FIRE and MUD Contents
1U.S. Geological Survey.
2Philippine Institute of Volcanology and Seismology.
In late April and early May 1991, staff from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey / U.S. Agency for International Development Volcano Disaster Assistance Program installed a ruggedized nine-component telemetered seismic network to monitor Mount Pinatubo. They also installed a low-data-rate telemetry network of two tiltmeters and a rain gauge.
The Mount Pinatubo network provided real-time geophysical data to the on-site monitoring team, who could evaluate it immediately in the context of geologic observations. This permitted the team to give the civil and military authorities the best possible eruption forecasts quickly and contributed to the timely decisions to evacuate large areas around Mount Pinatubo, including Clark Air Base.
Seismic stations were located from 1 to 17 kilometers from the summit and in all quadrants surrounding the volcano. Our biggest installation problems were the difficulty of site access and a lack of viable repeater sites in the rugged, heavily vegetated terrain around Mount Pinatubo. We also had problems with poor batteries and cable, observatory wiring, and timing. The problems that we encountered with the Mount Pinatubo network did not greatly affect our ability to monitor the volcano for public safety but have placed some constraints on the usefulness of the data for posteruption studies. The network was destroyed by the June 15 eruption, except for one station we had installed in a safe, easily accessible place at a distance of 17 km from the volcano. Had there been thermal protection for the antenna cables, one or two of the lost stations might have been saved.
The Mount Pinatubo network demonstrated some important lessons for future volcano-monitoring networks. Networks must be reliable, because they are a key monitoring tool on which public safety may depend. They must be designed to capture a wide range of seismic events, from small long-period events and tremor to large rock-breaking events. It is important to keep at least one station in a safe, easily accessible place to be able to maintain rudimentary monitoring capabilities if the rest of the network is destroyed. The narrow bandwidth of the analog telemetry system can limit the usefulness of the seismic data. This limitation can be largely overcome with dual-gain voltage-controlled oscillators or by digital systems now under development.
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On April 2, 1991, Mount Pinatubo awoke from 6 centuries of tranquility with steam explosions and vigorous fumarolic activity. Staff from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) immediately established an observatory west of Mount Pinatubo and began seismic monitoring with several portable seismic recorders (Sabit and others, this volume). On April 5, staff from the Volcano Disaster Assistance Program (VDAP), a cooperative program between the U.S. Geological Survey (USGS) and U.S. Agency for International Development (USAID) (Miller and others, 1993) based at Cascades Volcano Observatory (CVO) were alerted, and on April 23 a three-person USGS team flew to the Philippines to assist PHIVOLCS in monitoring Mount Pinatubo and to advise the U.S. Air Force at Clark Air Base, located at the eastern foot of the volcano. The USGS team carried seismic equipment designed around the standard USGS analog telemetry system and modified for use in volcanic monitoring. The USGS monitoring cache also included telemetered electronic inclinometers, which were incorporated into the network after the seismometers had been emplaced (Ewert and others, this volume).
The telemetered seismic network provided the joint Pinatubo Volcano Observatory (PVO) monitoring team with critical information that was used to forecast volcanic activity and which led to the successful evacuation of the area around Mount Pinatubo. This included evacuation of the majority of personnel from Clark Air Base 5 days before the climactic eruption on June 15,1991. The portable seismic network and base-station equipment were installed in slightly less than 3 weeks. With the telemetered network in place, seismic energy levels, hypocentral locations, spectral information, and types of seismic events could be quickly analyzed by the monitoring team and the team's analysis could be acted upon by public officials during the crisis. After the climactic eruption we were able to continue monitoring seismic activity despite destruction of most of the network because one station (CAB) had been deployed at a greater distance from Mount Pinatubo and remained operational.
Along with those aspects of the seismic network that contributed to the successful prediction and evacuation, we had some problems with the network that limited its effectiveness: bad batteries and antenna radio-frequency (RF) connections that caused numerous helicopter trips for station maintenance; dependence on a military communications repeater site that was vandalized before the main eruption, leaving us unable to locate events in the last days of the buildup; the uncalibrated seismometers; and the narrow dynamic range of the telemetry system, insufficient to effectively transmit data across the full signal-amplitude range; the tendency of the event-recording software to ignore emergent events like long-period (LP) events and teleseisms (Murray and others, this volume); and the lack of thermal protection from hot ash clouds for exposed components at the field sites.
Both the successes and failures we encountered provide valuable lessons for future volcano-monitoring networks. In this paper we describe the seismic equipment installed around Mount Pinatubo and discuss the factors that influenced installation and configuration of the network. This is followed by a short description of the telemetry, repeater sites, observatory wiring, and the site problems we encountered in operating the Mount Pinatubo network. We conclude with a review of the Mount Pinatubo network and recommendations for improvements and alterations to seismic equipment for future rapid deployments at restless volcanoes.
