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New road route investigation

Experience in the use of a wide range of geophysical techniques enables Zetica to offer the most effective solution to a particular client’s problem every time.

In 1999 Zetica was asked to undertake a survey along the route of a proposed bypass near Cork in the Republic of Ireland. The objectives of the survey were to map the location of solution features near the top of the limestone bedrock and provide information on the depth to rock head and bedrock rippability. The bedrock was overlain by up to 40m of glacial overburden.

Mapping of potential solution features was carried out through a two-phase approach, using microgravity profiling to determine areas of mass deficiency that might be associated with voids or infilling of the bedrock with the lower density overburden, and an electrical resistivity imaging survey to provide profiles of bedrock depth across identified gravity anomalies, which were used to constrain the gravity models. One such resistivity cross-section is illustrated below. Greens and blues denote low resistivity overburden whilst the high resistivity bedrock appears as yellow to magenta. A thick sequence of overburden infilling a suspected solution hollow is evident on the left-hand side of the image.

The determination bedrock rippability and overburden thickness, in areas previously identified as having relatively thin overburden cover, were achieved using seismic refraction profiling. Rippability can be approximately related to the P-wave velocity of a particular rock type.



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