Case Study: Delta Geophysics is Selected for One Day Bedrock Seismic Refraction Survey

Single 800-foot seismic line

Determine rock excavation quantity along a proposed sewer line.

The Method:
Seismic Refraction

The Scope:
Single profile using delay-time data reduction.
Delta Geophysics’ contributes to value engineering by responsive and cost effective analysis.

In October 2008 Delta Geophysics was retained to complete a one-day refraction survey to map bedrock depth along a proposed sewer right-of-way. The approximate depth of the proposed 800 foot sewer line was 15’-20’ below ground surface, and rock excavation quantities were critical to determining alternative costs for force main versus gravity sewer installation.

The Approach

Delta Geophysics’ team opted to use the seismic refraction method.

Seismic refraction is one of the most commonly used methods for identifying bedrock at depths of less than 100 feet. This method, when used with a hammer energy source and the delay time method of data reduction is the simplest and often the most cost effective method for determining rock velocities. Correlations between seismic velocities and excavating machinery requirements are well documented, and used to quantify the amount of rock that must be blasted prior to excavation, and hence more accurately determine construction costs.

Delta mobilized and completed the project in one day using seismic refraction.  Data quality was monitored in the field and stacked until data was acceptable for processing.  The survey was conducted in two directions to best resolve dipping beds and processed using the delay-time method.  Based on the local geology, a velocity of 9,500 feet per second was selected as the hardest rock that could be excavated without blasting, and the depth of this bedrock layer shown on a profile along the proposed right-of-way (see above cross section).  This depth to bedrock was used by the engineer to determine rock excavation quantities and associated costs.

Client Benefit
The seismic profile provided reliable rock excavation quantities for construction estimating, and resulted in selecting the force main alternative as the most cost-effective building solution.