Nova Scotia

Geologists begin testing the Oxford sinkhole to predict its final size

The hole, which appeared last summer in a park in the Nova Scotia town, is now nearly as big as an Olympic-sized pool.

Team will take two weeks to study Nova Scotia site in two phases

The team of geologists will probe the ground around the sinkhole to determine what lies beneath. (Paul Palmeter/CBC)

A team of geologists will begin testing the ground near the Oxford, N.S., sinkhole this week, hoping to learn how big it could get and if it could threaten nearby buildings and roads.

The hole appeared last summer in a park near the Lions Club, across the street from a Tim Hortons and gas station.

When it was first spotted, it was the size of a dinner plate. Now it's nearly as big as an Olympic-sized pool.

Dan Parker, a geologist with engineering firm GHD, leads the team that will probe the ground around the hole. He said his main goal is to find out how big the hole might get, and what could be at risk.

Dan Parker, a geologist with GHD, is leading the team that will analyze the Oxford sinkhole. (Brett Ruskin/CBC)

"We use properties of the subsurface to figure out what the subsurface conditions are," Parker said. "And that will help optimize subsequent subsurface investigations."

Parker said doctors don't start by cutting a patient open to see what's wrong inside — they use X-rays and other imaging tests to inform their diagnosis.

Seeing beneath the surface

If the patient is the sinkhole, then Parker's box of steel electrodes is the X-ray machine.

The first phase of the sinkhole's diagnosis will test the resistivity of the ground around the hole.

"We'll be laying out cables and inserting steel electrodes into the ground. There's a combination of 80 electrodes that will be active at any given time," Parker said.

"Small currents are injected into those electrodes and the resistivity properties are measured in the subsurface."

Parker's team sends electricity through the steel stakes. By measuring how dirt, rocks — or empty caverns — affect the electrical current, the team gets a two-dimensional view of what's hiding beneath the surface.

An example of the two-dimensional image obtained from a resistivity survey. The different colours show different layers of dirt, gravel, shale, bedrock or open space. (Submitted by Dan Parker)

The next phase will use sound instead of electricity. 

Seismic testing will send sound waves underground. Based on how the different underground materials reflect that sound, the team can get a new view.

"Together, the two datasets will give us a really good image as to what's going on in the vicinity of the active sinkhole," Parker said.

The work stems from an announcement last month that the federal government would put $68,500 toward the geophysical testing.

The team will be on site this week and next week. They hope to deliver a report to the town of Oxford with their findings by mid-summer.


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