Estimates of Basement Depth: Magnetotelluric and Gravity Modeling across Bonanza Creek Ranch, New Mexico

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Estimates of Basement Depth Magnetotelluric and
Gravity Modeling across Bonanza Creek Ranch, New
Mexico

• Nancy Helen Natek
• The University of New Mexico
• SAGE 2001

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Project Description

• The goal of this project is to understand the
  subsurface structure in the Bonanza Creek Ranch
  area.
• Specifically, use magnetotelluric and gravity
  data to estimate the depth of the electrical
  basement.

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Magnetotelluric Method

• The magnetotelluric method measures naturally
  occurring electric and magnetic fields at the
  Earths surface.
• Magnetotelluric sources can be high-frequency
  fields such as worldwide thunderstorms or
  low-frequency fields such as solar activity.
• By using these sources we can map electrical
  properties of the subsurface structure of the
  Earth from the surface to the lower crust.

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Magnetotelluric Modeling

• In MT modeling we seek to obtain values of
  resistivity of the subsurface.
• Since the resistivity of the subsurface is not
  uniform, we measure the apparent resistivity, ra
  as a function of frequency.
• We present MT data in plots of log ra versus log
  T which are called MT sounding curves.

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1-D Sounding Curves and Their Interpretation

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

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

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2-D Rift Model and Sample MT Sounding Curves

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

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MT Sounding Curves and 1-D model for Station
s0104
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Location of MT Sites
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2-D Stitch of a 1-D Inversion
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Gravity Model
(Model from Jounada Oueity)
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Conclusions

• We find that, in general, the magnetotelluric and
  gravity models are consistent with each other for
  basement depth in the Bonanza Creek Ranch Area.
• The magnetotelluric model estimated the depth of
  the electrical basement ranging from 3.2 to 3.6
  km while the gravity model estimated 4 km.

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Acknowledgements

• I am grateful to SAGE 2001 for giving me the
  opportunity to participate in this program and to
  National Science Foundation (NSF) for supporting
  this work.
• I would like to thank Shawn Biehler, George
  Jiracek and David Alumbaugh for their support in
  data processing and analysis.

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• Happy the man whose lot is to know
• The secrets of the Earth.
• --Euripides (480-405)