Imaging the subsurface using seismic waves

Just over three years ago, a sinkhole caused significant structural damage to a bridge in the Philadelphia suburb of Plymouth Meeting. The 45-foot-wide bridge—which previously carried more than 15,000 vehicles-per-day prior to the collapse—required more than $2 million in emergency repairs.

Can this type of situation be predicted? Maybe. One Temple Engineer believes there’s data in the dirt beneath our feet.

Trained geotechnical engineer Dr. Joseph Coe, a member of the Civil Engineering department at Temple College of Engineering, is currently leading research on how to non-destructively capture this data through multi-channel analysis of surface waves (MASW).

“You’re basically figuring out what’s going on underground based on measurements at the surface. You take something like a sledgehammer and impact the ground to introduce this seismic stress wave that propagates through the soil,” Dr. Coe said. “Sensors then measure this wave and we can use this information to ‘see’ what's going on below the surface. For example, you may have a utility line or a soft zone that indicates a possible sinkhole hazard. That will affect the data we collect.”

The non-destructive nature of this research has many upsides, but cost is always a concern—particularly when planning a construction project. Though, there’s an argument for the up-front investment preventing costlier remediation later.

“Often, there's only a certain amount of money available for observing the subsurface, usually pennies on the dollar compared to overall project cost,” Dr. Coe said. “However, in that regard, this MASW technique is actually quite cost effective and provides information over a wider area relative to traditional subsurface investigation methods where we drill into the ground.”

Bridges and roads are only two examples, but they are costly ones. Recent estimates for completely repairing the backlog of America’s bridges are an estimated $123 billion. Other striking possibilities for MASW in geotechnical engineering include earthquake research.

For example, the U.S. Geological Survey is seeking more expansive data on soft soil zones to characterize potential seismic hazards, something Dr. Coe is also pursuing with the MASW method.

Villanova University joined Dr. Coe on the proposal to complement the surface wave methodology with database work. The goal? Obtain full funding from the USGS and pursue seismic mapping for the City of Philadelphia.

“Philadelphia Office of Emergency Management told us this would be useful information to potentially include in response plans for emergency scenarios,” Dr. Coe said. “Softer soils amplify the ground motions resulting from an earthquake. So, a map showing where these soft soil zones are across Philadelphia would be tremendously beneficial in determining which areas might experience stronger shaking during an earthquake.”

Earthquake preparedness may be less a hot topic in Philadelphia planning discussions than, say, in the Pacific Northwest, where building codes reflect the need to design for more-frequent earthquakes. However, the takeaway is more about impact than frequency.  

“Earthquakes on the east coast, for much smaller magnitude, are felt much farther away,” Dr. Coe said. For example, the 2011, 5.8 magnitude Virginia earthquake was reportedly felt as far north as Quebec. Couple those facts with Philadelphia’s aging structures and building boom, getting an accurate picture of the region’s soils starts to come into focus.

One answer for Philadelphia’s seismic and infrastructure preparedness might be right beneath our feet.