Picture of the month prepared by Andrew Stone, Hydrogeologist, andrewstone@gmail.com
WELLS IN BEDROCK FRACTURES
How often do you get to see a cross-section of what was until very recently a subsurface drill hole intercepting a bedrock fracture?
The photo of 1.4 billion old Silver Plume granite was taken on the I-70 frontage road in Mount Vernon Canyon just west of Golden, Colorado.
Photo Credit: Peter Barkmann, Colorado Geological Survey
The photograph shows a now exposed section view of a drill hole intercepting bedrock fractures in granite bedrock. When occurring below the water table, saturated fractures in bedrock can transmit and store water. To be successful, a bedrock well should intercept saturated fracture systems that are interconnected. In many cases, fracture systems that are connected to recharge zones, such as overlying layers of sediment, have the best chance of producing a reliable sustainable yield. There are millions successful bedrock wells providing water to homes, farms and businesses throughout the USA. Some bedrock wells have limited connection to groundwater and yield may decline seasonally or in times of drought. Aerial imagery and geophysical techniques can be effective in identifying the orientation and density of fractures and optimizing the chances of selecting a site for a successful water well.
Picture of the month prepared by Andrew Stone, Hydrogeologist, (andrewstone @ gmail.com)
THREE ROCK TYPES IN ONE PHOTOGRAPH
1. Dolerite (igneous – intrusive volcanic)
2. Mudstone (sedimentary)
3. Where the dolerite as molten magma “cooked” the sedimentary rock, (metamorphic)
Photo Credit: Andrew Stone
The sedimentary layers in the photograph are in the Beaufort Group of the Karoo Sedimentary Basin in South Africa and were deposited about 250 million years ago. The igneous dolerite dike (1.) resulted from volcanic activity about 180 million years ago. Dikes can serve as barriers or as conduits for groundwater movement. Geophysical techniques are typically used to help characterize the geometry of subsurface geology in areas where dykes and sills occur.
Sill or Dike?
A sill of igneous rock does not cut across preexisting rock beds and is described as concordant. An igneous dike (as shown in the photograph) is described as discordant and does cut across the layers of older rocks.
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The Colorado Groundwater Association has invited Ron Bell, Senior Geophysicist, to be the luncheon speaker for their monthly meeting. On November 14, 2018, Ron will be presenting “Applying Groundwater Geophysics in the Prairies and Mountains of Colorado”. In his discourse, Ron highlights the geophysical data and cost effective benefits of integrating geophysical methods in groundwater resources exploration and how they contribute to the success of the project(s).
Ron has over thirty years of experience in the application of geophysical technology to groundwater, mineral, geothermal, and hydrocarbon resources exploration as well environmental and geotechnical subsurface site characterization. His experience includes the acquisition, processing, visualization, and interpretation of terrestrial, borehole, and airborne potential field data including direct current resistivity, induced polarization, time and frequency domain electromagnetic, magnetic, and gravity methods.
The luncheon will be from 12:00 PM – 1:00 PM
Geotech Environmental Equipment, Inc.
2640 East 40th Ave. Denver, CO 80205.
For more details click here.
