Case Study: A Local Ambient-noise Array

September 2007

Introduction

As data processing techniques have advanced in recent years, the power of seismic noise as an analysis tool has been recognised.

Previously only signals, e.g. earthquakes and active sources, that comprised only a small amount of the total data recorded were perceived as useful. Now, the ambient seismic noise recorded for the vast majority of the time is also being studied.

In an ambient noise study the important information is stored in the coherent signals that cross arrays. These signals can be from a variety of sources: long period surface waves produced in distant earthquakes, ~5s period microseisms initiated by ocean waves and high frequency cultural noise.

Once the coherent signals between two stations in an array are stacked together, they show how various frequencies are affected by the ground between the stations. This information gives a picture of the crust between the stations — the higher the frequency being studied, the shallower the structure imaged.

For a picture of the surface of the crust in a region, an array of sensors can be used. Stations in the array need to be stationed reasonably close together, in order to record the highest frequencies before they are attenuated. They also need to be arranged such that many crossing propagation paths are produced. Such a network need only run for a small amount of time (a few weeks) in order to produce enough data to work with.

In Thessaloniki, Greece, GSL demonstrated how easy such a network can be to install and run for up to a week on internal power supplies. Similar networks, with more permanent power supplies, can also be used in volcano monitoring situations.

Scream! Spectrogram of the cultural noise at 3 stations in a city.

Equipment

In total eleven stations were set up in the array. Each station consisted of:

One CMG-6TD an ultra-lightweight, three-component digital output seismometer ideally suited for rapid installation. This has an on-board CD-24 digitizer.
One CMG-SSU (serial server/UPS), which provided communications links. The SSU is designed to give your installation the greatest possible autonomy. An optional internal UPS allows the whole station to operate for up to 72 hours from an internal battery. Serial Servers can also operate from either 10–24V DC power or 110/240V AC mains (outlet) power without additional hardware.
One WiFi OMNI 2.4 Ghz Outdoor Antenna. This has a range of 300-500m depending on conditions, line of sight obstructions, and power. 5GHz antennae are also available for equivalent distances.

Station Set-up

The CMG-6TD has the option of both an RS232 and RS422 port directly out of the on board digitizer. In this case the CMG-SSU was used as well, in order to provide the station with 72 hours worth of power, without the need of solar panel, mains or external batteries.

Idealised setup of a such a system, with maximum distances between points shown. An external battery provides additional power at each station, and the data is ultimately transmitted to a data centre away from the array.

The CMG-SSU connected to the WiFi aerial and a laptop.

The CMG-6TD was connected via its 10-way military-spec connector on its lid to a break-out box, to which the GPS and SSU were attached. The GPS is used to provide accurate time stamps to the data. The CMG-SSU provided power, and forwarded data.

The CMG-SSU has RS232, RS422 and WiFi connectors. This allows a laptop to be plugged directly into it, checking data at its source. The WiFi antenna is also connected, and put in a location with a reasonably unobstructed line of sight to the receiver.

Array Set-up

The array was located in a small area of park in central Thessaloniki. Ten of the stations were placed at varying radii and azimuths away from a central station. Six of the stations were located every 60 degrees around a 10m radius circle around this central point, whist the remain four stations were up to 150m away.

Set up of a number of stations within the local array. All stations are within 150m of the central hub. The CMG-6TD sensor and CMG-SSU are next to each other on the ground. The GPS receiver can be anywhere with a clear sky view. The WiFi aerial was elevated to improve its range.

The central station consisted of the same instrumentation as the satellite stations, connected to a recording hub. The central hub was a hard wearing, waterproof case containing a Lantronix wireless port, an Ethernet access port and an Afar radio link. The Ethernet can be linked to either a Laptop running the Scream! Software for recording and viewing the incoming data, or to a CMG-DCM. The CMG-DCM has a 40 Gb storage capability, so the data can be stored locally, as well as being re-transmitted over the radio link, over distances of up to 50 km.

Layout of the inner ring of instruments in the array.

In other environments, away from tall buildings, such as on a volcano, the WiFi can be expected to work up to 500m away. This allows for a broader array, which can be used in the detection and location of local earthquakes, as well as ambient noise surveys.

Scream! waveviewer for raw vertical data from seven of the stations over a 30 minute period. The coherency of much of the noise is instantly noticeable.



