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Borehole Siting

Borehole strainmeters are sited in groups of four to eight instruments to image a target geophysical process. These arrays are typically designed based on geophysical models of the target process, for example tectonic or volcanic deformation. While idealized locations are easy to predict and, in some cases obtainable, the most difficult aspect of borehole strainmeter site selection is finding a site with a section (10-20 ft) of competent, unfractured rock 450-800 ft below the surface. The basic selection criteria used to identify a good strainmeter site are:

  • Competent unfractured rock at a depth of 450 ft to 800 ft
  • The absence of fracture zones through which water may be flowing
  • Low probability of nearby pumping which may disturb the local strain field
  • A well drained surface site to avoid flooding
  • Secure from any possible vandalism
  • Access to power and telemetry

Based on these selection criteria, the Plate Boundary Observatory developed the following list of guidelines to assist in determining the viability of possible borehole strainmeter sites.

  • Geophysical Importance
  • Geology
  • Rock Quality
  • Proximity to Wells
  • Topography
  • Seismic and Cultural Noise
  • Sky View for Telemetry and GPS
  • Solar
  • Access
  • Relative Location to AC Power
  • Security

Geophysical Importance

This refers to how well a prospective site fits into the proposed array of instruments. Some sites may be more important in an array design due to their ability to image the target process or operate in conjunction with other geophysical instrumentation.

Geology

Rock type is important for strainmeter sites. Ideally, the rock would have identical properties as the strainmeter sensor, but in the real world this is not possible without designing each instrument to match the rock in each individual borehole. The preferred rock type would be a granitic rock type due to its hardness and massive non layered characteristics. This would be a crystalline rock with a hardness of 5-6 (Mohs hardness scale). In practice, igneous, metamorphic, and sedimentary rock are all possible. The array design and possible rock types available in a region may require compromise in the overall array design.

Rock Quality

This refers to rating the quality of rock based on surface outcrops and/or well logs. Rock type, hardness, presence of fractures, and defined lineations are all features to note. The vertical thickness of competent sections and the inclinations of any layers or bedding planes etc are also to be noted. The more hard and massive a formation, the more desirable it is. Ideally, we would use massive, unfractured intrusive granites. Often, however, we must use rocks with layering such as sandstones, or metamorphic foliations like gneisses. Layering that is more horizontal is more desirable. Steeply dipping beds or vertical fractures are less desirable.

Proximity to Wells

Active pumping of water wells near a strainmeter site may be detected in the strainmeter data. These effects will vary depending on properties of the aquifer being pumped and the well proximity. Strainmeter sites should not be located near any actively pumped wells. Any wells near potential sites should be noted by distance and size of well pumps.

Topography

Certain topographical effects may cause noise in deformation data. The preferred site would be on level ground in the middle of a plain. This is not always possible. The worst location would be on the top of a steep narrow ridge. See figure below for a topographic rating profile. Location of strainmeter sites should be at least 100 m from any streams.

Seismic and Cultural Noise

Strainmeter sites should be selected away from (>1.0 Km) signifigant highway noise and/or railroad tracks. Other cultural noise sources should be avoided, some example would be water tanks, reservoirs, wind generators, quarries, land fills or anything which causes vibration or surface loading near a potential site. Despite cultural noise concerns, stations may often be sited near quarries in heavily wooded areas, where a quarry is a sure way to tell that there is near-surface rock of a suitable type.

Sky View for Telemetry and GPS

This relates to possible telemetry links (radio, satellite dish etc) and co-located GPS receivers. Open sky view 15 degrees above the horizon is good. The view to the south is more critical than the north, as that is where most satellites orbit. Measure the angle above horizon in the geographical directions (N, NE, E, SE, S, SW, W, & NW) plus the direction and angle of least amount of sky view.

Solar

How well the site is suited for solar power. Base this on the amount of southern expose and the number of hours of direct sunlight during a typical winter day. Trees, mountains, and buildings may all degrade the amount of sunlight.

Access

Conditions of the access into and the working space at a potential site. The drilling of a borehole for strainmeter installation requires access with large heavy equipment (10-20 ton vehicles). Access routes must be capable of supporting these vehicle weights and steep grades, side hill angles, and turning radius are importance considerations. If access is on dirt roads there may be different concerns based on dry and wet conditions. Any locked gates can be issues with multiple personnel accessing the site. Access would also include the space at the site suitable for a drill rig and all support equipment. The minimum space would be 30 ft X 80 ft and the preferred space would be at least 100 ft X 100 ft. The working space should be relatively level (< 3 ft of grade in 80 ft).

Last modified: Wednesday, 08-Jun-2016 22:43:09 UTC

 

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