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FHWA Unknown Foundations Summit

Improving 3D Imaging of Underground Structures
Joe Descour, President, C-Thru Ground Inc.

Our vision very similar to Seismic surveys. Both use seismic waves. Typical approach

Cross-hole tomography has some drawbacks. Results not always satisfactory, hard to interpret.

Conventional cross-hole velocity tomography

Plus use reflected signals using data imaging

Simulation 3d data piles
energy radiated from hole

Source at tips of piles—acted like separate source

[image]
Result of cross-hole tomography, may be piles at spread of footing.
Velocity low on side, nothing indicated like piles.

Reverted to reflective waves. Inside triangular form is shadow zone. Anomalies between two holes easier to detect.

Source was sending receivers out. Got images matches

Side piles are battered piles. Can count number of piles.

Tried parallel seismic, two abutments.

East abutment—spread footing
Had no piles
Big surprise for two holes—new source at bottom of picture, was energy that traveled thru pile
Tip of pile becomes source when energy travels through it.

Parallel Seismics Survey

Magnetic Gradiometer survey
Shows strong anomaly on side of abutment with piles
Both sides looked almost same

Case where people wanted to show size of caves themselves

Piles under bed, Pittsburgh
3 holes, sources and receivers

Study in Japan, cross-hole tomography produced images.

More effective showing boundary between mud stone and sand stone.

Anomaly was turned around and showing another boundary

Sewer, Pittsburgh

People wanted to know where sewer line was. 4 holes, survey done along 4 sides. Tomographic analysis helped discover distribution. Trying to imagine the things we want to see under ground, with minimal training. Similar to ultrasound.

Questions

Q: how use tomography to measure H-piles. How did you get individual travel times.
JD: piles reflect energy, but wavelength has to be properly constructed. If not, won't see wavelength. Using sweep frequency source. Contributes to lack of energy. Pulse shows changing. Sensors pick up 2 kinds of signals and merge it. H-piles resonate and produce image.

Q: What separation between bore-holes did you have?
JD: Source and receiver string were side-by-side. Energy traveled along the hole. Calculates xxx

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