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

Depth

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?

-6a

3a

2a

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

40m20m10m3.7m

60mdepth

Midpoint

30m depth

15m depth

5.5m depth Exploration

Depth

Coil spacing

Sounding

EM34

EM31EM34

EM34

Surface

Northernmost site - site 1 (see earlier location map) Line 1 is highlighted in red below

From Werner and Rauch

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

N S

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.

N S

Northernmost site - site 1 (see earlier location map) Line 1 is highlighted in red below

From Werner and Rauch