In the 20 days between April 25 and May 14, 1991, we installed a seven-station, three-repeater, nine-component seismic network around Mount Pinatubo (fig. 1 and appendix 1) and set up a recording site at the observatory on Clark Air Base. Seismic stations were placed in each quadrant around Mount Pinatubo and in a line extending east from the volcano toward the observatory on Clark (fig. 1). The network was configured to yield high-quality hypocenter solutions in the expected zone of seismic activity beneath the volcano and to rely as little as possible on repeaters located in rugged jungle terrain. The existing PHIVOLCS stations were not telemetered and were not routinely incorporated into this network. However, before the telemetered network became fully operational, S-minus-P phase arrival time differences from all stations around the volcano were used to determine rough locations.
Figure 1. Station locations and radiotelemetry links in the Mount Pinatubo area.
The telemetered seismic equipment we installed at Mount Pinatubo came from the VDAP cache at CVO (Lockhart and others, 1992a) and had been especially designed to be both durable and portable for responding to volcanic crises worldwide. Because the telemetered seismic equipment had been prepared in advance for deployment in harsh environments (Lockhart and others, 1992b), site preparation and installation rarely took more than 2 or 3 hours for a three-person field crew. Derived from the standard USGS analog telemetry system (Eaton, 1976), the stations used Mark Products L-4 1-Hz and L-22D 2-Hz geophones and USGS seismic telemetry employing UHF radio links.
The voltage-controlled-oscillator (VCO) board and radio were housed together in a waterproof plastic instrument case. We used environmental connectors sealed to the instrument case for the power, geophone, and antenna connections. The VCO board itself was bolted inside a metal box inside the instrument case, for protection from stray RF signals. We did not encounter any problems with radio interference of the VCO circuitry. We placed desiccant in each of the instrument cases and placed them with the batteries in a larger, nonwatertight equipment enclosure, usually a large plastic cooler with a lid. The coolers at all remote sites were buried in soil, in part to avoid the extreme daytime heat but also for protection against theft and vandalism. We cut large holes in the bottoms of the coolers to drain condensed moisture. The soil was initially permeable, loose and well drained at all of the sites, and there were no problems with seepage before the eruption. However, the accumulated tephra caused poor drainage at some sites after the eruption and some equipment got wet, but not that in waterproof cases (T. L. Murray, USGS, oral commun., 1991).
In order to reduce effects of wind noise on the geophones, we buried them at least 0.5 m deep. At each site we encapsulated the geophone in a 10-in section of 3-in plastic tubing capped on both ends and used a gland connector to pass the cable through the top endcap. The 3-in diameter L-4 geophone fit tightly into the tubing, which was set into a small fast-setting cement pad for a solid connection to the ground; the geophone could still be removed from the tube once the cement had set. The plastic sheath provided electrical isolation of the geophone from the earth, eliminating a potential source of electrical ground loops.
The VCO boards were designed at CVO by T.L. Murray (T.L. Murray, USGS, unpub. data, 1991); he used the basic circuit design in the USGS J502 VCO (F. Fischer, USGS, unpub. data, 1986) but with a much simpler calibration circuit than the one used by the J502. Individual station gain settings (see appendix 1) were set so that the ambient seismic noise was 100 mV peak-to-peak (p-p) measured at the input of the VCO chip. The VCO boards drew 15 mA at 12 V. All sites used 100 mW UHF radios and transmitted through 9-dB-gain yagi antennas. The radios drew around 85 mA, so the total current drain for a single-component station was 100 mA, or 2.4 ampere-hours/day. To power remote sites, we used pairs of locally purchased 80-ampere-hour lead-acid automobile batteries and a regulated 18 W solar panel. The batteries had a high failure rate, the result of which was lost data and frequent maintenance trips to sites accessible only by helicopter.
Each site was grounded from the negative battery terminal to a ground rod that was driven into the soil next to the equipment enclosure. The radio outputs were protected from lightning by plasma-discharge tubes grounded to the negative battery terminal. The VCO signal input and output lines and power lines were all protected from transient voltage surges by avalanche diodes. Despite the many fierce lightning storms that passed over parts of the network, only one station was lost: BUG was damaged by what appeared to have been a nearly direct strike, which destroyed the radio, damaged the VCO, and left arc burns on the VCO board and the metal box that contained it.
In this report we refer to the station locations by a three-letter code (such as CAB) and we refer to the recorded signal from that station by a four-letter code that indicates the orientation of the sensor; for example, CABZ is the signal from the vertical geophone at CAB.
