[Geomag-data] Regional Resistivity

[Edward Oughton]

Dear Josh,

Thanks for your response. I take on board the caveats around this data and will consequently treat it with caution.

Best wishes,

Edward

From: Rigler, Erin (Josh) [mailto:erigler at usgs.gov]
Sent: 23 February 2016 21:05
To: geomag-data at geohazards.usgs.gov
Cc: Edward Oughton <e.oughton at jbs.cam.ac.uk>
Subject: Regional Resistivity

Edward,
Your recent inquiry to the USGS geomagnetism program was forwarded to me. We are currently transitioning to an email list-serve to help field questions about our operations and data products, so I hope you won't mind if I CC your question, and my response, to this server. If you want to continue this conversation, or think you may have reason to receive very occasional future emails related to our data products, you might consider registering yourself: https://geohazards.usgs.gov/mailman/listinfo/geomag-data.
With that out of the way, here is my response to your inquiry (copied below):
Short answer: we don't have conductivity data available for the regions you mentioned below; for reasons unknown they were not included in the report from which these data were extracted (i.e., P. Fernberg 2012, One-Dimensional Earth Resistivity Models for Selected Areas of Continental United States and Alaska<http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001026430>, EPRI Technical Update 1026430, Palo Alto, CA).
Longer answer: the data we *do* have are hypothetical 1-D models of conductivity constructed from geological/geophysical research articles published in the past. They are organized by physiographic provinces that only roughly approximate the geographic bounds of conductivity ("physiographic" pertaining to surface features, *not* the actual underlying geology). Some of the depth-dependent conductivities are measured (e.g., using magnetotelluric or magnetic sounding techniques), but most are estimated from knowledge of the underlying geology, and laboratory measurements of the conductivity of similar rock types. These 1-D conductivities, with large uncertainty bounds, should be considered representative of their region, and not necessarily accurate for any particular location. For this reason, it is not unreasonable to choose a nearby region that *does* have conductivity data to serve as a proxy for a region where it is missing, especially if there is reason to believe the two regions have similar underlying geology.
Long-term plans: We are working to convert magnetotelluric measurements made in the United States' NSF Earthscope program into a self-consistent, and much more physically realistic, 3-D model of conductivity. There is no firm delivery date for this, but it is probably safe to say it will go public within a 1-2 year time frame, with beta releases somewhat earlier, and regular updates following on.
Thanks for your interest in the USGS Geomagnetism Program.
Kind Regards,
Josh Rigler

---------- Forwarded message ----------
From: Edward Oughton <e.oughton at jbs.cam.ac.uk<mailto:e.oughton at jbs.cam.ac.uk>>
Date: Sun, Feb 21, 2016 at 10:41 AM
Subject: Regional Resistivity
To: "geomagdata at usgs.gov<mailto:geomagdata at usgs.gov>" <geomagdata at usgs.gov<mailto:geomagdata at usgs.gov>>

Dear Sir/Madam,

I noticed a couple of regional gaps in this data with no model being assigned. This includes:

  *   Northern Rocky Mountains
  *   Middle Rocky Mountains
  *   Wyoming Basin
  *   Southern Rocky Mountains
  *   Ozark Plateaus
  *   Ouachita
  *   Interior low plateaus
  *   Eastern Maryland, Delaware and New Jersey
  *   Northwest Massachusetts, Vermont, New Hampshire and Northern Maine
Is this because we don't have data available for these locations, or do they serve some sort of baseline?

Any help would be very much appreciated.

Thanks and best wishes,

Edward

--
Dr Edward J. Oughton
Senior Risk Researcher
Centre for Risk Studies
Judge Business School
University of Cambridge
T: +44 (0) 7920 401 571



--
E. Joshua Rigler
Research Geophysicist
Geomagnetism Program
U.S. Geological Survey
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