We have designed an adapter to connect a Choke Ring antenna and SCIGN D3 adapter to a 6 5/8" diameter steel borehole casing for use at Bay Area Mini-PBO sites. The borehole adapter accommodates access to the top of the borehole for maintenance of the down hole instrumentation and can be repositioned with a high level of accuracy both in the vertical and horizontal axes using a force-centering system that ensures correct azimuthal alignment.
The first design, currently employed at Ohlone (OHLN), consists of a ~3" wide by 6.5" rectangular steel plug rounded on both ends to fit inside the borehole casing. This plug is bolted through the casing with 2 bolts on either end (4 total). A 1 1/4" threaded plate is welded to the top. However , when it became clear that the adapter would occassionaly need to be removed to allow maintenace at the downhole instrumentation, such as the pore pressure sensor, we realized that the lack of machining precision at holes tapped into the borehole casing and tapped into the plug ends allowed about 5 mm of horizontal motion at the phase center when using a 7" pipe, SCIGN adapter and Choke Ring antenna. The axis of rotation is located along the long axis of the rectangular adapter between the two attaching bolts on either end of plug.
This first borehole adapter would not be acceptable should it have to be removed to access to the borehole. We therefore rejected this design because it cannot ensure long term positioning stability at the antenna.
The current design, which will be installed at SBRN during January 2003, consists of two modified 11" diameter stainless steel flanges. The lower slip-and-weld type flange will be welded onto the top of the 6 5/8" borehole casing providing a level surface for the second flange (see photo). The upper blind-type flange rests on the top surface of the lower flange and will be bolted in place using four 3/4" by 3" stainless steel bolts. This upper flange also provides a secure mounting point for the 1 1/4" Stainless Steel (SS) pipe used to connect to the SCIGN DC3 Adapter.
Two half-inch stainless steel dowels are press fit with a high location precision (radius 7.500" +/-0.001" ) into the lower flange on the ring defining the contact patch with the top flange. Two matching holes are machined into the upper flange with a high location precision (radius 7.500" +/-0.001" ) and hole diameter precision ( +0.005"/-0.000"). These determine the reinstallation accuracy, so close tolerances are required. One of the dowels is off-set insuring directional alignment.
The combination of upper flange and attachments (Choke Ring antenna, SCIGN adapter and pipe) results in a heavy load that can be cumbersome to handle. The complete setup weighs about 40-45 lbs, with the bulk of the weight composed of the 25 lbs upper flange. To minimize the chance of damaging the contact ring of the upper flange and to aid in alignment, the tops of the dowels are rounded.
Type 304 Stainless Steel was chosen for the flanges for its affordability, weldability, resistance to corrosion, and resistance to scratching.
Slots are cut from matching bolt holes on the upper and lower flanges to allow the GPS antenna cable access when using an 11" shroud or cover. These slots are necessary when using stiff low-loss cables (hard-line) in lieu of the more flexible manufacturer cables. An alternative approach would be to drill a large hole slightly off center on the top flange for cable access. However, removal of the upper flange and antenna would be more difficult without severely damaging the hardline cable.
POSSIBLE SOURCES OF ERROR:
There are a number of potential error sources that might affect the location, or apparent location, of the phase center.
Horizontal "slop" when repositioning the upper flange will be limited to the fitting space needed for the two holes in the upper flange and the precision of the outer surfaces of the two 1/2" dowels used in the lower flange. Realistically, this should be fairly small, about 0.005" (or 0.013 mm).
The contact ring between the upper and lower flanges has an inside diameter of 6.72" and an outside diameter of 8.5". Flange surfaces were not machined flat. The two dowels are not symetrical so the flanges must be reinstalled in the same direction, therefore the contact points on the flanges will not change. The introduction of dirt or other foreign material can present vertical offset of the upper flange and offset the phase center of the antenna.
For example, a 0.5 mm particle on the inside edge of the contact patch would translate into a horizontal shift of the phase center of about 1 mm using a 7 inch, 1 1/4" diameter pipe (12.6" total length from upper flange to phase center) to connect the borehole adaptor to the SCIGN adaptor.
Uneven tightening of the four bolts could also result in a shift in the phase center, especially if some of the bolts are left loose. The magnitude of this problem has yet to be determined. Uniform tightening of the bolts in a criss-cross manner is advised to equally destribute the forces on the flanges contact patch.
Another source of error could be related to having a flat metallic plane located below the phase center. This could include near field signal scattering and changes in the antenna field pattern due to electromagetic coupling. In this case, signal scattering should not be factor due to a vertical metal shroud used to surround the perimeter of the borehole adapter between the borehole adapter and the antenna base. Electromagnetic coupling from the borehole adapter and the metal shroud will likely be a problem that will have to be tested. Whenever a conducting material is placed in the near field of the antenna there will coupling from induced currents (Elosegui et al. 1995).
We plan to install and field test the borehole adapter at the San Bruno (SBRN) site in January of 2003. The installation, similar to the OHLN setup (see photo), will include a shroud constructed from 12" diameter steel pipe extending from a rectangular base up to just below the Choke Ring antenna. Additionally, a SCIGN adapter and SCIGN tall radome will be used for increased security and receiver duties will be handled by an Ashtech MicroZ. The factory GPS cable will be replaced with a 1/2" low-loss coaxial cable due to the long distance between receiver and antenna.
We plan to test possible near field signal scatter and electromagnetic coupling using two simultaneously running temporary sites in close proximity. One monument will replicate the Mini-PBO monument including borehole casing, borehole adapter, 1 1/4" pipe, SCIGN adapter, and steel shroud. The second, will be a simplified monument consisting of a Choke Ring antenna on a vertically mounted threaded rod.
Various iterations of the test will be run with consideration given to:
1. Near field multipathing from the horizontal planes including the borehole adapter's upper flange and the lower shroud casing.
2. Near field multipathing from the upper vertical cylindrical section of the shroud.
3. Coupling between GPS antenna and steel shroud and adapters.
Upper and lower flanges
Underneath upper flange
Underneath lower flange
Top of lower flange
215 McCone Hall, UC Berkeley
Berkeley CA 94720-4760
Last Modified: Fri Dec 20 13:05:44 PST 2002