We installed the first telemetered seismic station, LIL, immediately upon our arrival at Clark Air Base in order to quickly establish baseline seismic monitoring in a secure, accessible location. LIL was on Lily Hill at Clark Air Base near a large water tank. The site was selected because of ease of access and line-of-sight telemetry to our observatory. After a few days, it was clear that operation of the water pump in the tank generated excessive seismic noise, and the equipment was moved to a quieter site (CAB), which was still on the air base and readily accessible, 17 km from Mount Pinatubo. CAB was on a hill 100 m away from another water tank. The CAB site was not as noisy as LIL, but vibration noise of the pump for the nearby water tank limited CAB's usefulness until the evacuation on June 10 reduced water use on the air base, and, consequently, water-pump noise at CAB. We searched for alternative sites for the CAB station but found none better.
Nevertheless, CAB turned out to be one of the most critical stations in the network. During the most intense phase of volcanic activity, CABZ served as the most reliable indicator of Real-time Seismic Amplitude Measurement (RSAM) seismic energy (Endo and Murray, 1991; Endo and others, this volume) because CAB's great distance from the volcano resulted in smaller recorded amplitudes and less signal clipping than was observed at closer stations. It was the only station to survive the June 15 eruption, so it was the only seismic monitoring available to observatory staff until new stations could be installed some time after June 15.
We installed PIE and UBO as line-of-sight stations that, with CAB, formed an east-west line from Mount Pinatubo to Clark (fig. 1). PIE was installed on an elephant-grass-covered ridge 6 km east of Mount Pinatubo, at a site where we could land a helicopter, but which was otherwise inaccessible except by a long, steep hike through the jungle. UBO was located on an abandoned geothermal drill pad just east of the volcano's summit, 1.5 km from the vigorous fumaroles on the northeast side of the summit. Although there had been road access to this pad a few years before, landslides had made the road impassable, and the site was only accessible by helicopter. Burgos (BUR) and Patal Pinto (PPO) were sites north of Mount Pinatubo with road access. PPO was chosen because of road access, azimuthal coverage of the sector north of Mount Pinatubo, proximity to the active fumaroles, and the presence of a secure building we could use as a site. BUR site was selected because of its road access, azimuthal coverage of the sector to the northwest of Mount Pinatubo, and because it was line-of-sight to PPO. Mount Negron (GRN) was located on a sharp, brushy peak that provided azimuthal coverage on the south side of the volcano and that could serve as a repeater for stations to the southwest. At GRN, the site noise was very high until we re-set the geophone in a 1.5-m-deep hole. Sitio Buag (BUG) was chosen because it closed an azimuthal gap on the southwest side of the volcano, enjoyed helicopter access, and was line-of-sight to both GRN and Mount Cuadrado.
Our observatory was east of Mount Pinatubo at Clark Air Base. Before the June 15, 1991, eruption, Mount Pinatubo appeared from Clark as a craggy summit visible through a broad notch in a north-south line of jungle-covered peaks and ridges forming a wall just east of the summit. To locate events beneath the volcano, we had to ring the volcano with seismometers and transmit signals from the west over the wall of peaks and ridges down to the observatory at Clark. There were no unforested, helicopter-landable repeater sites for the seismic stations north and northwest of Mount Pinatubo that were line-of-sight to Clark. The sector to the south and southwest of Mount Pinatubo was line-of-site to the summits of both Cuadrado (10 km from Mount Pinatubo) and Negron (5 km from Mount Pinatubo). We installed a seismic station and repeater on Negron (GRN site) because we thought it's proximity to Mount Pinatubo would make it a good site to colocate a seismic station and because of a fortuitous landing pad that had been constructed a few years before on the otherwise inaccessible peak.
It was very difficult to relay the signals from BUR and PPO, the stations north of Mount Pinatubo, because of the expanse of jungle-covered hills between them and Clark. Before the eruption, a telemetry link to Clark would have required several long and tenuous repeater shots. Initially we considered Crow's Nest as a possible repeater site, a peak 16 km north of Mount Pinatubo. Crow's Nest had been used as a temporary military site a few years before and was considered a high-risk site for theft and vandalism. This plus poor telemetry test results from Crow's Nest to PVO convinced us to use a microwave telephone link from a nearby military communications site, Bunker Hill, which the U.S. Air Force had provided for our use. The Bunker Hill site was line-of-sight to PPO, equipped with generator power and guarded around the clock. This seemed to be the best choice for repeating telemetry signals from the north side of Mount Pinatubo back to Clark. In fact, it served well until the guards were evacuated on June 10 and the generators were stolen. After this, attempts were made as late as June 14 to install an alternative repeater on Crow's Nest with a 5-W UHF radio transmitting yagi-to-yagi to a receiver some 60 ft up an antenna mast on Clark, but the signal could not be received; the radio signal path was not line-of-site.
After the eruption, there were many good repeater sites because the eruption had removed all the jungle from the hills (T.L. Murray, oral commun., 1993). After the destruction of the first telemetered seismic network at Mount Pinatubo on June 15, 1991, USGS and PHIVOLCS staff installed a second seismic network around the volcano. Some of the sites of the first network were re-occupied, and seismic repeaters were installed at sites that had been inaccessible to us before the eruption.
