RESISTIVITY SOUNDING West Tahoe Fault & UNR North Parking Lot MiniRes L&R Instrument
Jan 04, 2016
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
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 21377121.48 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 30545921.48 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 103877481.48 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 1.5No 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