Monitoring changes in soil moisture
during artificial infiltration with
geophysical methods
Derek Lichtner1, Jonathan Nyquist1,
Laura Toran1, Li Guo2, and Henry Lin2
(1) Earth and Environmental Science, Temple University, Philadelphia, PA 19122
(2) Crop and Soil Sciences, Penn State University, University Park, PA 16802
From Lin, 2010.
Funding was provided by NSF EAR-0725019 for the Susquehanna/Shale Hills Critical Zone Observatory and the Temple CST URP
Flow in the Vadose Zone
Important for:
1)
Making agricultural decisions
2)
Understanding contaminant propagation
3)
Describing groundwater recharge
4)
Understanding soil formation
The Critical Zone
Vadose zone
Unconfined
Aquifer
Susquehanna Shale Hills CZO
• 7.9-ha forested research site in Huntingdon County, PA
• Ephemeral stream runs roughly east to west
• Weikert series soil: a well-drained, shallow soil
Geophysical Methods
•Surface reflection GPr
•Ground wave GPR
•Electrical resistivity tomography
Artificial Infiltration Experiments
Artificial Infiltration Experiments
• 53 L or 26.5 L of water at constant head
• Horizontal flow
• Geophysical data were collected at 15 minute intervals
Ground-Penetrating Radar (GPR)
GPR signal trace
Two-way travel time
Reflective interfaces
Tx Rx
GPR images:
• Reflective interfaces
with contrasting
dielectric
permittivities, e.g.
soil layers, moisture
• Scattering objects,
e.g. rocks, tree roots
800 MHz, 1 GHz, and 2.3 GHz antennas were used
GPR Data Processing
Software:
• MatLab scripting
• MatGPR
opensource add-on
• Reflex2DQuick
• Surfer 11 Gridding
Strong reflectors (weathered shale)
Heterogeneous soil/root fabric
Example radargram of Weikert soil site, pre-infiltration
New Approach: Surface Reflection GPR
Tx Rx
Tx Rx
Elevated GPR
unit
Stronger
surface
reflections
air
dry soil
wet soil
• GPR unit is elevated
• Increased reflection
amplitudes where
the soil is moist
• Water content is
proportional to the
surface reflection
coefficient
• Travel time
proportional to
microtopography
Position, N to S (m)
Microtopography from off-ground GPR
Subsurface flow followed this topography
Position, W to E (m)
•Relative elevations determined from off-ground GPR travel times
•5 cm contour interval
Time-lapse GPR Surface Amplitude
(%)
1. At end of 26.5 L (7 gal) injection
2. 15 minutes after injection ended
(%)
3. 45 minutes after injection ended
4. At end of additional 26.5 L (7 gal) injection
•Perspective is overhead map view
•Blue = percent increases in soil moisture, orange = background
•Water appears rapidly and subsequently fades
(%)
(%)
Another Approach: Ground Wave GPR
air
Tx
air wave
ground wave
layer ε1
layer ε2
refracted wave
Propagation paths of GPR waves in
soils. After Huisman et al., 2003.
Rx
• Air wave arrives first
• Ground wave arrives
second
• Water content is
proportional to the
difference in arrival
times
• Lower velocity ground
waves indicate higher
moisture content
Ground Wave GPR
Ground Wave Delay due to Moisture
Unwetted Trace:
Wetted Trace:
Normalized Amplitude
Normalized Amplitude
Time (ns)
Time (ns)
•Left: Unwetted
GPR signal trace is
very reproducible
After wetting:
Ground wave delay
Before
Before
After
After
•Right: Ground
wave in wetted soil
shows delay and
amplitude increase
Ground Wave Arrival Time Picking
Air wave
Distance (m)
Slow ground wave = moist soil
Time (ns)
Air wave echo
•Air wave (top red line) and ground wave arrivals (bottom red line)
•Ground wave velocity is dependent on moisture
Change in Water Content (m3/m3)
Time-lapse Soil Water Content
Large increases at grid’s
center fade with time
2nd infiltration
more to E with
microtopography
Position, W to E (m)
Map view
Time-lapse water contents
calculated with ground wave GPR
Trench
Ground wave line
Soil grid
Electrical Resistivity
• Super Sting R8 Resistivity meter
with 28 electrodes
• Changes in resistivity are
proportional to changes in
saturation
voltmeter
battery
V
subsurface with apparent
resistivity ρ
electric field lines
equipotential lines
Time-lapse Resistivity
Depth (m)
Trench
Water promotes nearsurface current, creating
positive inversion artifact
Position, W to E (m)
Conductive anomaly
from water
Depth (m)
At end of 53 L injection
Anomaly fades slightly
with time, spreads
Depth (m)
30 minutes after injection ended
Greater extent after
2nd infiltration
At end of additional 53 L injection
•Negative percent-changes in resistivity (blue) correspond to increases in soil moisture
•Increased water contents appear quickly and subsequently fade, indicating rapid infiltration
Conclusions
Off-Ground GPR
• Infiltration followed the site’s
microtopography to the S and SE
• Infiltration was rapid, with
geophysical signatures strongest at
the conclusion of injection and
fading with each subsequent 15
minute measurement
Ground Wave GPR
Resistivity
At end of 53 L injection
30 minutes after injection ended
At end of additional 53 L injection
• A second infiltration
pulse utilized already
activated flowpaths
Questions?