Installed first as a single vertical-component seismic site, PPO was soon augmented to serve as a repeater for BUR. Later, we replaced the single vertical-component 1-Hz geophone with a three-component 2-Hz geophone. Then we added a two-component tiltmeter controlled by a low-data-rate (LDR) digital-telemetry platform (Murray, 1988, 1992, Ewert and others, this volume), which we patched into the seismic telemetry system. Next we added a rain gauge to the LDR platform (Marcial and others, this volume). With this addition there were seven data components transmitting over one radio link from PPO. As we had no multiple-input summing amplifier, signals from the three VCO's, the LDR platform, and the BURZ receiver were summed by wiring together the secondary windings of all the output transformers. The signal out from each platform was adjusted so that the amplitude of the summed signals was 1 V peak-to-peak and all signals were strong enough to break squelch on their respective discriminators. At PPO, the technique of hard-wiring the transformer secondaries worked as well as a multiple-input summing amplifier would have. In order to keep the LDR platform from spiking the seismic lines every time it transmitted, we kept the LDR board's modem chip turned on and transmitting a carrier tone at all times.
The PPO tiltmeter (Ewert and others, this volume) had initially been installed on the concrete floor of the hut at PPO on June 2. Wiring problems at PVO kept us from being able to analyze the PPO tilt data for a number of days; it became clear that our simple installation had been inadequate. Several more days passed while we worked on voice-communications problems between PVO and the west side of Mount Pinatubo. We returned to PPO to make a standard installation of the tiltmeter on June 10, but increasing activity at Mount Pinatubo cut our efforts short, so tilt data from the PPO tilt site were never reliable.
The PVO observatory computerized data-acquisition system is described by Murray and others (this volume). We received the telemetered seismic signals with 9-dB-gain yagi antennas and UHF radios. Signals went from the radios to USGS J101 and J110 discriminators and from there to drum recorders and the computer system. As the telemetry system developed, the observatory wiring became more complicated. Soon there were three telemetered seismic signals received directly (PIEZ, UBOZ, CABZ) and one radio link with two signals (Negron line, GRNZ, BUGZ). A telephone link from Bunker Hill carried four seismic signals (PPON, PPOE, PPOZ, BURZ) and two signals from an LDR digital transmitter (PPO tilt and rain gauge). The two LDR signals from PPO and the UBO tilt LDR line went to an LDR-receiver computer modem. The Bunker Hill telephone link was patched into both the discriminator rack and through an isolation amplifier to the LDR-receiver computer modem.
The network's location in the Philippines resulted in timing problems that we were not able to overcome completely. Although we had a GOES satellite clock for timing the seismic arrivals, the GOES satellite signals were not received over that part of the globe, so the satellite clock was not used. Instead, we timed the drums by manually synchronizing the drum's clocks to WWV broadcasts. Similarly, the PC clocks on both the seismic data-acquisition computer and the RSAM computer were manually synchronized to WWV. This was supposed to have been done daily, but often was not, especially during the first hectic weeks of the response. Manual timing to WWV gave a relative timing error no better than +-0.1 s. However, on several occasions we found the PC clock or RSAM clock to have drifted by several minutes, and this drift complicated posteruption studies. Either a GPS or Omega clock, which works anywhere in the world would have been a better solution. Even without a GPS or Omega clock, our timing problems would have been minimized had we had a stable time-code generator like that built by Jim Ellis for the USGS 5-day seismic recorder (Criley and Eaton, 1978).
Battery failures and loose RF connections caused the majority of our site visits for maintenance. The batteries and some RF cable had been purchased locally. Because of air-freight restrictions against shipping batteries, we have always found it much easier to purchase batteries locally than to try to ship them by air to distant locations. We have had problems with battery quality in other places too, but at Mount Pinatubo poor-quality batteries cost us dearly in lost data and maintenance visits, which required a helicopter.
The battery problems that plagued the rest of the network were also a problem at CAB, but we were able to maintain CAB easily because of it's accessibility and proximity to the observatory. The water-pump noise reduced the quality of the seismic data from CAB, and as the rest of the network was sufficient to locate events and to monitor volcanic seismicity like fumarolic noise, CAB battery maintenance was a low priority until just before the most intense phase of volcanic activity. Then we replaced the batteries and brought CAB back online.
We used RG-8 coaxial cable for antenna cable and environmental `N' type RF connectors. The locally purchased RG-8 turned out to be thinner than the cable we had been using, so the barrels of our crimp connectors were loose on the cable. On several occasions we had problems with the cable pulling out of the crimp connector barrels and ruining the connection; this problem required site visits. Later, we replaced suspect connectors with clamp connectors. Although the cable was still not held securely and could be pulled loose, heavy taping of the connector and first 6 inches of the cable with cold-shrink tape usually provided enough strain relief to make the connection serviceable.
