Validation of an Inverse Procedure for estimating soil

moisture using GPR

Dr. Hamed ParsianiElectrical & computer Engr.

University of Puerto Ricoparsiani@ece.uprm.edu

Collaborator: Dr. Eric HarmsenAgricultural Engr. & Biosystems

Students: Daniel Rodriguez &

Richard Diaz

Ground Penetrating Radar (GPR)

GSSI-SIR-20

• SIR-20 equipped with its own computer and T/R antennas (eg. 1.5 GHz)

• GPR images used to determine moisture content (of sand here)

3D Radiation Pattern of the Antenna. It can be observed that the radiation pattern obtained is not uniform. In this experiment the widest part occurs at about 8 inches.

A typical horizontal slice of the 3-D Radiation Pattern. It can be observed that the intensity is higher at the center and it diminishes radially. The highest overall intensity occurred at one inch

slice.

1.5 GHz GPR Antenna Radiation Pattern

Avg. Velocity & Dielectric Constant Determination from Hyperbolas

22

21

2

tt

xV

Avg. velocity determined from hyperbolas

Dielectric constant

2

V

Vc Dry and moist layers of sand and reflectors

Time Domain Reflectometer

• A Tektronix 1502 time domain reflectometer (TDR) was used to estimate the soil dielectric constant for comparison with estimates of the GPR.

• Measuring the dielectric constant of soil is accomplished by use of a wave guide, of 20 centimeter long, which is pushed into the soil.

Sandbox Experiment

0

3.9

7.9

Sand Layers with Different Moisture Contents

Depth in Inches

Dielectric constant = 3

Moisture content <1%

GPR- measured Dielectric constant = 8.85Gravimetric-measured Moisture content =16%

Mixing Model calculated Moisture content= 17%

Wt S C( ) 0.49 WP 0.165 8( )

x2 i w i 7( )

with

6( )v Wt

2 v Wt x2 v Wt w v a 1 s

5( )WP 0.06774 0.00064 S 0.00478 C

0.57 WP 0.481 4( )

x1 v i w i v

Wt 3( )

with

2( )v Wt

1 v v x1 v v a 1 s

Mixing Model of Wang & Schmugge

where θv = volumetric moisture content ε1 = dielectric constant for moisture content less than or equal to Wtε2 = dielectric constant for moisture content greater than Wtεa = dielectric constant of air (1)εi = dielectric constant of ice (3.2)εs = dielectric constant of dry soil or rock (4)εw = dielectric constant of pure water (81)φ = porosity WP = moisture content at the wilting point (pore water pressure = 15 bars)S = sand content in percent of dry soilC = clay content in percent of dry soil Wt = transition moisture content at which the dielectric constant increases steeply with increasing moisture content γ = fitting parameter which is related to WP.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40

6

12

18

24

30

Volumetric Moisture Content

Die

lectr

ic C

onst

ant

.

Moisture measurement based on Gravimetric Method:

Actual soil moisture was measured in each of the soil layers by the gravimetric method. In this method, the volumetric moisture content of the soil is obtained by eq.9.

(ρb/ρw)[(Wwet – Wdry) /Wdry ]

(9)where Wwet is the soil wet weight,

Wdry is the dry weight of the soil, ρb is the soil dry bulk density (1.6 gm/cm3), and ρw density of water (1 gm/cm3). The soil was dried in an oven at 105 oC for 24 hours. The soil dry bulk density was obtained dividing the soil dry weight of an undisturbed soil core of known volume.

Table 1. Volumetric moisture (%) by gravimetric, GPR and TDR methods

DepthGravimetric GPR TDR Benedetto

0 – 10 cm 0. 87 0 3.02 0

10 – 20 cm 16.03 15.04 14.71 14.5

Dielectric values vs. water contents for 1.5GHz

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

% Water contents

Die

lectric

valu

es

Results

Conclusions & Future Work

Conclusions:• The percent moisture content obtained by GPR corresponds well to other methods, and • verifies the results of the Benedetto model for 1.5 GHz antenna. • calculation of the depth-specific moisture average was realized, using the inverse procedure.

Future Work:• The average moisture content calculated at depth-specific based on balk-average GPR measurements must be extended, in the future, to more layers and tested with TDR. • Measure soil moisture content of loam and clay• Compare larger area moisture values with the ATLAS measurements