[Geomag-data] Regional Resistivity

[Rigler, Erin (Josh)]

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>
Date: Sun, Feb 21, 2016 at 10:41 AM
Subject: Regional Resistivity
To: "geomagdata at usgs.gov" <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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