In the last days before June 15, the utility of the data was reduced because the recordings from increasing numbers of stations clipped on high signal amplitudes. With station gains set to ambient noise levels early in the precursory sequence, the overall seismic energy saturated our instruments just at the time when it was too dangerous to visit most sites and reset station gains. CAB was easily accessible, and it would have taken only minutes to drive to the site and reduce the gain. However, doing so would have changed the RSAM values and the appearance of the seismogram to which we had become visually "calibrated" in making quick estimates of levels of background seismicity. With the rapidly developing eruptive sequence, we did not want to have to recalibrate our eyes to a different, much quieter seismogram, so we did not reduce gain at any station.
Except for CAB, the Mount Pinatubo network was destroyed by the June 15 eruption. The Bunker Hill repeater, which tied into a U.S. Air Force microwave link to Clark (with the signals BURZ, PPOZ, PPON, PPOE, and the PPO tiltmeter and rain gauge) was lost to vandalism and theft on June 11. Station UBO was knocked out by a small volcanic ash cloud apparently related to an explosion signal at about 0341 local time June 12. Coincidentally, UBO seismic telemetry had failed several hours before the blast signal was registered at PIE. We collected data from PIE, BUG, and GRN until the climactic eruption on the afternoon of June 15, when pyroclastic flows or related surges damaged or annihilated these stations. We lost UBO, GRN, PIE, and PPO completely. UBO was at or near the edge of the caldera that formed on June 15. The PPO site was destroyed by the eruption. No remnants of the concrete hut were found during a visit to the site in early 1992 (R.P. Hoblitt, USGS, oral commun., 1993). GRN was either destroyed or buried. Nothing could have been done to save these sites. Two stations (BUG and BUR) were not destroyed but merely damaged, their RF cables melted through and shorted out. In addition, solar panels melted and the plastic insulators used on the yagi antennas melted. Neither the loss of the solar panels nor (probably) the antenna insulators would have been immediately fatal to the sites, but the melted antenna cables shorted out the radio signals to the antennas. After the climactic eruption, BUG required only a new antenna, antenna cable, and fresh battery (T.L. Murray, oral commun., 1993).
The principal reason for monitoring volcanoes is the safety of the local populace, so seismic volcano monitoring requires seismic monitoring networks of great reliability. The network must also be flexible enough to capture a wide range of volcanic seismicity amplitudes, from the small but important rock-breaking and LP events and tremor to some of the large signals that accompanied the Mount Pinatubo eruption.
The destruction of all but one station in the Mount Pinatubo network during the major eruption raises the question of network design. The lesson from Mount Pinatubo is to maintain at least one station in a safe place far enough from the volcano to survive any likely eruption and record large-amplitude seismic activity on-scale. The station must be readily accessible. In anticipation of large eruptions like at Mount Pinatubo, it would be desirable to have an outer network of three or four stations in safe places up to 30 km from the volcano. This would allow location of large events during periods of high seismic activity, when the closer stations might be saturated or destroyed. At Mount Pinatubo the ability to locate events during the height of the eruption and afterward might not have had any immediate public-safety benefits, but the posteruption analysis of such data might have been of great scientific value.
We were very lucky to have had good helicopter support. The telemetered network may not have been possible at all without helicopter access to otherwise inaccessible sites. Because of the difficulties in telemetering seismic data from a widely spread network in mountainous terrain like that near the Mount Pinatubo area, more attention should be given to developing flexible and reliable alternatives to low-power ground-based line-of-sight telemetry networks. The major problem with low-power ground-based line-of-sight telemetry nets is the dependence on line-of-sight repeater sites, which are frequently difficult to access. Failure of an important telemetry link can eliminate a significant part of the network, as exemplified by the loss of the Bunker Hill military microwave link on June 11 when a major eruption was clearly imminent. There were no line-of-sight repeater sites that could have replaced the Bunker Hill repeater.
Satellite telemetry is occasionally suggested for use in mountainous terrain like this, but systems like Argos (Collecte Localisation Satellites, 1992) and the Geostationary Operational Environmental Satellites (National Oceanic and Atmospheric Administration, 1984) transmit only a small amount of data as few as eight times daily. For a volcano observatory to receive the signals from data collection platforms that are not line-of-sight, one must either have a telephone link to an earth station, or purchase one at some expense. Because of these limitations, Argos and GOES platforms are better suited to monitor potentially active volcanoes rather than active volcanoes that threaten populations. We feel that low-power ground-based line-of-sight telemetry networks are currently a more effective monitoring tool than satellites. Another argument for the use of low-power ground-based line-of-sight telemetry networks is that the standard seismic telemetry techniques are widely known to technicians in geological institutions all over the world. Productive collaborations like that between the USGS and PHIVOLCS during the Mount Pinatubo crisis are much easier to coordinate and carry out when both groups use and know the same equipment.
