Analysis Of Magnetic Gradiometer Surveys - 1998

1996 excavation at Fort Clatsop
1996 Excavation at Fort Clatsop

NPS Photo

Process Of Discovery
An Analysis of Magnetic Surveys at the Fort Clatsop, Oregon: The Second Season

John W. Weymouth
University of Nebraska
Lincoln, Nebraska

January 23, 1998

Fort Clatsop, near the town of Astoria, Oregon, is the site of Fort Clatsop erected by Lewis and Clark in 1805. This site has been tested by excavation at various times. There is a reconstruction of the Fort at the Monument but the precise location of the Fort is not known. As part of the ongoing archaeological study of the Fort, a magnetic survey was conducted over part of the site area in 1996 (Weymouth, 1997). That survey covered about 600 square meters south-west and about 200 square meters north-east of the reconstructed fort. In 1997 another magnetic survey was carried out covering a little over 800 square meters north-east of the reconstructed fort and north of the area covered in 1996. About 33 square meters of this survey overlapped the 1996 survey. This is a report of the 1997 survey.


On November 7, 1805 the Lewis and Clark expedition reached the estuary of the Columbia River (Ferris, 1975). For several days, and in bad weather, they explored the north shore of the estuary for a suitable winter camp site. Finding the north shore inhospitable and lacking in game they crossed over to the south shore on November 26 and eventually on December 7 they located a site 3 miles up what is now known as the Lewis and Clark River. They started to build a fort Called Fort Clatsop after a local Indian tribe. It was a 50 feet by 50 feet enclosure with two rows of cabins inside facing each other. Although not finally completed until December 30 the men moved in on Christmas eve.

The Lewis and Clark expedition left Fort Clatsop March 23, 1806 for their return trip. The abandoned fort was all but gone by the 1850's. In 1856 a Mr. Shane, who built a cabin at the site, reported that there existed remains of two of the Lewis and Clark cabins. In June and July of 1948 Caywood conducted some exploratory excavations. He reported that there was evidence of "charcoal and then later some thin layers of orange- red burned earth and burned stones of about the size of baseballs and larger". These excavation units are indicated on the GIS maps produced by Keith Garnett (Garnett, 1996, 1997) (referred to here as the KG maps). Schumaker dug several trenches in 1956, 1957 and 1961. These are marked on the KG maps.

Today, in spite of several archaeological studies the exact location of the Fort is unknown. A exhibit of the fort was built on the site in 1955 by the Oregon Historical Society. In 1958 the National Park Service acquired the site and named it the Fort Clatsop.

James Bell (Bell, 1990, 1996) conducted some radar surveys at the fort site in 1990. In 1996 (Bell, 1996) he conducted a more extensive survey over regions south-west and north-east of the fort exhibit. These transects as well as areas of possibly significant reflections are marked on the KG maps.


In general a magnetic survey consists in measuring the magnetic field of the earth a few centimeters above the surface on a grid of evenly spaced points. Slight differences in concentrations of weakly magnetic iron oxides beneath the surface can give rise to anomalies in the mapped data. Such concentration differences can have anthropogenic causes such as filled pits, fired or burned earth including bricks, intrusive walls and cellars or privies (see Weymouth, 1986).

There are two ways to measure the earth's field, total field or gradient. In the total field method the magnitude of the field, regardless of direction, is measured. In the gradient method the gradient of the field or the difference between two readings separated by a short vertical distance is measured. A gradiometer emphasizes near surface features and tends to cancel deeper of longer range features but the signals are slightly weaker than those of the total field method. It is best to use total field data for quantitative analysis of individual anomalies. All of the 1996 data were obtained using a Geoscan FM36 Fluxgate Gradiometer.

An area 20 m by 40 m was laid out north-east of the reconstructed fort in units of 10 m by 10 m blocks. Figure 1 is a layout of the blocks. These blocks were surveyed with two Geometric G856 total field proton magnetometers with a sensitivity of 0.2 nT, one placed on the grid points spaced 1/2 m by 1/2 m, the other positioned off the grid and fired simultaneously for diurnal correction. The G856 data were obtained with a crew under the supervision of Ken Karsmizki, Museum of the Rockies, Montana State University. The surveys were done on July 26, 27, 28 and 30, 1997. The rows and columns between the blocks were done twice, once for each block surveyed, thus producing 11 pairs of "common rows". An average and standard deviation was calculated for the difference between the corrected values of each member of a pair of common rows. The standard deviations give a measure of the operator "noise" . The average of the standard deviations is 3.2 nT. This sets a lower limit for significant contour intervals.

A smaller block, Block A, was surveyed at the south end of the large area (see Figure 1) with a Geometric G858 cesium gradiometer operating continuously with a cycle time of 0.5 seconds and a sensitivity of 0.05 nT. The grid interval was 25 cm in the east direction and 20 cm in the north direction. The G 858 was also used to survey Blocks L and P in the main group. These latter surveys were discrete, not continuous, with a grid interval of 50 cm by 50 cm. The G858 data were obtained with a crew under the supervision of John Weymouth Since the L and P surveys did not add much to the G856 results these surveys will not be presented in this report. The software of the gradiometer provides not only the gradient but also separate values for the top and the bottom sensors. The bottom sensor values can be used for total field values uncorrected for the diurnal variation.


Last updated: February 28, 2015

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