RESISTIVITY SOUNDING

West Tahoe Fault & UNR North Parking Lot MiniRes L&R Instrument

OBJECTIVES

Use resistivity to obtain a profile across the West Tahoe Fault

Constrain the source of poor data

IP & EARTH RESISTIVITY METER(MINIRES)

MEASURES: Earth resistivity Induced polarization

USES: Ground Water Exploration Geotechnical Engineering  Locating & Categorize Fractures      Archaeology Water Well

Fractures       Contaminated plumes                   Geologic Structures  Hydrogeologic Characterization     Surface Resistivity for M.T. Agricultural Applications  Corrosive Soil Identification Electrical Grounding

ECHO PASS

• Base of Echo Pass near South Lake Tahoe• Centered on LIDAR image of the fault trace• Site conditions were snowy and wet

ECHO PASS OBJECTIVES To obtain a profile across the fault trace Soundings parallel and perpendicular to fault

trace Used Wenner Array to collect four soundings Resistivity values were unrealistically high

UNR NORTH PARKING LOT North parking lot near medical center Site conditions were favorable except for nearby

fence & culvert

UNR RESULTS Used Wenner Array Complex Profile Simple Profile Instrument is Functional

END

TAKING A TYPICAL FIELD READING

Four electrodes are used for field readings and they are placed along a straight line, usually at uniform intervals (A-Spacing’s).

The instrument (MiniRes) is placed near the middle of the line.

The far two electrodes are used to put electric current into the earth.

The middle two electrodes are used to measure the voltage in the earth.

The far electrodes are connected to the MiniRes with cables. The cables are connected to the C+ and the C- binding posts

on the right side of the MiniRes. The middle two electrodes are connected to the P+ and the

P- binding posts on the left side of the MiniRes. Be sure the C+ and P+ cables go to the electrodes on the

same side of the MiniRes.

WENNER ARRAYThis is the most common method of sounding in the United States and has the greatest signal to noise ratio and is least susceptible to near-surface homogeneities. The four electrodes are in a straight line and are at uniform separation. The separation between adjacent electrodes is designated “a”.

SCHLUMBERGER ARRAYThis is the most common method of sounding in Europe and is common in the United States. It saves moving the potential electrodes every time the current electrodes are moved. The electrodes are in a straight line and like the Wenner array, the outer electrodes are the current electrodes and the inner electrodes are the potential electrodes. The potential electrodes, usually designated M and N, should never be separated by more that one-fifth the separation between the current electrodes. The current electrodes are usually designated A and B.

Echo 1A

Spacing

A/2 (P) 3A/2 (C) Res (Ohms) Apparent Resistivity

(2πPA)1 0.5 1.5 340400 2137712

1.48 0.74 2.22 329000 3057857.62.16 1.08 3.24 261200 3543125.763.16 1.58 4.74 215700 4280523.364.64 2.32 6.96 215700 6285325.446.8 3.4 10.2 112100 4787118.410 5 15 86200 5413360

14.8 7.4 22.2 46900 4359073.621.6 10.8 32.4 11.52 1562.6649631.6 15.8 47.4 2.299 456.231952

Echo 2A

Spacing

A/2 (P) 3A/2 (C) Res (Ohms) Apparent Resistivity

(2πPA)1 0.5 1.5 486400 3054592

1.48 0.74 2.22 378400 3517000.962.16 1.08 3.24 285600 3874106.883.16 1.58 4.74 201100 3990789.284.64 2.32 6.96 167500 48808166.8 3.4 10.2 129500 553016810 5 15 98600 6192080

14.8 7.4 22.2 83200 7732940.821.6 10.8 32.4 skipped31.6 15.8 47.4 skipped

Echo 3A

Spacing

A/2 (P) 3A/2 (C) Res (Ohms) Apparent Resistivity

(2πPA)1 0.5 1.5 1654100 10387748

1.48 0.74 2.22 1626100 15113623.842.16 1.08 3.24 1182900 16045801.923.16 1.58 4.74 892200 17705530.564.64 2.32 6.96 953700 27790055.046.8 3.4 10.2 507100 21655198.410 5 15 281200 17659360

14.8 7.4 22.2 135200 12566028.821.6 10.8 32.4 52600 7135084.831.6 15.8 47.4 20200 4008649.6

Echo 4A

Spacing

A/2 (P) 3A/2 (C) Res (Ohms) Apparent Resistivity

(2πPA)

1 0.5 1.5 No Reading (Too Big)

1.48 0.74 2.22 1726600 16047711.042.16 1.08 3.24 1465100 19873788.483.16 1.58 4.74 1080800 21448259.844.64 2.32 6.96 640500 18663657.66.8 3.4 10.2 328100 14011182.410 5 15 247200 15524160

14.8 7.4 22.2 125500 1166447221.6 10.8 32.4 36300 4924022.431.6 15.8 47.4 10000 1984480