Environmental and Exploration Geophysics I tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Resistivity IV Characteristic Curves and Case Histories
2 Layer 4 Layer 3 Layer Characteristic Curves were developed for a variety of different models.
Characteristic curves and survey responses
The effect of variations in depth to a layer of higher resistivity L=AB/2 (Schlumberger Array)
The effect of resistivity variations - positive resistivity contrast
Three layer curves- varied depth to top of middle layer. Three layer curves illustrate the effect of varied depth to base of middle layer.
The method of Characteristic Curves (Two layer case) Summary of steps Set 1 = a1 Construct the ratios a / 1 for each spacing. Guess a depth Z ….
Plot a/Z ( the Wenner spacing divided by your guess of the depth) versus a / 1 In the graph at right, we have the variations in a / 1 plotted for three different guesses of Z – the depth to the interface.
Recall, that once you have determined k, it is straightforward to compute 2 1 = a (shortest a-spacing)
High resistivity stream channel gravels are delineated by the 250 ohm-meter contour Data acquired with constant a-spacing on a 2D grid.
Melted areas in permafrost Constant a-spacing
Exploration depth remains constant and the measured variations in ground conductivity provide a view of relative variations in conductivity at the exploration depth Profiling DepthDepth
Location of gravel deposits in a clay alluvium Constant a-spacing
Tri-potential resistivity method Can you compute the geometrical factors for these various electrode configurations? -6 a 3a3a 2a2a
Increase in apparent resistivity measured by the CCPP electrode array is actually an indicator of a low resistivity – perhaps water filled - fracture zone.
Case History Resistivity Profiling Surveys on the Hopemont Farm in Terra Alta, WV Survey performed by Eb Werner for Dr. Rauch
Survey was conducted for the City of Terra Alta to locate a water well. From Werner and Rauch
CCPP CPPC CPCP The tri-potential resistivity response over an air-filled fracture zone. Model data From Werner and Rauch
The Terra Alta surveys conducted by Werner and Rauch employed measurements at three different a-spacings - 10ft, 20 ft and 40 ft. Lines were positioned to cross a photolineament and were from 250 to 500 feet in length. Readings were made at 10 foot intervals. Things to avoid- Conductive materials buried or in contact with the ground. Buried telephone cables and metallic pipelines, fences, metallic posts and overhead power lines
The apparent resistivity measurements made by Werner and Rauch were interpreted within the context of the tri-potential response predicted by the Carpenter model (see earlier figure). “.. The present problem involves only the confirmation of the existence and exact location of a fracture zone mapped from other information. ” “ … it is only necessary to locate anomalies characteristic of vertical discontinuities …” “ … the graphic plots were inspected visually for those anomaly responses..” Anomalous areas were plotted on location maps. “Alignments of such anomalies at or near the location of the postulated fracture zone were accepted as confirmation of the existence of the fracture zone. Interpretation approach
40m 20m 10m 3.7m 60mdepth Midpoint 30m depth 15m depth 5.5m depth ExplorationExploration DepthDepth Coil spacing Sounding EM34 EM31 EM34 Surface
Northernmost site - site 1 (see earlier location map) Line 1 is highlighted in red below From Werner and Rauch
The anomaly around 100 feet is considered to be “data noise” The feature at 320 feet is interpreted to be the “fracture zone” response. Note that this feature is not marked by highs in the CPPC and CPCP measurements The 20 foot a-spacing profile reveals a more pronounced fracture zone anomaly at about 320 feet along the profile. The response suggests high reistivity Line 1 NS
Red dots locate prominent “fracture zone” anomalies observed on all three a-spacings From Werner and Rauch The blue line indicates the probable location of a major fracture zone. Given the 10 foot station spacing location of the zone is accurate to no more than ±5 feet Line 1 Line 6
From Werner and Rauch Line 6 The fracture zone anomaly appears consistently on Line 6 at approximately 125 feet along the profile The anomaly broadens as the a-spacing increases because electrodes in the array extend over the anomalous region at greater and greater distances from the array center point. NS
Northernmost site - site 1 (see earlier location map) Line 1 is highlighted in red below From Werner and Rauch
Your report should be a minimum of 3-4 pages double spaced (12pnt with 1 inch margins and the length does not include figures). AGAIN - Number your figures and make specific reference to them in the text of your report. Be sure to label important features on those figures when they are mentioned in your text. Use of captions is recommended.