Principal Facts of Gravity data in the Northern Willamette Valley and Vicinity, Northwestern Oregon and Southwestern Washington

Gravity data were collected from 2004 through 2006 to assist in mapping
subsurface geology in the northern Willamette Valley and vicinity, northwestern
Oregon and southwestern Washington. Prior to this effort to improve the gravity
data coverage in the study area, very little regional data were available. This
report gives the principle facts for 2710 new gravity stations and 1446
preexisting gravity stations. Much of the study area is now covered with data
of sufficient density to define basin boundaries and correlate with many of the
larger fault systems.

The study area lies between 44° 52.5' and 46° N latitude and between 122°
15' and 123° 37.5' W longitude. Although this is a continuing project and more
gravity data is expected to be collected, this report is being published to
show the progress of the data collection. The majority of these data are spaced
at about 1.6 km (1 mile), but three closely spaced profiles were measured in
the Portland area across several faults. To obtain a 1.6 km grid of data points
would require about 5120 gravity stations. To date we have collected 2710
stations. Including the preexisting data points, the total number of stations
is 4156, and complete regional coverage is about 80 percent at this time.