Summary of Geophysical Results From the NSF Funded Biocomplexity Project:

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Summary of Geophysical Results From the NSF
Funded Biocomplexity Project Chris Hawkins, MS
Thesis Imaging the Shallow Subsurface Using
Ground Penetrating Radar at the Nyack Floodplain,
Western Montana. Nate Harrison, MS Thesis,
Gravity, Radar And Seismic Investigations To Help
Determine Geologic, Hydrologic, And Biologic
Relations In The Nyack Valley, Northwestern
Montana 12/5/2004SDS
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The Problem

• What is the depth to bedrock?
• What is the 3D shape of the Quaternary fill in
  the valley?
• To what extent (if any) do Tertiary sediments
  exist in the subsurface of the valley?
• Are there any major stratigraphic variations in
  the Quaternary sediments in the valley?

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A Geophysical Solution

• Collect gravity data in grid like fashion
  throughout the valley to model the configuration
  of the Quaternary and/or the Tertiary sediments.
• Collect GPR data in open areas to detect deep
  variations in the young stratigraphy, and depth
  to bedrock values us longer wavelength than
  Hawkins.
• Augment the above with seismic refraction data,
  as possible, to measure stratigraphic variations
  in the young stratigraphy, and depth to bedrock
  values.

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We measure gz, the vertical component of gravity
Integrate over all mass in a distant volume to
get the anomalous gravity at a point, P
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Gravity anomaly from equal bodies, different
depths. Area under the curves is equal.
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• 150 Gravity points.
• 79 GPR (50 mHz) lines

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Gravity Observations - collect and process

• Collect observations, GPS gives /- 30 cm
  elevation control
• Correct observations for
• instrumental and tidal drift (/- 0.002 mgal)
• latitude (/- 0.001 mgal)
• elevation above mean sea level (/- 0.1 mgal)
• local/regional deviations in topography

d/d(horizontal) /- 0.05 mgal/km for terrain
correction
Thus total error is /- 0.11 milligal
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Color is terrain correction contours are
topography
Terrain corrections are largest source of error
but not random error
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The Post-corrections Result is the Complete
Bouguer Anomaly

• The complete Bouguer anomaly correlates with
  shallow density variations.
• Low density material surrounded by a high density
  medium results in an low in the anomaly.
• This anomaly has not been corrected for
  long-wavelength gravity changes due to the
  isostatic effect.

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Complete Bouguer Anomaly on Topography
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Observed Gravity - Regional Gravity Residual
Gravity

• Processing ends and interpretation begins
• A subjective step
• Probably the most important step in gravity
  methods

Knowns for the Nyack Valley

• We are looking for the anomaly caused by the
  lower density valley fill. Thus at the bedrock
  contacts at the valleys edge, the residual
  gravity must be near zero
• Bedrock density is around 2800 kg/m3
  (experience)
• Glaciation post dates faulting - valley is
  roughly U-shaped
• Model results must fit gradients and volume of
  anomaly values

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Modeling the Crust-Mantle (Regional) Effects

• The crust-mantle effects are responsible for the
  large (or long-wavelength) variations in gravity.
• This is due to density variations in an uneven
  surface at the crust-mantle boundary.
• The regional anomaly was modeled with gravity
  points compiled by the National Geophysical Data
  Center (NGDC/NOAA)
• The NOAA points surround the Nyack Valley by
  about 36 kilometers.

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Gravity from Beyond Planar
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Regional Gravity as Best Fit Plane
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Residual at Small Scale
Residual, from shallow sources, centers on zero
milligals.
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Density Estimates (cont.)

• Previous work near the Nyack Valley
• Precambrian basement rocks (Belt Supergroup)
  density estimate of 2650 kg/m3.
• Tertiary rocks (Kishenehn formation) density
  estimate of 2350 kg/m3.
• 2D analysis of the residual gravity anomaly
• Quaternary rocks density estimate of 1950 kg/m3.

