LIDAR and Aeromagnetic Technologies

Two recent advances in technology led to the identification of faults in the region, and especially the Mac's Pond Scarp (the official name of the IslandWood trench):

LIDAR and Aeromagnetics.

LIDAR (LIght Distance And Ranging, also known as Airborne Laser Swath Mapping or ALSM) is a relatively new technology that employs an airborne scanning laser range finder to produce accurate topographic surveys of unparalleled detail. ALSM technology uses an airborne laser rangefinder, a differential GPS, and an inertial navigation system to produce 30,000 points per second at about 15cm accuracy. The cost is about $500/mi2, and with 106 points/mi2 that equates to about 0.05 cents/point. Moreover, it is possible, with the aid of several algorithms and geometric filtering to remove the tree canopy (Haugerud, and Harding, 2001, Harding and Berghoff 2000, PSLC 2002).

Virtual deforestation over a selected area both before and after (Haugerud and Harding 2001). The same effect can be seen on the southern end of Bainbridge Island when we compare before and after (Haugerud and Harding 2001). The accuracy of this technology leads us to the identification of faults that we might not otherwise have seen, for example Mac's Pond Scarp (the red arrow pointing down to the left indicates the location of the IslandWood excavation (Sherrod 2003b)). If we zoom in closer to the IslandWood property, we see the fault scarp more clearly (Image created by Matthew John Brewer, adapted from t24nr023_ne.jpg and t25nr02e_se.jpg available on the web PSLC 2002 and Instruction Trail Map 10-03.pdf).

Furthermore, with LIDAR technology the North-South oriented hills on Bainbridge Island created by glacial advance and retreat become more evident (Harding and Berghoff 2000:3).

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Aeromagnetics maps the region using the magnetic fields present in volcanic (igneous) rocks to identify faults. Where the bedrock has been uplifted, there is an increase in magnetic resonance, and this accurately depicts the boundaries of faults in the Puget Sound Lowland (Blakely et al. 2002, Blakely 2003).

The magnetic method exploits the fact that some rocks are more magnetic than others. A volcanic rock, for example, is typically more magnetic than sediments. Very small magnetic forces emanate from places where magnetic rocks are in contact with less magnetic rocks. In this example, a fault has offset a thin volcanic layer, and the offset produces a very small magnetic field. We can detect that field with sensitive instruments. Typically we put these sensors in airplanes, fly them close to the ground, and call them aeromagnetic surveys. What we are actually measuring is the magnetic force above the earth (Blakely et al. 2002, Blakely 2003).

Magnetic resonance, coupled with LIDAR technology, provides accurate locations of the Seattle Fault Zone, and once such magnetic anomaly is evident with the IslandWood fault (Blakely et al. 2002, Blakely 2003).

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These pages were created by Matthew John Brewer on March 23, 2004 as part of the Graduate Program at IslandWood and fulfillment of the Independent Study Project. Last updated on May 18, 2004.

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