RESISTIVITY STUDY SPRING 2010 FIELD EXERCISE APPLIED GEOPHYSICS 492/692 Amie Lamb, Katie Ryan, Justin Skord and Nicole Shivers.
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RESISTIVITY STUDY SPRING 2010 FIELD EXERCISE APPLIED GEOPHYSICS 492/692 Amie Lamb, Katie Ryan, Justin Skord and Nicole Shivers
Background Resistivity is used to determine subsurface geology at shallow depths. Resistivity can locate fault zones, mineral content and the presence of fluids. Also measured by resistivity is the porosity and degree of saturation of the subsurface. Resistivity is quantified by Ohms Law: J = σ E (Current density (J), conductivity of the medium (σ) and electric field intensity (E).)
Background The primary structures being targeted for identification during the Spring 2010 survey were faults. Faults are identified due to changes in resistivity through bedrock and sediments. Resistivity in igneous and metamorphic rocks ranges from 100 to 1 million ohm m Sedimentary rocks ranges from 10 to 5000 ohm m.
The figure above shows general values of resistivity for various rock types and other materials.
Introduction During the Spring 2010 geophysics field study resistivity was used to determine changes in the electrical properties of the subsurface. The MiniRes by L and R Instruments, Inc was used to conduct resistivity surveys in Reno, NV. One survey was done along the Truckee River to detect changes in resistivity due to fault zones. A second survey was done near Manzanita lane in Reno, NV to see if the resistivity is effected by a known hydrothermal system in that area
Introduction A generalized configuration of the resistivity surveys used throughout the Spring 2010 field survey. 30m C1 P1 P2 A-spacing=10m
Soundings and Profiles Two types of resistivity surveys were conducted: soundings and profiles. Sounding’s: The general location of the survey remains in place but the electrode spacing is varied. Profiles: The electrode spacing remains the same but the locations change along a predetermined path. Two types of sounding methods: Wenner and Schlumberger.
Wenner Array Wenner Array: The current and potential electrodes are evenly spaced and apparent resistivity is calculated at each a-spacing with the following equation. ρ a = 2πAR (R is the resistance reading on the MiniRes, and A is the A-spacing)
Schlumberger Array Using the Minires apparent resistivity is calculated using the following equation: π A 2 R/4B
Data and interpretation’s Modeling of Wenner soundings was done with the Resis computer program which comes attached to the Berger text: Introduction to Applied Geophysics. This program allows modelers to model several layers in the subsurface at several unique apparent resistivity.
Truckee River Sounding The Truckee river profile consists of 6 measurements each taken at an a-spacing of 3.16 meters. There is one anomalously high resistivity reading at station 24. This station shows an apparent resistivity of close to 700 ohm-meters.
TRK24 easting: 258681 Map location of TRK 24 Sounding
Manzanita 6 profile The Manzanita-six profile consists of nine measurements taken at a-spacing of 6.81 meters. There are two anomalous sections of this profile. Station 10 is anomalously high and stations 14 and 16 are anomalously low.
Conclusions: Manzanita Lane The Manzanita sounding, in general, has much lower apparent resistivity’s than the Truckee sounding. The top layer of the Manzanita model is one meter thick with a resistivity of 39 ohm-meters. The middle layer is five meters thick with a resistivity of 29 ohm-meters. The bottom layer begins at a depth of six meters with an increase in resistivity up to 39 ohm-meters
Conclusions The Truckee and Manzanita sounding models differ greatly from each other. The sounding along the Truckee, in general, had much higher apparent resistivity values than the soundings done on Manzanita Lane. While it is clear that these two soundings are very different in their electrical properties there is similarity in that each sounding shows a lower resistivity middle layer bounded at the top and bottom by layers of higher resistivity