North American Database of Archaeological Geophysics
North American Database of Archaeological Geophysics
Abstract/Summary:
Project Name: Resistivity Soundings at Fort Vancouver, Washington;
Reference: DeVore, Steven L. (2002). Resistivity Soundings at Fort Vancouver, National Historic Site, Clark County, Washington, Report by Midwest Archeological Center, Lincoln, Nebraska, submitted to Douglas Wilson, Fort Vancouver National Historic Site, National Park Service, Vancouver, Washington.
Vertical Electrical Sounding (VES) survey at the Fort Vancouver National Historic Site was carried out with a Gossen Geohm 40D earth tester in a Wenner array of four electrodes from May 13-16, 2002, as part of a National Park Service training workshop. The VES is done at a location by measuring several resistance values with increasing electrode separation. The following increments were used: 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, and 5.0 meters in both directions from the center probe. The distance of the separation approximate the depth of the survey.
The results of the survey indicate a three-layer curve of resistance for both line measurements. The upper layers have a very low resistance or inversely high conductivity associated with clay, however they are thin. The lower layers have a much higher resistivity or low conductivity associated with silty sand or loamy soils. This information suggests that a GPR (Ground Penetrating Radar) survey should be successful. Using a 500 mHz antenna would allow penetration down to ~2.0 meters and better resolution than low frequency antennas.
Magnetic survey results indicate areas of high magnetic gradients in both grid units. These areas are probably related to the spruce mill that was in operation during the early 20th century. These anomalies may also be related to Army activities associated with the Vancouver Barracks. Areas of less dynamic anomalies may be associated with village activities of the trading posts residents. Further analysis of the magnetic gradient data will provide additional information.