Portland State University |
U.S. Geological Survey |
Portland State University |
U.S. Geological Survey |
Marvin Beeson (PSU) |
Ansel Johnson (PSU) |
The Department of Geology at Portland State University in collaboration with the U.S. Geological Survey and with the cooperation of Tri-Met conducted a gravity survey through the northern light-rail tunnel through the Portland Hills. Below are a series of workbooks that contain the raw data and preliminary analysis of the data. THIS DATA IS PRELIMINARY, IT SHOULD NOT BE USED OR CITED. It is placed here for your information only. The final data will be available from this site.
We will add more figures and charts when final approved versions are available.
BLAKELY, Richard J., U.S. Geological Survey, Menlo Park, CA
94025, blakely@usgs.gov; CRUIKSHANK, Kenneth, JOHNSON, Ansel,
BEESON, Marvin,
Department of Geology, Portland State University, Portland, OR
97207; WALSH, Kenneth, Parsons Brinckerhoff, 2140 Jefferson St.,
Portland, OR 97201; WELLS, Ray E., U.S. Geological Survey, Menlo
Park, CA 94025
Recent construction by Tri-Met of a twin-bore, light-rail tunnel through the late Cenozoic Tualatin Mountains (Portland West Hills) allowed us to conduct a detailed gravity transect through the interior of the northwest-striking, fault-bounded, asymmetric anticline forming the mountain range. The tunnel extends east-west a distance of 4.5 km, obliquely crossing the anticline and intersecting several concealed faults inferred at the surface. At least one of these faults, the Oatfield fault, is well expressed in aeromagnetic data. Three M„3 earthquakes and numerous smaller earthquakes occurred about 20 km northwest of the tunnel in 1991, suggesting that the Oatfield fault or other faults in the Portland Hills may be seismically active.
Gravity measurements were made along the entire length of the tunnel at spacings ranging from 23 to 46 m and with an estimated precision of 0.01 mGal. The position and altitude of each station relative to construction benchmarks at each portal were surveyed with a 1-sec total-station. The gravitational effects of overburden and terrain were subtracted using a 30-m digital elevation model assuming a range of densities for the overburden. In comparison to ground-based gravity measurements, the tunnel transect provided a more detailed view of density variations at tunnel depths. On the other hand, interpretations were complicated by the ambiguities posed by having mass both above and below each measurement.
The first-order feature of the terrain-corrected, Bouguer-anomaly profile is a gradual increase from -28.5 mGal at the west portal to a maximum of -15.4 mGal at about 3.8 km from the west portal. This west-east gradient is due to the tunnel's location between a broad gravity low over the Tualatin basin to the west and a gravity high over Eocene basalt (basalt of Waverly Heights) to the southeast. Superimposed on this broad gradient are a number of step-like anomalies with magnitudes of 1 to 3 mGal and characteristic widths of 100 to 300 m. The most pronounced of these anomalies occurs beneath Sylvan Creek, where a fault has been identified and mapped inside the tunnel. Another gravity anomaly occurs 1200 m from the west portal at the intersection of the tunnel and the surface trace of the Oatfield fault. A steeply dipping fault identified inside the tunnel at this same location has placed 15 Ma Columbia River basalt 20 m above 1 Ma Boring Lava. We believe the fault identified inside the tunnel at 1200 m from the west portal is a segment of the Oatfield fault and is the cause of the associated gravity anomaly.
The Excel workbooks are a working draft of the data collected on Sunday 27th October 1996. There are three workbooks available: Survey data, PSU gravity work-up, and USGS gravity work-up. This data is preliminary, and should not be cited.
Other geophysical work in the Portland area includes an Airborne Geophysical Analysis of the Portland Area by Richard Blakely.
Section 1: Survey Data
This section contains coordinates for gravity stations along the tunnel. The coordinates for the starting position were obtained from the U.S.G.S. Linton 7.5’ quadrangle. The coordinates for the final station were obtained from the U.S.G.S. Portland 7.5’ quadrangle. These will be updated with GPS positions soon.
The locations of the gravity stations is shown on the Portland Geologic quadrangle (Beeson & others, 1991, Oregon Department of Geology and Mineral Industries Map GMS-75)
Section 2: PSU Gravity
These worksheets are the various corrections and calculations performed on the raw gravity data. The ground elevation (p. 8) above gravity stations was obtained by plotting gravity stations of the Portland 1:1200 topographic maps. The magnitude of the terrain correction was obtained by Rick Blakely (USGS) using the program BOUGUER.
The gravity worksheet contains an some extra columns (such as "Adjusted Terrain Correction"). These were put in place to experiment with different densities above and below the tunnel. The current worksheet uses a density 2.67.
Section 3: USGS Gravity
These are the values obtained by Rick Blakely. The worksheets also include some comparisons with the PSU data set
