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Seismic Vaults

IRIS/PASSCAL Intern Caps Busy Summer with Alaska Deployment at Poker Flat

It's been a busy summer for IRIS/PASSCAL intern Kasey Aderhold, a PhD Candidate at the Department of Earth & Environment, Boston University. Kasey has been heavily involved in an ongoing comparison of seismic vaults to direct burial installations at two sites: the Dotson Ranch near Socorro, NM, and Poker Flat in Alaska. 

 

PASSCAL's Polar Group Revs Up for Antarctic Summer

This is the time of year when the Polar group at PASSCAL is busy deploying teams to the Antarctic. Since polar conditions make for difficult site installations, PASSCAL is pleased to present the following instructive videos of Antarctic station deployments.

Courtesy of Audrey Huerta of Central Washington University, here is a time-lapse video of the difficult installation of a POLENET seismic system on a rock surface at Miller Ridge in the Transantarctic Mountains. This installation took about 4 hours in real-time. You will see shadows encroaching as the sun sets toward the end of the four-minute video.

Insulated Sensor Vault Drawings

Vault Equipment Drawings Vault Double wall UV resistant polyethylene shell 2" thick indexed phenolic sensor base 3" thick polystyrene insulation Trillium 240 or Guralp T3 seismometers Detailed Drawings 3" Thick Insulation 2" Thick Indexed Phenolic Base Double Wall UV Resistant Polyethylene Shell

Station Enclosure Drawings

Station Enclosure Drawings   Large Enclosure:  Insulated Hardigg AL3434-2807 External dimensions:  37" x 37" x 37" Internal dimensions:  24" x 24" x 21" 2" thick vacuum insulation panel (VIP) sub-enclosure Protective polypropylene barrier between VIPs and electronics Integrated cable harness with external bulkhead connectors Year round operation with rechargeable AGM batteries   Detailed Drawings Hardigg AL3434-2807 Insulated:  Assembly Drawing with BOM

Broadband Vault Construction (Manual)

 Broadband Vault Construction

Diagram of Vault

Construction of the vault for broadband seismometers has a direct impact on data quality. Before construction can begin there are two other important considerations as well; location, and setting. A short discussion of these topics precedes the construction details because you should really pick the right site before going to all the work of building a vault. It takes a long time to find good sites which balance the competing requirements of low noise, access, security, position within the array, power, permission, etc. Finding one site per day is fairly productive.

 

Location:

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Experiment Overview Manual

Experiment Overview Types of Experiments

The PASSCAL program supports a wide diversity of seismic experiments, each with unique aspects. Broadly speaking, the experiments fall into two categories, active source and passive network experiments. Active source experiments include refraction / reflection surveys.  Passive network experiments utilize natural earthquakes as sources and include studies of upper mantle and lower crustal structure or monitoring arrays for locating earthquakes. Both active source and passive network experiments are supported out of the Passcal Instrument Center (PIC) located in Socorro, New Mexico.

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Vaults for Year Round Polar Programs

Seismic Sensor Vaults

The type of seismic sensor vault and the amount of sophistication it requires, is dependent on the type of seismometer.  A geophone does not require a “vault” per se, it can be buried in dirt or snow and be kept reasonably level in the process. A sensitive broadband seismometer requires a quiet, temperature stable and level vault.  In the Antarctic two main styles of vaults are used for broadband seismometers:  the rock vault and the snow vault.

The Rock Vault

In rocky environments where digging is nearly impossible, a weatherproof enclosure with a rigid bottom, typically aluminium, is secured to the ground.  The seismometer is then set inside, oriented properly and leveled.

To reduce noise caused by wind, and to thermally stabilize the vault and protect the instrument from flying debris, a double walled dome is placed on top of the vault and anchored to the ground as well.

Vaults for Summer Only Polar Programs

For summer only deployments, either short-period seismometers and geophones or broadband seismometers are typically installed. In the case of short-period sensors or geophones, a “vault” is not required and the instrument is often buried in the snow at a shallow depth and reasonably leveled.  If broadband seismometers are used, a vault needs to be built to hold the instruments. This involves more work as the sensors have to be accurately oriented, leveled and protected from wind noise and thermal changes.  For a detailed description of broadband seismic vaults for polar environments, see the Specialized Polar Equipment section.

Seismic Vaults

Introduction:

The construction of the vault for broadband seismometers has a direct impact on the data quality. The principle of broadband sensor vault design is to minimize temperature changes, and to distance the sensor pad from the surficial layer which tilts due to temperature, precipitation, solar insolation, wind, etc.

Nearby sources of ground noise, like footsteps or acoustic noise, are reduced by shallow burial (2-5 ft.) The construction must be adapted to the site and materials available. A decision will have to be made to balance the cost and labor of different vault designs against the length of deployment, resources available, and quality of data expected.

Finding a pre-existing enclosure is a tremendous savings in time and effort. This might include missile silos, power stations, abandoned mines, caves, or root cellars. Consider the details of cables, power, and locating a GPS antenna for timing at these sites.

Guideline Documents

 

May 7, 2009

 

Here are links to documents that will help you to think about seismic station siting considerations and seismic vault construction during planning stages, field reconnaisance, and instrument deployment. Also included here are links to guidelines and procedures for station installation and station servicing.

 

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