March 2003

Geophysics Beats Digging Holes

New techniques and processing software together with the refinement of data collection instruments mean the initial investigation of sub-surface structures and services can be undertaken quickly, effectively and safely without the need for potentially hazardous intrusive work.

What's more, if geophysical surveying were regularly employed as a failsafe procedure it might enable authorities to identify potential problems well before they emerge as a full-blown crisis. Savings on the public purse not to mention peace of mind could be significant. Just imagine the positive impact of an early warning, trouble shooting system for our highways!
Real scenario 1: Unexpected cavities open up during repairs to city center sewer system. Digger sinks into the hole. Repair work, road network and commercial center shut down. A Microgravity investigation is conducted (this geophysical field technique provides highly detailed analysis of changes in the earths gravity caused by reduced density in the earths crust caused by voiding, for instance), and a contour map showing variation in sub-pavement density is produced. Fig 1 shows areas of low density caused by voiding marked in blue and areas of high density (representing solid ground) marked in red.

As can be seen from the Fig1, the most obvious 'lows' in density is caused by two tunnels running beneath the street, and where the street crosses a bridge at the eastern end of the T-Junction. However, the results also show where the density of the sub-surface is lower than the surrounding area.
Fig.1
Concern is also raised by an anomalous contour pattern running along the edge of the tunnel, believed to be caused by water eroding the fill material behind a partially collapsed wall. Armed with this information, the 'lows can be directly and safely investigated by coring - which itself reveals poorly consolidated ground (in the marked zones).

Geophysics is particularly valuable to construction and civil engineering where the growing need for non-intrusive investigation and greater accuracy now make it a realistic and cost-effective alternative to physical excavation.
Real scenario 2: A burst water main sends large volumes of water through the structure of a masonry arch bridge, which may have caused cavities to form within the bridge, fill material. Ground penetrating radar is chosen as the best way to investigate potential voiding.

Collected GPR data to a depth of around 1.4 meters into the fill materials reveals internal construction detail and additionally some anomalous subsurface features. It is possible that voiding may be the cause of the anomalous zones shaded in black (Fig 2/2a).
Interpretation of GPR Data Plotted onto CAD
plan of bridge
In either eventuality, the potential has been assessed without recourse to physical intrusion that would ultimately have closed the bridge and associated road network.

Geophysics has several advantages over traditional intrusive investigation methods, primarily the speed at which relatively large areas can be covered, but also the reduced costs associated with keeping intrusive investigation to a minimum.