The use of airborne LiDAR enabled a team of scientists led by Ian Madin at the Oregon Department of Geology and Mineral Industries (DOGAMI) to detect previously undiscovered fault scarps on Mt. Hood, Oregon. These faults are being further investigated with the help of the U.S. Geological Survey, Portland Water Bureau, Oregon Department of Transportation, Portland State University and Oregon State University. A recent article in the Oregonian (http://www.oregonlive.com/pacific-northwest-news/index.ssf/2011/08/hidden_earthquake_faults_revealed_at_mount_hood_oregon.html) discusses this effort. Trenches were dug across the faults to allow the scientists to examine the soil layers. Seeing how the soil layers have shifted and settled will allow scientists to gain insight regarding the location, frequency and magnitude of past earthquake events, providing more information about related to potential earthquake hazards. This information can be used to ensure that the necessary safety precautions are taken for nearby infrastructure.
To assist with trench logging and measurements, researchers from Oregon State University collected terrestrial LiDAR scans at the two Mt. Hood trench sites. The trench sites were small, less than 100 meters in length. Due to the need for detail in extracting soil layers, it was very important to obtain high resolution data so every rock could be seen in detail. Six scan positions were completed at one site and three at the other. Scans with resolutions of a few millimeters were obtained at each position, which left roughly 20 minutes for us to take in the scenery and pick huckleberries during each scan. Reflective targets were used for scan alignment and static GPS data were collected at each position. DOGAMI also provided RTK GPS coordinates on small red caps which had been previously placed on the trench face as control points. These points are all visible in the high resolution scans.
The scanning process was difficult due to the geography of the area, limiting access and visibility for the scanner. The trenches were surrounded by trees and bushes as well as a mound of soil from the excavation. As a result, the scan positions were often located less than 15 meters from the face of the trench instead of the optimal 50 meters to avoid oblique scanning. The scanner and target positions had to be carefully planned prior to obtaining the first scan to be sure that all targets would visible from each of the scanner positions.
The data was used to create triangulated surface models and photo texture mapping to be used in the analysis of the fault scarp to unfold its geologic story. DTMs from these scans have also been combined with Gigapan imagery to enable high resolution logging, and have enabled improved interpretation and analysis compared to traditional techniques which use a pre-laid grid of strings across the trench face for measurement reference. More importantly, the information is preserved digitally so that scientists can continue to ask questions and study the trench long after it has been filled in.