RESISTIVITY STUDYSPRING 2010 FIELD EXERCISEAPPLIED GEOPHYSICS 492/692Amie 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 C1P1 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.

ρa= 2πAR

(R is the resistance reading on the MiniRes, and A is the A-spacing)

Schlumberger ArrayUsing the Minires apparent resistivity is calculated

using the following equation: π A2R/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

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

Any Questions?