SCIGN Short Drilled-Braced Geodetic Monument
SCIGN Short Drilled-Braced Geodetic Monument (a.k.a. "Mini Monument")
By S. Dockter, D. Elliot, and F. Wyatt (UCSD/SIO), J. Galetzka and K. Hudnut (USGS Pasadena)
SCIGN utilizes a smaller, hand-drilled geodetic monument known as the short drilled-braced geodetic monument in the following situations:
- Remote locations only accessible by helicopter or 4WD vehicle
- Environmentally sensitive sites
- Rapid deployment after large earthquakes
- Funds are not available to install a SCIGN deep-anchored drilled-braced monument
The monument (figure 1) consists of 1-inch diameter stainless steel rods forming a quad-pod (one vertical and three diagonal legs) that stands 45-60 inches above the surface and is anchored 3 to 6 feet into bedrock. A generator-powered handheld hammer drill using a 1.5-inch diameter drill bit to prepare the holes. Epoxy or anchoring cement is used to anchor the stainless steel rods in the holes.
The disadvantage of using short drilled-braced monuments that they are probably less stable than the standard deep-anchored monuments, which may prevent us from seeing very subtle transient tectonic movements. Their great advantage, however, is to be able to determine precise tectonic velocities at sites such as islands and remote mountain ranges. Permitting is also easier because of the smaller station footprint and minimal site disturbance during installation.
A mast assembly (figure 2) together with a short drilled-braced geodetic monument assembly make a completely autonomous GPS station. The two assemblies typically occupy a 5’ by 30’ rectangular area and are connected by a coaxial antenna cable protected in flexible metal conduit laid on or just below the surface of the ground. With good training, planning and preparation, three people can construct and fully complete a GPS station in about 10 hours once at a site.
Examples of sites where SCIGN has installed this type of GPS monument include all islands of the Channel Islands National Park, 29 Palms Marine Training Center, Isla Guadalupe and the Mojave National Preserve.
Five short drilled-braced monuments were installed during the
Materials and Equipment List
(Refer to Mike Jackson’s work here)
Digital Video Collage of Installations
Warning: this Quicktime file is large (~60 MB)! Short Drilled-Braced Monument Movie