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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.

Active Source Experiments:

Active source experiments utilize over a hundred instruments at short intervals (50-1000 meters) along a single line or set of lines. The station (often called the receiver) consists of a recorder, sensor and battery. These are deployed overnight or for a few nights while the line is shot with explosives. Teams of 2-3 people are responsible for up to 60 stations, including the deployment and retrieval of equipment . An experiment may have 3-5 shooting periods and require different station locations for each period. Recorders are programmed for TIM triggers corresponding to the shooting schedule. The entire exercise takes about a week.

A variation of the active source experiment is the onshore-offshore collaboration. In this experiment a vessel with airguns provides the source and shoots either at set time intervals or at set distance intervals. In the first case, the recorder can be programmed with TIM triggers. In the second case, the recorder is usually programmed for CON triggers, (continuous recording), and the data is sliced up later when the shot times are known. Often, an experiment will use land-based explosives in conjunction with the ship-based airguns so that the recorder must be programmed with a datastream for each type of source.

The complete dataset is usually available in some form when the experiment is finished and packed up. Interpretation and assimilation of other datasets can take several months.

Passive Network Experiments:

Passive networks utilize up to 100 stations, consisting of recorder, sensor, batteries, solar panels, and a housing for the equipment. The stations are deployed for 3-24 months over the area of interest, spanning 100 km to 1500 km in remote terrain. A team of 2-3 people returns to the station periodically to upload data and check on its well-being, usually servicing 5-10 stations in a trip. In the case of broadband networks, a more extensive team is required for site construction and installation. Recorders are usually programmed with continuous recording at a sample rate of 1-40 sps to record teleseismic events.

Earthquake monitoring networks are similar operations except that the spatial scale is reduced to a few hundred kilometers and the data rates are much higher- usually 100-200 sps. These arrays are often deployed following significant earthquakes to isolate the area of rupture, count the number of aftershocks, determine focal mechanisms, etc. Each team of 2-3 persons can deploy 5-8 short-period stations per day and then visit the sites weekly.

Data is processed at a central site, or in some cases shipped back to the host university. In addition to archiving the raw data, each station's data is converted to trace files and these are organized into network volumes. That is, for a given time period all the data from all stations are grouped into a single volume. Because each station contributes 100-800 Mbytes of data per service visit and because these servicing trips are asynchronous, building the network volume is a major chore. The data must also be screened for quality including checking the clocks, seismic background noise, trigger efficiency, and proper recorder operation. This QC should be done before the next service interval so that corrective action can be taken at the station. A schedule of three weeks servicing, one week processing data is typical for passive networks. Due to the length of some experiments, pacing is important for the moral of the team. The PI should plan for reserve assistance or to sacrifice some capability should the effort prove to be beyond the resources available.

Staffing

The number of people required to carry out an experiment varies with number of instruments to deploy, size of the array, terrain, time allotted, and skill of the crew. In the section above, we listed what a team of 2-3 people could be expected to accomplish. For safety reasons, teams should never be less than two individuals. If extensive driving is involved at least two team members should be able to drive. Here we summarize some additional tasks which must be performed and the skills required.

In all experiments, it is much easier if those persons installing the station do not have to actually locate a suitable site and secure the proper permission to install it. This should be done well in advance of the arrival of equipment and is most successful when approached by a professional looking academic. The site installer rarely maintains such an appearance for long. Prepare a letter on university letterhead granting permission for access to the site and have it signed by the landowner. In the process, directions to the site can be established and the exact spot marked. Permitting often takes months for active source experiments, and can take a day or two per station for passive networks. Some experiments have become frustrated by this process and adopt guerilla deployment tactics, especially on federal or state lands. PASSCAL does not endorse this practice and may hold a PI liable for equipment seized as a result.

Depending on the size of experiment, it may be necessary to designate a person responsible for data management and quality control. They should not be obligated to deploy or service sites, if appropriate. This person should be adept with UNIX and familiar with PASSCAL Software or any other software required. One person has a much easier time keeping the archive process streamlined than several part-time operators. Because fieldwork can be exhausting and archiving tedious, the same people doing both may produce grave errors. It cannot be emphasized enough that the key to quality data is a timely review while there is still a chance to do something about it. Staffing the data processing position should be one of the first, not the last position filled.

Clearing customs in foreign countries can be encouraged somewhat by a representative of the project being in attendance to provide information, move paperwork or alert the shipping agent if things are not proceeding as planned. Other team members may be able to adjust their arrival to the actual delivery of equipment when notified by the advance party and avoid waiting days for it to clear. Coordinating the arrival of team members and the equipment in foreign countries is difficult, but it is cheaper to have the equipment sit idle a few days and wait for the team members.

Professional drivers are required in some countries and helpful in most all foreign experiments. Traffic accidents remain the single biggest hazard when traveling overseas and, given the extensive driving most PASSCAL experiments require, a good driver is essential. They know the roads, hotels, cafes, markets and are usually pretty good hands to help with the station. Most of them can also fix whatever they're driving. Do yourself a favor and hire one. If you get a reckless, uncooperative driver, fire him on the spot and get another one.

Site Locations

In active experiments the receiver site location is governed entirely by the geometry of the array. The line geometry is sometimes distorted from the ideal to maintain convenient access to roads or to avoid natural barriers. Here locating the site means flagging a spot where the sensor should be placed, give or take a few meters, and securing permission to use this site and any access roads from the landowner. The sites are usually surveyed before, during or after the experiment to determine the true station geometry. Because the actual deployment of instruments involves setting up as many stations per team as possible, accurate directions to the sites should be established so the team does not waste time getting lost.

In passive arrays the site location is loosely governed by the array geometry but the specific place is selected based on expected seismic noise levels, security, access to phone or power, etc.. Try to avoid sites that are visible from the road. Seismic noise levels are very difficult to assess by simply walking around a site. Still, guidelines to follow include:

  • Install the seismometer on bedrock
  • Bury the sensor with soil, 30 cm for short period, 1-1.5 meters for broadband
  • Maintain 30-50 meters distance from footpaths
  • Avoid large overhead powerlines or trees due to wind induced motion
  • Maintain 3 km distance from major roads, railroads, irrigation pumping stations
  • Avoid large power generating stations with big turbines (10km)
Shipping

The experiment is ultimateley  responsible for arranging shipping all shipping costs, except insurance.  PASSCAL personnel can assist you in arranging the shipping.  IRIS insures all PASSCAL equipment; no additional insurance is warranted. An estimate of the number of pieces and total weight for your experiment is available from the PIC. While the shipping budget is often determined, the method is not.

Within the US, four shipping methods are used; Federal Express Cargo, commercial air cargo and driving, ground freight, and a rental truck.

International shipping is of course more complicated. The PIC and PI will select a freight agent and determine who will be responsible for negotiating the deliveries. This depends on the size of the overall experiment and experience of those involved. Often items from various institutions should be consolidated prior to overseas shipment. Delays, inbound or outbound, are possible and clean paperwork is essential.


This page contains support documentation for the "Seismic Vault" page.

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