Monitoring Volcanic Eruptions with a Wireless Sensor Networks

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Monitoring Volcanic Eruptions with a Wireless
Sensor Networks

• Geoffrey Werner-Allen, Jeff Johnson, Mario Ruiz,
  Jonathan Lees, and Matt Welsh
• Harvard University
• EWSN05
• 
  Presented by Tim

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Outline

• Introduction
• Background
• System Design
• Deployment
• Distributed Event Detection
• Evaluation
• Conclusion

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Introduction

• Volcanic monitoring has a wide range of goals,
  related to both scientific studies and hazard
  monitoring.
• Volcanologists currently use wired arrays of
  sensors to monitor volcanic eruptions.
• Wireless sensor networks have the potential to
  greatly benefit studies of volcanic activity.

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Background

• Infrasound (Infrasonic wave)
• Sound with very low frequency (150Hz)
• Very high amplitude but not audible
• Seismic wave
• Wave travels through the Earth, often as the
  result of an earthquake or explosion

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Volcanic Monitoring
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Challenges and Issues

• Existing data loggers store data locally
• e.g., 1 or 2 Gb microdrives, store about 15 days'
  worth of data
• Must trek up to the station to retrieve the data
• Usually very inaccessible can take several hours
  to drive/hike in
• Very high power consumption
• Two car batteries plus solar panels to recharge
• Very expensive
• Individual data logger costs thousands of
• Still need PCs/laptops to process and store data
  permanently
• Hard to deploy large number of stations
• Size, cost, power requirements,...

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Opportunities for wireless sensor networks

• Data sampling rates of 100 Hz
• Very small, low power, easy to deploy
• Can put out a larger number of sensors in an area
• Can customize software on the motes for capture,
  preprocessing, etc.

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Outline

• Introduction
• Background
• System Design
• Deployment
• Distributed Event Detection
• Evaluation
• Conclusion

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System Architecture
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Infrasound Node

• Sample data continuously at 102.4Hz
• A set of 25 consecutive samples is packed into a
  32-byte packet and transmitted at approximately 4
  Hz.
• The aggregator will send acknowledgement back. If
  source node does not receive ack, itll
  retransmit up to 5 times.

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Aggregator Node
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GPS Receiver Node

• Motes record sample and GPS time seq in
  message
• Can be used to align samples from each mote

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Time Regression

• Uncertainties
• The sampling rate of individual note may vary
  slightly over time, due to changes in temperature
  and battery voltage.
• The log do not record the precise time.
• Apply a linear regression to the data log stream
  and map individual sample to a true time.

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Physical Packaging
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Outline

• Introduction
• Background
• System Design
• Deployment
• Distributed Event Detection
• Evaluation
• Conclusion

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Volcano Tungurahua

• Active volcano in central Ecuador 5018 m
• Site of much ongoing seismological research

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Deployment

• Three infrasound nodes, one central aggregator
  node and a GPS receiver.
• The GPS receiver and FreeWave modem were powered
  by a 12 V car battery. All other nodes were
  powered by 2 AA batteries.
• The distance between sensors and observatory is
  about 9km.
• The deployment was active from July 2022, 2004
  and collected over 54 hours of infrasonic
  signals.

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Deployment
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Deployment
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Deployment
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Data Analysis- Loss Rate

• Weather conditions (e.g., rain) affected radio
  transmission.
• Mote 4 experienced very low loss, due to its
  position with line-of-sight to the receiver.
• Mote 3 experienced higher loss, probably due to
  antenna orientation.

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Data Analysis- Correlation

• The result of wireless sensor array shows high
  correlation with wired station.

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Outline

• Introduction
• Background
• System Design
• Deployment
• Distributed Event Detection
• Evaluation
• Conclusion

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Distributed Event Detection

• The initial deployment is not feasible for larger
  arrays deployed over long period of time.
• To save bandwidth and energy, it is desired to
  avoid transmitting signals when the volcano is
  quiescent.

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Mechanism

• Each node samples data continuously at 102.4 Hz.
• When the local event detector triggers, the node
  broadcasts a vote message.
• If any node receives enough votes from its
  neighbor nodes, it initiates global data
  collection by flooding a message to all nodes in
  the network.
• Token-based scheme for scheduling transmissions.
• The order depends on node ID.

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Local Detector Design

• Threshold-based detector
• Exponentially weighted moving average based
  detector

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Local Detector Design

• Threshold-based detector
• Triggered whenever a signal rises above Thi and
  falls below another Tlo during some time window
  W.
• Because it relies on absolute thresholds, it is
  sensitive to particular microphone gain on each
  node.

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Local Detector Design

• Exponentially weighted moving average based
  detector
• For each sample, calculate two moving averages
  with different gain parameters, ashort ,along
  ,and compare the ratio of the two averages.
• e.g., (ashort 0.05,along 0.002)
• If the ratio exceeds some threshold T (i.e., the
  short-term average exceeds the long-term average
  by a significant amount), the detector is
  triggered.

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Outline

• Introduction
• Background
• System Design
• Deployment
• Distributed Event Detection
• Evaluation
• Conclusion

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Evaluation

• Use 8 mica2 nodes in the lab, but only 4 nodes
  with infrasound sensor board.
• The infrasound signals were produced by closing
  the lab door.
• Three parts
• Energy usage
• Bandwidth usage
• Detector accuracy

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Energy usage

• Each node exhibits a baseline current draw of
  about 18mA and supply voltage is 3 V.
• Assuming that nodes detect a correlated signal
  every ½ hours, and locally vote at twice this
  rate.

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Bandwidth usage

• Continuous sampling scheme consumes nx4x32
  bytes/sec of bandwidth
• (n of nodes, each node transmit one pkt every ¼
  sec, size of pkt 32bytes)
• Because of the low frequency of eruptions,
  distributed event detection uses less bandwidth.

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Detector Accuracy

• Fed the detectors with the complete trace of data
  recorded on Tungurahua.

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Future Work Conclusion

• Seismology presents many exciting opportunities
  for wireless sensor networks.
• To expand the number of sensors in the array and
  distribute them over a wider aperture.
• The long-term plans are to provide a permanent,
  reprogrammable sensor array on Tungurahua.

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My Comments

• The idea is simple but its hard work to deploy
  the motes in such a place.
• To do research needs lots of passion.
• The first mote-based application to volcanic
  monitoring!
• Provide a wealth of experience to develop more
  sophisticated tools.