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Poor fit - density contrast too low
High gradients and short-radius curvature require
high density contrast and help bound density
contrast. This was delta-rho -250 kg/m3
higher delta rho means shallower basin
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Profile A-A with delta rho -700 kg/m3
Maximum depth 109 meters
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Cross Section D-D
Tertiary rocks in this area do not show a
decrease in thickness
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Conclusions Drawn From 2D Models

• The density contrast between the Precambrian
  basement and Quaternary sediments is 700 kg/m3.
• Greatest depth to bedrock is 220 meters in cross
  section A-A.
• Tertiary rocks increase in thickness to the north
  (B-B to D-D) from 50 to 260 meters.
• The Nyack Fault (western contact between Kt and
  the Belt) increases in dip from 6º east (B-B) to
  28º east (C-C).

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Initial Model
Original residual anomaly
3D depth to bedrock model
Density contrast 700 kg/m3
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Depth to Bedrock Model Quaternary Thickness

• Nate modeled the depth to Tertiary sediments with
  2D models.
• These depths were used with inversion to produce
  a modified depth to bedrock model.
• In the new model
• Bedrock Tertiary or Precambrian (whichever
  comes first)
• Thus, it is actually a Quaternary thickness model.

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Nates GPR seismic data might show where the
glacial deposits start

• Nates 79 50 mHz GPR lines, undertaken to confirm
  depth to bedrock and determine intra-basin
  stratigraphy, added little useful information.
  The penetration is too shallow (lt50m) to really
  help with the gravity inversion and we found no
  consistent stratigraphy. Although Nate did find
  a sporadic assortment of boulders at depths of
  20-30 meters.
• Nates five seismic lines indicate that, in
  general, the upper few meters (1-4m) is loosely
  compacted sediment and soil (866 m/s) over more
  consolidated material, 1660 m/s. Below that at
  23 /- 10m there is a second velocity increase to
  2,263 /- 621 m/s. This increase in velocity may
  be from an increase in the grain size of fluvial
  units Cains well HA-4 indicates such near one
  of the seismic lines.
• The depths to the third seismic layer are
  compatible to the depths at which Nate saw large
  boulders in his GPR surveys. This may be the top
  of glacial material.

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Nates Schematic, longitudinal profile, north to
the left
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Geologic Map
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• Publications
• C. R. Hawkins, S. D. Sheriff, and M. Lorang,
  Using Ground Penetrating Radar to Search for
  Preferential Groundwater Flow and Nutrient
  Delivery, Nyack Valley, Western Montana,
  submitted to River Research and Applications,
  2004, in review.
• M. Lorang and S. Sheriff, Synthesis of subsurface
  morphology and fluvial modifications maybe we
  made some progress yesterday?
• Presentations with Published Abstracts
• N. E. Harrison and S. D. Sheriff, 2004, Gravity,
  Radar And Seismic Investigations To Help
  Determine Geologic, Hydrologic, And Biologic
  Relations In The Nyack Valley, Northwestern
  Montana, Geological Society of America Abstracts
  with Programs, Vol. 36, No. 4, p. 32.
• C.R. Hawkins and S. D. Sheriff, 2003, Preliminary
  GPR investigation of an Intermontane Floodplain,
  Northwestern Montana, 2003 INRA Subsurface
  Science Symposium, October 5-8, INRA 2003 CD.
• C.R. Hawkins and S. D. Sheriff, 2003, Shallow
  Subsurface Imaging with Ground Penetrating Radar
  of the Nyack Floodplain, Montana, Geological
  Society of America, Abstracts with programs,
  V.35, 6 Abstract 123-8.
• Theses
• Chris Hawkins, Imaging the Shallow Subsurface
  Using Ground Penetrating Radar at the Nyack
  Floodplain, Montana. M.S. 2003
• Nathan Harrison, Gravity, Radar And Seismic
  Investigations To Help Determine Geologic,
  Hydrologic, And Biologic Relations In The Nyack
  Valley, Northwestern Montana, M.S. 2004
• Future Experiments We now have high frequency
  GPR (500 mHz) and are experimenting with using
  electrical resistivity to trace saline
  injections. We need an area with
  known/demonstrated paths of preferential flow