analysis and Data management Network (ROADNet - was developed with NSF funding and now forms the basis for the NSF Ocean Observatories Initiative (OOI) Cyberinfrastructre. Past and Future Plans As noted earlier, the original plan to install this system at H2O was not possible following the failure of the H2O junction box and the NSF decision to discontinue efforts to restore the system to service. A proposal was accepted by IODP to drill an observatory borehole in Monterey Bay (IODP Leg 312), which would include the installation of this borehole system within the MARS observatory. However, this leg was postoned late in

6 2005 because of permitting concerns in Monterey Bay. While there was a possibility this would be returned to the drilling program in 2007, this did not happen and is not likely to be approved in the foreseeable future. Through a separate NSF cooperative agreement (LOOKING; Orcutt PI) a broadband seismometer will be installed at MARS in 2008 as a prototype end-to-end experiment in support of the NSF Ocean Observatories Initiative. The Ocean Observatories Initiative Conceptual Network Design (CND) was updated in March 2007 and was used as a basis for responses to the Joint Oceanographic Institutions (JOI) RFP for the Coastal/Global Scale Nodes (CGSN) Implementing Organization (IO). The CND included a requirement for a global node in the Mid-Atlantic, which had to include an advanced spar buoy. In response we, as part of the OSU/SIO/WHOI proposal team, developed a relationship with industry (Technip USA) to help provide the funding needed for an advanced platform. With $8M in matching funds, the OSU/SIO/WHOI consoritum won the contract for the CGSN IO. One of the anticipated sensors for the resultant Extended Draft Platform (EDP; see Figure 10) was the seismic sonde developed under the terms of this grant. Unfortunately, just prior to the Preliminary Design Review (PDR) in December, the decision was made to reduce the funds available for global systems and the advanced EDP was removed from the PDR. The EDP will be constucted in 2008, with industry funding, for a launch in We intend to pursue funding necessary to install power and satellite communications equipment on the EDP in 2009 and partcipate in the testing of the EDP as an advanced platform for scientific measurements during Matching funds ($405K) to this prospective proposal will provide the funds to design and construct a suitable seafloor junction box. Hopefully, an advanced platform will be considered suitable for the OOI at that stage and it will be possible to deploy the EDP as well as the seismic sonde developed through this grant. References Butler, R., A.D. Chave, F.K. Duennebier, D.R. Yoerger, R.A. Petitt, Jr., D. Harris, F.B. Wooding, A.D. Bowen, J. Bailey, J. Jolly, E. Hobart, J.A. Hildebrand, and A.H. Dodeman, The Hawaii-2 Observatory (H2O), EOS, 81, 157&162-3, Chave, A.D., F.K. Duennebier, R. Butler, R.A. Petitt, Jr., F.B. Wooding, D. Harris, J.W. Bailey, E. Hobart, J. Jolly, A.D. Bowen, and D.R. Yoerger, H2O: The Hawaii-2 Observatory, in L. Beranzoli, P. Favali, and G. Smriglio (eds.), Science- Technology Synergy for Research in the Marine Environment: Challenges for the XXI Century, Developments in Marine Technology Series 12, Amsterdam: Elsevier, pp , Collins, J.A., F.L. Vernon, J.A. Orcutt, R.A. Stephen, K.R. Peal, F. B. Wooding, F.N. Spiess, and J.A. Hildebrand, Broadband seismology in the oceans: lessons from the Ocean Seismic Network Pilot Experiment, Geophys. Res. Lett., 28, 49-52, Kasahara, J., and Stephen, R.A., Leg 200 synthesis: A broadband seismic station in oceanic crust at the Hawaii-2 Observatory and coring into the Nuuanu Landslide. In Kasahara, J., Stephen, R.A., Acton, G.D., and Frey, F.A. (Eds.), Proc. ODP, Sci. Results, 200, 1 44, Stephen, R. A., F. N. Spiess, J. A. Collins, J. A. Hildebrand, J. A. Orcutt, K. R. Peal, F. L. Vernon, and F. B. Wooding, The Ocean Seismic Network Pilot Experiment, Geochem. Geophys. Geosyst. 4, 1-38, Shipboard Scientific Party, Leg 200 summary. In Stephen, R.A., Kasahara, J., Acton, G.D., et al., Proc. ODP, Init. Repts., 200: College Station TX (Ocean Drilling Program), Sutherland, F.H., Vernon, F.L., Orcutt, J.A., Collins, J.A. and Stephen, R.A., Results from OSNPE: Improved teleseismic earthquake detection at the seafloor, Bull. Seismol. Soc Am., 94, , Wilcock, W. S. D., Webb, S. C. and Bjarnason, I. Th., The effect of local wind on seismic noise near 1 Hz at the MELT site and in Iceland, Bull. Seis. Soc. Am., 89, , Wysession, M.E., How well do we utilize global seismicity?, Bull. Seismol. Soc. Am., 86, , 1996.

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8 Figure 10: The Extended Draft Platform (EDP) provides power (>500W) and bandwidth to the seafloor and water column. The nominal platform bandwidth is 1Mbps. The EDP is designed for a decadal lifetime with annual servicing.