Home Products New installations Customer support About us Latest news The company Addresses and directions Local distributors Vacancies Live seismic activity	Printable version Case Study: A Local Ambient-noise Array September 2007 Introduction As data processing techniques have advanced in recent years, the power of seismic noise as an analysis tool has been recognised. Previously only signals, e.g. earthquakes and active sources, that comprised only a small amount of the total data recorded were perceived as useful. Now, the ambient seismic noise recorded for the vast majority of the time is also being studied. In an ambient noise study the important information is stored in the coherent signals that cross arrays. These signals can be from a variety of sources: long period surface waves produced in distant earthquakes, ~5s period microseisms initiated by ocean waves and high frequency cultural noise. Once the coherent signals between two stations in an array are stacked together, they show how various frequencies are affected by the ground between the stations. This information gives a picture of the crust between the stations — the higher the frequency being studied, the shallower the structure imaged. For a picture of the surface of the crust in a region, an array of sensors can be used. Stations in the array need to be stationed reasonably close together, in order to record the highest frequencies before they are attenuated. They also need to be arranged such that many crossing propagation paths are produced. Such a network need only run for a small amount of time (a few weeks) in order to produce enough data to work with. In Thessaloniki, Greece, GSL demonstrated how easy such a network can be to install and run for up to a week on internal power supplies. Similar networks, with more permanent power supplies, can also be used in volcano monitoring situations. Scream! Spectrogram of the cultural noise at 3 stations in a city. Equipment In total eleven stations were set up in the array. Each station consisted of: One CMG-6TD an ultra-lightweight, three-component digital output seismometer ideally suited for rapid installation. This has an on-board CD-24 digitizer. One CMG-SSU (serial server/UPS), which provided communications links. The SSU is designed to give your installation the greatest possible autonomy. An optional internal UPS allows the whole station to operate for up to 72 hours from an internal battery. Serial Servers can also operate from either 10–24V DC power or 110/240V AC mains (outlet) power without additional hardware. One WiFi OMNI 2.4 Ghz Outdoor Antenna. This has a range of 300-500m depending on conditions, line of sight obstructions, and power. 5GHz antennae are also available for equivalent distances. Station Set-up The CMG-6TD has the option of both an RS232 and RS422 port directly out of the on board digitizer. In this case the CMG-SSU was used as well, in order to provide the station with 72 hours worth of power, without the need of solar panel, mains or external batteries. Idealised setup of a such a system, with maximum distances between points shown. An external battery provides additional power at each station, and the data is ultimately transmitted to a data centre away from the array. The CMG-SSU connected to the WiFi aerial and a laptop. The CMG-6TD was connected via its 10-way military-spec connector on its lid to a break-out box, to which the GPS and SSU were attached. The GPS is used to provide accurate time stamps to the data. The CMG-SSU provided power, and forwarded data. The CMG-SSU has RS232, RS422 and WiFi connectors. This allows a laptop to be plugged directly into it, checking data at its source. The WiFi antenna is also connected, and put in a location with a reasonably unobstructed line of sight to the receiver. Array Set-up The array was located in a small area of park in central Thessaloniki. Ten of the stations were placed at varying radii and azimuths away from a central station. Six of the stations were located every 60 degrees around a 10m radius circle around this central point, whist the remain four stations were up to 150m away. Set up of a number of stations within the local array. All stations are within 150m of the central hub. The CMG-6TD sensor and CMG-SSU are next to each other on the ground. The GPS receiver can be anywhere with a clear sky view. The WiFi aerial was elevated to improve its range. The central station consisted of the same instrumentation as the satellite stations, connected to a recording hub. The central hub was a hard wearing, waterproof case containing a Lantronix wireless port, an Ethernet access port and an Afar radio link. The Ethernet can be linked to either a Laptop running the Scream! Software for recording and viewing the incoming data, or to a CMG-DCM. The CMG-DCM has a 40 Gb storage capability, so the data can be stored locally, as well as being re-transmitted over the radio link, over distances of up to 50 km. Layout of the inner ring of instruments in the array. In other environments, away from tall buildings, such as on a volcano, the WiFi can be expected to work up to 500m away. This allows for a broader array, which can be used in the detection and location of local earthquakes, as well as ambient noise surveys. Scream! waveviewer for raw vertical data from seven of the stations over a 30 minute period. The coherency of much of the noise is instantly noticeable.