Download Nyack

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
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?
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.
Take a couple of Newton's laws:
f mg
GMm
f
r
2
Equate them:
mg
GMm
r
and you get:
g
2
GM
r
2
Think of g being the gravitational effect
of some mass M at a distance r from the
point of observation.
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:
g( P)
G
dM dx dy dz
r
2
Gravity anomaly from equal bodies, different depths. Area
under the curves is equal.
• 150 Gravity
points.
• 79 GPR (50
mHz) lines
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
Color is terrain correction; contours are topography
Terrain corrections are largest source of error but not random error
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 longwavelength gravity
changes due to the
isostatic effect.
Complete Bouguer Anomaly on Topography
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 valley’s edge, the
residual gravity must be near zero
• Bedrock density is around 2800 kg/m^3 (experience)
• Glaciation post dates faulting - valley is roughly U-shaped
• Model results must fit gradients and volume of anomaly values
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.
Gravity from Beyond ~ Planar
Regional Gravity as Best Fit Plane
Residual at Small Scale
Residual, from shallow sources, centers on zero milligals.
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.
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/m^3; higher delta rho means shallower basin
Profile A-A’; with delta rho = -700 kg/m^3
Maximum depth ~ 109 meters
Cross Section D-D’
Tertiary rocks in this area do not show a decrease in thickness
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’).
Initial Model
Original residual anomaly
3D depth to bedrock model
Density contrast = 700 kg/m3
Depth to Bedrock Model
• 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.
– Quaternary Thickness
Nate’s GPR & seismic data might show where the glacial deposits start
• Nate’s 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 (<50m) 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.
• Nate’s 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 –
Cain’s 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.
Nate’s Schematic, longitudinal profile, north to the left
Geologic Map
•
Publications
–
–
•
Presentations with Published Abstracts
–
•
•
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?
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