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

MiniRes by L and R Instruments, Inc.

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: Map location of TRK 24 Sounding

Model of the TRK24 Sounding

Profile along Truckee River

Truckee Resistivity Profile 13-27

Model of MNZ1 Sounding

Manzanita 4 Profile  The Manzanita four profile consists of six measurements each taken at a-spacing 6.81 meters.  There is an low anomalous reading at station three along this profile

Locations of the MNZ4 Profile

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.

Locations of the MNZ6 Profile

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

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