Geometrics Showcases New Atom Technology in New Orleans
Dr. Koichi Hayashi of Geometrics/OYO Corporation recently flew to New Orleans, Louisiana and participated in the field component of a joint study between us, Dr. Juan M. Lorenzo and his two students from Department of Geology and Geophysics, Louisiana State University and Dr. Mitchell Craig from Department of Earth and Environmental Sciences, California State University, East Bay (Figure 1). The purpose of the study is to develop non-invasive investigation methods for levee inspection. Dr. Hayashi’s team collected passive seismic data using both our upcoming ATOM wireless system (Figure 2), along with our traditional Geode acquisition system and SeisImager software. Since the team collected passive waves using the Microtremor Array Measurement (MAM) method, they did not require a source, thus reducing field effort and potential damage to urban environments. The researchers collected resistivity data using an Ohm-Mapper TR-4. Using the two different geophysical methods, researchers are able to determine areas within the levees that are at higher risk during storm events.
Dr. Koichi Hayashi of Geometrics/OYO Corporation, Dr. Juan M. Lorenzo and his two students from Department of Geology and Geophysics, Louisiana State University and Dr. Mitchell Craig from Department of Earth and Environmental Sciences, California State University, East Bay
To facilitate the work, Dr. Hayashi deployed ATOM, our wireless seismic data acquisition system. The geometry used to acquire the array would have been nearly impossible if the researchers had used a cable system. Figure 3 shows the unusual geometry, with a variety of spacings, a 90 degree bend and a number of road crossings. Weather conditions during the field work were extreme, with temperatures running as high as 100 degrees Fahrenheit and with thunderstorms and heavy rains occurring on occasion. The system worked as designed, and even during heavy rain the units maintained GPS lock and continued recording time-synced data. Data from the two-hour surveys were downloaded wirelessly from the seven boxes in 1-2 minutes. The whole process, including downloading the data, calculating dispersion curves from 3 different sized arrays, and performing velocity inversions took approximately 5 minutes and could be performed in the field. According to Dr. Hayashi, the system worked beautifully and the system’s performance and data quality are an improvement over traditional cabled systems.
ATOM in the field. The white line is the tape measure.
The team worked at four different locations, the 17th Street Canal, London Avenue Canal, Marrero Levee and the Lower 9th Ward. Two sites sustained damage from Hurricane Katrina in 2005 and have since been rebuilt. The work centered on identifying zones beneath the levees or canal walls having lower shear-wave velocities, corresponding with saturated, unconsolidated sands and low rigidity clays (Figure 4). Soils of this type are more likely to fail during a heavy storm event or, in seismically active areas, during an earthquake.
Map showing ATOM array. Note the unique spacings of the geophones, which would be difficult to achieve using a regular wired system.
Results from this work will go a long way to providing better, faster, and less expensive methods for hazard evaluation and to assist civil entities in mitigating these hazards. Passive seismic methods work well in urban environments where the source used in traditional measurements may cause damage to surrounding structures. Wireless technology is the answer for when your survey crosses roads or driveways, where expensive cables can be damaged.
Results showing low velocity zones
Geometrics’ thanks John Monzon of Southeast Louisiana Flood Protection Authority West for providing access to sites, and Gerry Gillen of Orleans Levee District for providing access to sites and accommodation. We would like to thank Juan M. Lorenzo, Martial Morrison, and Terrance DeLisser of Louisiana State University (IRIS REU intern), and Mitchell Craig of California State University, East Bay, for all their efforts for data acquisition.
For more information on the work Dr. Hayashi and his team are performing, please contact us at firstname.lastname@example.org.