Void Detection

Geophysical methods are widely used for void detection in an effort to non-invasively image subsurface cavities, and other subsidence features relevant to engineering, geotechnical, and environmental considerations.  Voids can occur in numerous forms and dimensions with differing causal effects, including:

• Washouts and settlement gaps beneath roadways
• Mine workings and abandoned mines
• Tunnels
• Natural caves and solution features (karst limestone systems)
• Leaking pipes
• Utilities and storm-water systems
• Buried archaeological features

HGI has experience in providing a wide variety of void detection surveys at differing scales and site conditions.

The choice of geophysical method is dependent on the void characteristics (size, shape, depth, and overburden), survey scale and objectives, and site conditions and environment.  Microgravity, seismic, and electrical resistivity methods are routinely used in mapping targets such as mine workings, tunnels karstic features, and large washouts.  Seismic surveys used can include reflection, refraction, and surface-wave methods.  High resolution surveys can be conducted on near-surface voids or cavities, especially roadway washouts and leaking pipes, using ground penetrating radar.

HGI has experience in providing a wide variety of void detection surveys at differing scales and site conditions/environments.  Examples include; identifying and mapping voids associated with 19^th Century well network along the proposed route of the Channel Tunnel Rail Link in London, UK.  Mircogravity and ground penetrating radar surveys were combined to identify several targets, after tunnel boring machines had caused several areas of subsidence to form in this densely populated urban area.  HGI was approached by Arizona State Parks to assist with mapping out caverns and passageways associated with the Kartchner Caverns State Park system.  The example electrical resistivity profile below displays a number of the resistive targets associated with the air filled caverns of the state park.

Electrical resistivity profile over the Kartchner Caverns State Park, AZ. The air filled voids and the caverns and passageways show up as resistive targets in the more conductive limestone bedrock background.