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

Funding was provided by NSF EAR-0725019 for the Susquehanna/Shale Hills Critical Zone Observatory and the Temple CST URP

From Lin, 2010.

Flow in the Vadose ZoneImportant for:1)Making agricultural decisions2)Understanding contaminant propagation3)Describing groundwater recharge4)Understanding soil formation

Vadose zone

Unconfined Aquifer

The Critical Zone

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

Infiltration

Ground-Penetrating Radar (GPR)

Tx Rx

Two-

way

trav

el ti

me

GPR signal trace

Refle

ctive

inte

rfac

es

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

Software:•MatLab scripting•MatGPR opensource add-on•Reflex2DQuick•Surfer 11 Gridding

Example radargram of Weikert soil site, pre-infiltration

Strong reflectors (weathered shale) Heterogeneous soil/root fabric

GPR Data Processing

New Approach: Surface Reflection GPR

Tx Rx

air

dry soil

Elevated GPR unit

wet soil

Stronger surface reflections

Tx Rx• 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

Microtopography from off-ground GPR

Subsurface flow followed this topography

•Relative elevations determined from off-ground GPR travel times•5 cm contour interval

Position, W to E (m)

Posi

tion,

N to

S (m

)

Time-lapse GPR Surface Amplitude

•Perspective is overhead map view•Blue = percent increases in soil moisture, orange = background•Water appears rapidly and subsequently fades

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

(%) (%)

(%) (%)

Another Approach: Ground Wave GPR

• 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

RxTxair

layer ε1

layer ε2

air wave

refracted wave

ground wavere

flect

ed w

ave

criti

cally

refr

acte

d w

ave

Propagation paths of GPR waves in soils. After Huisman et al., 2003.

Ground Wave GPR

Ground Wave Delay due to Moisture

After wetting: Ground wave delay

•Left: Unwetted GPR signal trace is very reproducible

•Right: Ground wave in wetted soil shows delay and amplitude increase

Normalized Amplitude Normalized Amplitude

Tim

e (n

s)

Tim

e (n

s)

Unwetted Trace: Wetted Trace:

Before

After

Before

After

Ground Wave Arrival Time Picking

•Air wave (top red line) and ground wave arrivals (bottom red line)•Ground wave velocity is dependent on moisture

Air wave echo Slow ground wave = moist soilDistance (m)

Tim

e (n

s)

Air wave

Time-lapse Soil Water Content

Time-lapse water contents calculated with ground wave GPR

2nd infiltration more to E with microtopography

Large increases at grid’s center fade with time

Position, W to E (m)

Chan

ge in

Wat

er C

onte

nt (m

3 /m

3 )

Soil grid

Ground wave line

TrenchMap view

Electrical Resistivity• Super Sting R8 Resistivity meter

with 28 electrodes• Changes in resistivity are

proportional to changes in saturation

Vvoltmeter battery

equipotential lineselectric field lines

subsurface with apparent resistivity ρ

At end of 53 L injection

30 minutes after injection ended

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

Time-lapse Resistivity

Conductive anomaly from water

Anomaly fades slightly with time, spreads

Greater extent after 2nd infiltration

Water promotes near-surface current, creating positive inversion artifact

Dep

th (m

)D

epth

(m)

Dep

th (m

)

Trench Position, W to E (m)

Conclusions• 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

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?

Off-Ground GPR

Ground Wave GPR

Resistivity