A telemetered seismic network located on only one side of a volcano might be very useful as an alternative to the Pinatubo network configuration when monitoring a volcano where one side is inaccessible. Such a network trades hypocentral location accuracy for simplicity and ease of installation and might involve a tripartite array (see, for example, Ward and Gregersen, 1973), be a kilometer or more on a side, and use three-component geophones. It might be located off the volcano and be used in conjunction with other stations on the edifice in order to make better depth estimates. The tripartite array or some other design that maximizes the monitoring power of a network whose station sites are limited to one side of a volcano deserves further inquiry.
The most fundamental lesson from the telemetered Mount Pinatubo network was the importance of receiving the data locally so scientists could act on it immediately and advise the local officials in charge of the public's welfare. The presence of the monitoring team in the community near the volcano gave credibility and urgency to their statements and was a key factor in the pre-eruption evacuations. In turn, their minute-by-minute decisions would have been less well informed and less certain without the constant stream of telemetered tilt and seismic data. Realtime telemetered data were analyzed in the context of concurrent visual observations for rapid, informed predictions, which were then passed on to the officials in charge of the public's welfare. A monitoring scheme that transmitted seismic data by satellite to experts sitting in distant offices would not have worked nearly so well.
Heat caused the failure of those stations that survived or could be found. The stations had been buried, so the geophones and electronics were unaffected by the heat, but at both UBO and BUG the antenna cables had been melted and shorted out by hot pyroclastic currents. Future installations should continue to protect the electronics from heat, and the antenna cables might be protected from hot pyroclastic currents by flexible metal conduit.
The Mount Pinatubo experience showed a need for more dynamic range at the seismic stations to cover the wide range of seismic amplitudes encountered during the evolution of volcanic activity. Early in the activity, station gains were set to detect the dominant small, shallow earthquakes and the LP's, both of which were important in assessments and predictions. As activity increased, the high-gain VCO transmissions went off-scale during the larger events. At Mount Pinatubo, it was impractical to change the station gains because of safety concerns and for consistency of the RSAM data and visual monitoring of the drums. Since returning from Mount Pinatubo, the USGS staff at CVO have addressed this problem by constructing seismic stations that use dual VCO's for volcano-monitoring, similar to a VCO design used at some of the California network stations (J.P. Eaton, USGS, written commun, 1993). The VCO boards constructed at CVO are piggybacked so that the preamplifier output of a VCO board using one carrier frequency is fed into a second board that uses another carrier frequency. The two boards are set at different gains: one is set for maximum attenuation (44 dB gain) and the other is set to a more standard gain of 62-74 dB. The output carrier frequencies of the two VCO boards are summed together at the secondaries of the output transformers and are fed into a single radio. The dual-VCO system doubles the telemetry load but in theory can increase the bandwidth of on-scale signals from around 40 dB to as much as 70 dB or so, which is nearly the 72 dB bandwidth of the 12-bit-plus-sign analog-to-digital converter used in the data acquisition system. The dual-VCO system retains the flexibility of analog telemetry to mix signals from different stations in the field and adds considerable bandwidth.
Digital systems are in use in some areas and are under development in others. In theory, they will be an improvement over analog telemetry, but analog telemetry will continue to be important in volcano monitoring for some time to come, especially where low cost, simplicity of design and ruggedness are concerns. Gain-ranging VCO's, such as are used in parts of Alaska (Rogers and others, 1980), are an alternative to dual-gain VCO's. Because of our reliance on RSAM values and rapid visual observation of drum records in predicting eruptions, we would not have wanted an automatic gain-ranging VCO, which changes the telemetered seismic energy values and the appearance of the seismograms. Perhaps the next step in volcano-monitoring networks should be hybrid networks, with easily deployed and reliable analog stations to ensure good coverage of the volcano combined with a few digital stations for recording large events.
At Mount Pinatubo, we used four drum recorders for a quick analysis of the seismicity. Seismograms from a representative signal (PIEZ) were posted on a wall of the observatory where they made a dramatic display of the evolving seismic activity. Local officials who toured PVO could see this display of seismic records and understand the clear increase in seismic activity it showed. This simple display proved to be a powerful tool in educating local civil and military officials. Besides helping the monitoring team in public education, the seismograms were of great value in making rapid analyses of the state of the volcano. Critical volcano-seismic signals such as tremor and small, emergent LP's not captured by event recorders showed up on the seismograms, and as the system gains were kept constant, drum recorders permitted the overall level of seismic activity to be visually checked minute by minute. Event epicenters could be estimated rapidly by checking the order of first arrivals on the four drums.
