Estimates of Basement Depth: Estimates of Basement Depth: Magnetotelluric and Gravity Modeling across Magnetotelluric and Gravity Modeling across

Bonanza Creek Ranch, New MexicoBonanza Creek Ranch, New Mexico

Nancy Helen NatekNancy Helen Natek

The University of New MexicoThe University of New Mexico

SAGE 2001SAGE 2001

Project DescriptionProject Description The goal of this project is to understand the The goal of this project is to understand the

subsurface structure in the Bonanza Creek subsurface structure in the Bonanza Creek Ranch area.Ranch area.

Specifically, use magnetotelluric and gravity Specifically, use magnetotelluric and gravity data to estimate the depth of the electrical data to estimate the depth of the electrical basement. basement.

Magnetotelluric MethodMagnetotelluric Method The magnetotelluric method measures The magnetotelluric method measures

naturally occurring electric and magnetic fields naturally occurring electric and magnetic fields at the Earth’s surface.at the Earth’s surface.

Magnetotelluric sources can be high-frequency Magnetotelluric sources can be high-frequency fields such as worldwide thunderstorms or low-fields such as worldwide thunderstorms or low-frequency fields such as solar activity.frequency fields such as solar activity.

By using these sources we can map electrical By using these sources we can map electrical properties of the subsurface structure of the properties of the subsurface structure of the Earth from the surface to the lower crust. Earth from the surface to the lower crust.

Magnetotelluric ModelingMagnetotelluric Modeling

In MT modeling we seek to obtain In MT modeling we seek to obtain values of resistivity of the subsurface.values of resistivity of the subsurface.

Since the resistivity of the subsurface is Since the resistivity of the subsurface is not uniform, we measure the apparent not uniform, we measure the apparent resistivity, resistivity, aa as a function of frequency. as a function of frequency.

We present MT data in plots of log We present MT data in plots of log aa versus log versus log TT which are called MT which are called MT sounding curves. sounding curves.

1-D Sounding Curves and 1-D Sounding Curves and Their Interpretation Their Interpretation

Two layer MT sounding Two layer MT sounding curves where the basement curves where the basement is either perfectly resistive is either perfectly resistive or perfectly conductive.or perfectly conductive.

Observed TE apparent resistivity sounding Observed TE apparent resistivity sounding curves showing increase in basin depths.curves showing increase in basin depths.

(Jiracek, 1995)

Resistive

Conductive

(Jiracek, 1995)Short Periods Long Periods

ShallowBasin

DeeperBasin

2-D Rift Model and Sample 2-D Rift Model and Sample MT Sounding Curves MT Sounding Curves

(Jiracek, 1995)

Hypothetical 2-D geologic-Hypothetical 2-D geologic-geoelectric continental rift geoelectric continental rift model and sample MT model and sample MT sounding curves. sounding curves.

(Jiracek, 1995)

Site 2Range

Site 1Basin

TM

TE

ModeSplit

Resistive Basement

Basin

MT Sounding Curves and MT Sounding Curves and 1-D model for Station s01041-D model for Station s0104

TE

TM

Basin

ModeSplit

1-D inversion model of TE curve

Resistive Basement

Resistive BasementDepth

3.3 km

Location of MT SitesLocation of MT Sites

N

Bonanza Creek Ranch

Sant

o D

omin

go B

asin

La

Baj

ada

Faul

t

Ran

cho

Vie

jo

Sang

re d

e C

rist

o M

tns

2-D Stitch of a 1-D Inversion2-D Stitch of a 1-D Inversion

SW NE

Distance (km)

Ele

vatio

n (

m)

Bonanza Creek Ranch

Gravity ModelGravity Model

(Model from Jounada Oueity)

Basement DepthModeled by Gravity4.0 km

Bonanza Creek Ranch

ConclusionsConclusions

We find that, in general, the magnetotelluric We find that, in general, the magnetotelluric and gravity models are consistent with each and gravity models are consistent with each other for basement depth in the Bonanza other for basement depth in the Bonanza Creek Ranch Area. Creek Ranch Area.

The magnetotelluric model estimated the The magnetotelluric model estimated the depth of the electrical basement ranging from depth of the electrical basement ranging from 3.2 to 3.6 km while the gravity model 3.2 to 3.6 km while the gravity model estimated 4 km. estimated 4 km.

AcknowledgementsAcknowledgements

I am grateful to SAGE 2001 for giving me the I am grateful to SAGE 2001 for giving me the opportunity to participate in this program and opportunity to participate in this program and to National Science Foundation (NSF) for to National Science Foundation (NSF) for supporting this work.supporting this work.

I would like to thank Shawn Biehler, George I would like to thank Shawn Biehler, George Jiracek and David Alumbaugh for their Jiracek and David Alumbaugh for their support in data processing and analysis. support in data processing and analysis.

Happy the man whose lot is to knowHappy the man whose lot is to know

The secrets of the Earth.The secrets of the Earth.

--Euripides (480-405)--Euripides (480-405)