Nancy Helen Natek The University of New Mexico SAGE 2001 Estimates of Basement Depth: Magnetotelluric and Gravity Modeling across Bonanza Creek Ranch, New Mexico Nancy Helen Natek The University of New Mexico SAGE 2001

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.

Magnetotelluric Method The magnetotelluric method measures naturally occurring electric and magnetic fields at the Earth’s 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.

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.

1-D Sounding Curves and Their Interpretation (Jiracek, 1995) Resistive Conductive Observed TE apparent resistivity sounding curves showing increase in basin depths. (Jiracek, 1995) Short Periods Long Periods Shallow Basin Deeper Two layer MT sounding curves where the basement is either perfectly resistive or perfectly conductive.

2-D Rift Model and Sample MT Sounding Curves (Jiracek, 1995) (Jiracek, 1995) Site 2 Range Site 1 Basin TM TE Mode Split Resistive Basement Hypothetical 2-D geologic-geoelectric continental rift model and sample MT sounding curves.

MT Sounding Curves and 1-D model for Station s0104 TE TM Basin Mode Split 1-D inversion model of TE curve Resistive Basement Depth 3.3 km

Location of MT Sites N Bonanza Creek Ranch Sangre de Cristo Mtns Santo Domingo Basin La Bajada Fault Rancho Viejo Sangre de Cristo Mtns

2-D Stitch of a 1-D Inversion SW NE Distance (km) Elevation (m) Bonanza Creek Ranch

Gravity Model (Model from Jounada Oueity) Bonanza Creek Ranch Basement Depth Modeled by Gravity 4.0 km Bonanza Creek Ranch (Model from Jounada Oueity)

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.

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.

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