Observatory wiring at PVO became a more complicated system as the number of telemetered instruments increased. Problems with the wiring at PVO combined with other factors to cost us the effective use of the tiltmeter installed at PPO. Wiring modifications consumed a lot of time as stations were added to the network. Ideally we would have liked the wiring to have been simple to minimize "down time" by making it easy to modify or troubleshoot. Since returning from Mount Pinatubo, CVO staff has simplified observatory wiring with printed-circuit-board switch panels (A.B. Lockhart, unpub. data, 1992). The switch panels route seismic signals between receivers and discriminators, and from there to output devices: drum recorders, RSAM, digital data-acquisition systems and auxiliary devices.
Although it did not affect our monitoring, the absence of a good timing system at Mount Pinatubo complicated the posteruption reconstruction of events. Because we were not tied into a global time standard, eruption events recorded elsewhere in the Philippines are not easily correlated with data from the Mount Pinatubo network. Another problem is that the timing common to the SSAM data and the events recorded on the data-acquisition system was independent from the RSAM timing.
We would like to thank Tom Murray and John Ewert for their help installing and maintaining the telemetered network. Thanks are also due to the helicopter pilots and flight crews of the U.S. Thirteenth Air Force, Third Tactical Fighter Wing, especially Capt. Brett Nyander, for excellent logistical support in hazardous and uncertain conditions. This paper benefited mightily from discussions with Randy White and Tom Murray and from reviews by Jerry Eaton, John Lahr, and Emmanuel Ramos.
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Criley, Ed, and Eaton, J.P., 1978, Five-day recorder system (with a section on the Time Code Generator, by Jim Ellis): U.S. Geological Survey Open-File Report 78-266, 86 p.
Eaton, J.P., 1976, Tests of the standard (30 Hz) NCER FM multiplex telemetry system, augmented by two timing channels and a compensation reference signal, used to record multiplexed seismic network data on magnetic tape: U.S. Geological Survey Open-File Report 77-884, 61 p.
Endo, E.T., and Murray, T.L., 1991, Realtime Seismic Amplitude Measurement (RSAM): A volcano monitoring and prediction tool: Bulletin of Volcanology, v. 53, p. 533-545.
Endo, E.T., Murray, T.L., and Power, J.A., this volume, A comparison of preeruption real-time seismic amplitude measurements for eruptions at Mount St. Helens, Redoubt Volcano, Mount Spurr, and Mount Pinatubo.
Ewert, J.W., Lockhart, A.B., Marcial, S., and Ambubuyog, G., this volume, Ground deformation prior to the 1991 eruptions of Mount Pinatubo.
Lockhart, A.B., Murray, T.L., Ewert, J.E., LaHusen, R.G., and Hadley, K., 1992a, A USGS equipment cache for responding to volcanic crises: Eos, Transactions, American Geophysical Union, v. 73, no. 43, p. 68.
Lockhart, A.B., Murray, T.L., and Furukawa, B., 1992b, Operating low-power telemetry networks in severe environments, in Ewert, J.W., and Swanson, D.A., eds., Monitoring volcanoes: Techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-1990: U.S. Geological Survey Bulletin 1966, p. 25-36.
Marcial, S.S., Melosantos, A.A., Hadley, K.C., LaHusen, R.G., and Marso, J.N., this volume, Instrumental lahar monitoring at Mount Pinatubo.
Miller, C.D., Ewert, J.E., and Lockhart, A.B., 1993, The USGS/USAID Volcano Disaster Assistance Program: The next five years, U.S. Geological Survey Open-File Report 93-379, 7 p.
Murray, T.L., 1988, A system for telemetering low-frequency data from active volcanoes: U.S. Geological Survey Open-File Report 88-0201, 28 p.
------1992, A low-data-rate digital telemetry system, in Ewert, J.W., and Swanson, D.A.,eds., Monitoring volcanoes: Techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-1990: U.S. Geological Survey Bulletin 1966, p. 11-23.
Murray, T.L., Power, J.A., March, G.D., and Marso, J.N., this volume, A PC-based realtime volcano-monitoring data-acquisition and analysis system.
National Oceanic and Atmospheric Administration, 1984, GOES data collection system and data processing system: Washington, D.C., National Environmental Satellite Data and Information System User Interface Manual, 132 p.
Rogers, J.A., Maslak, Sam, and Lahr, J.C., 1980, A seismic electronic system with automatic calibration and crystal reference: U.S. Geological Survey Open-File Report 80-324, 130 p.
Sabit, J.P., Pigtain, R.C., and de la Cruz, E.G., this volume, The west-side story: Observations of the 1991 Mount Pinatubo eruptions from the west.
Ward, P.L., and Gregersen, S., 1973, Comparison of earthquake locations determined with data from a network of stations and small tripartite arrays on Kilauea volcano, Hawaii: Bulletin of the Seismological Society of America, v. 63, no. 3, p. 679-711.
