Norcal Geophysical Consultants INC

“Geophysical surveys provide another perspective of the subsurface conditions for correlation with other data ”

 

Case Studies

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5. Determine subsurface bedrock topography and delineation of gravel beds for water supply development

Seismic refraction and electromagnetic time-domain geophysical surveys were performed along a portion of the Russian River in northern California to aid in the initial planning and placement of municipal water supply wells. The general geology of the area is river alluvial deposits overlying Franciscan Formation bedrock. The river deposits consist of sand and gravel interbedded with floodplain silts and clays. The depth to bedrock is variable.

The seismic refraction survey consisted of a series of six profiles 600 to 1200 feet long oriented both parallel and perpendicular to the river. The purpose of the survey was to define the depth and configuration of the subsurface velocity interface that represents the contact between the alluvium and the bedrock along each profile. A sample seismic profile that was obtained during this survey is shown below. The profile shows the ground surface topography, the thickness of the unconsolidated soil, and the topography of the alluvium/bedrock interface.

A time domain electromagnetic survey was performed to measure changes in conductivity with depth as it relates to the river (sand and gravel) and floodplain (silt and clay) deposits. Typically, higher resistivity (low conductivity) values correspond to the sand and gravels and the lower resistivity relates to the finer grained materials. The survey coverage consisted of a series of measurement points spaced about 250 to 300 feet apart along each seismic refraction profile. The results of this survey were superimposed on the seismic refraction profiles to illustrate the thickness and continuity of the higher resistivity gravel deposits within the alluvial overburden section.

The seismic refraction bedrock data were digitized and a 2-D bedrock topographic map was compiled to show subsurface bedrock elevations and general trends.

Based on the findings of this investigation, test well locations were selected where the depth to bedrock was the greatest and the gravel deposits were the thickest in order to optimize ground water production for each well.