A synopsis of the first telemetered Mount Pinatubo seismic network, April 25 to June 15, from A.B. Lockhart's field notes. Bracketed remarks are not from the notes.
LIL (LILy Hill; LILZ) 15°11.62 N., 120°31.81 E.
4/25. Installed at Clark Air Base on Lily Hill adjacent to water tank. L-c4 buried 1 m deep in soil, set into Kwik-set pad. Attenuation 30 dB.
5/1. LIL removed.
PIE (Mount PInatubo East; PIEZ) 15°10.00 N., 120°25.73 E.
4/30. Installed in elephant grass on ridge 6 km east of summit. L-4 buried 1 m deep in soil, set into Kwik-set pad. Attenuation 18 dB.
6/15. PIE destroyed.
UBO (Mount PinatUBO; UBOZ) 15°08.33 N., 120°21.76 E.
[So-named because map station location is in the center of the last letter ("o") of "Mount Pinatubo" label on the 1:50,000-scale DMA map of the Mount Pinatubo quadrangle].
5/1. Installed on geothermal drillpad 1 km east of summit. L-4 buried in rocky fill, set into Kwik-set pad. Attenuation 36 dB.
5/31. Site visit to install tiltmeter.
6/11. Telemetry signal lost, 2200 local time.
6/12. At 0341, explosion signal recorded on PIEZ record; pyroclastic current apparently dusted UBO, noted during 0600 flyby, signal still weakly transmitting. Antenna cable damaged?
BUR (BURgos; BURZ) 15°13.41 N., 120°15.56 E. (May be as far west as 120°15.41 E.).
5/5. Installed near Barangay Burgos on ridge overlooking bridge. L-4 buried 1 m deep in soil, set into Kwik-set pad. Road access. Attenuation 24 dB.
5/7. Receiving BURZ at Clark.
6/11. Bunker Hill vandalized, telephone link to Clark lost.
PPO (Patal PintO; PPOZ, PPOE, PPON) 15°10.95 N. 120°20.50 E.
5/3. Installed in cinderblock guardhouse 2 km N of summit. in Aeta village (30 inhabitants). Road access. L-4 sits on floor inside structure, Attenuation 36 dB.
5/6. Receiving data at Clark.
5/12. Installed 3-component L-22D.
PPOZ, Attenuation 24 dB.
PPON, Attenuation 30 dB.
PPOE, Attn 30 dB.
5/30. Site visit, gains reset: PPOZ, Attenuation 30 dB.
6/11. Bunker Hill vandalized, telephone link to Clark lost.
BUG (Sitio BUaG; BUGZ) 15°05.05 N. 120°16.90 E.
5/11. Installed on grassy hilltop overlooking Sitio Buag, 10 km southwest of summit, L-4 buried 1 m deep in soil, set into Kwikset pad. Attenuation 18 dB.
5/14. Signal first repeated to base (through GRN).
5/24. Site visit; VCO, radio struck by lightning. Radio destroyed, VCO burned but reparable.
5/28. Site reinstalled: Attenuation 18 dB.
6/15. All exposed BUG components damaged by heat. Lose transmissions.
GRN (mt. neGRoN; GRNZ) 15°05.67 N., 120°21.98E.
[Note: Installation notes on GRN were very sketchy, and the attenuation setting was never written down during any of the trips made to the site. After the eruption one of us, Gemme Ambubuyog, used the GRNZ VCO calibration signals recorded on seismograms to suggest an attenuation setting of 24 dB. The same analysis corroborated the relative attenuation settings for UBOZ, PIEZ and LILZ.]
5/14. Install repeater and L-4-based seismic station on peak of Negron, several kilometers southeast of Mount Pinatubo. Site is nearly always clouded in and is inaccessible on foot but has a helicopter pad on the summit. Off the pad, the peak of Negron is brushy and knife-edged.
5/16. VO-COM repeater installed at Negron.
[Early June: Geophone disinterred and reburied 1.5 m deep.]
6/15. GRN annihilated.
CAB (Clark Air Base; CABZ) 15°10.76 N., 120°30.61 E.
5/1. Installed on Clark Air Base on hill 200 m from water tank. Site is in concrete hut with L-4 buried adjacent to WW II concrete emplacement about 10 m away. Location is compromise between security (afforded by concrete hut), radio shot to apartment (afforded by hill), and noise of water tank/pump. It is hoped that the water tank noise will not be overwhelming.
5/5. Attenuation reduced from 30 dB to 24 dB 1245 local time.
[During the latter part of May and early June, CAB was offline due to battery problems. Because of noise from water tank, data are of poor quality anyway; consequently repair of the problem is assigned a low priority.]
6/11. Station brought back online with battery change. Now that the base has been evacuated, there is no water pump activity and station is quieter.
FIRE and MUD Contents
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