Comparison of techniques for measuring the water content of soil and other porous media

Brendan Hugh George
BScAgr (Hons)

Department of Agricultural Chemistry & Soil Science
University of Sydney
New South Wales
Australia

Full copy of the thesis through Australian Digital Theses Program.
Abstract
Table of Contents

ABSTRACT

The measurement of water in soil on a potential, gravimetric or volumetric basis is considered, with studies concentrating on the measurement of water by dielectric and neutron moderation methods. The ability of the time-domain reflectometry technique to measure water content simultaneously at different spatial locations is an important advantage of the technique. The reported apparent dielectric by the TRASE® time-domain reflectometer and Pyelab timedomain reflectometry systems is sensitive to change in extension cable length. In some soil, e.g. a commercial sand, the response to increasing extension length of extension cable is linear. For other soil a linear response occurs for certain lengths of cable at different moisture contents. A single model accounting for clay content, extension cable length, time-domain reflectometry system, probe type and inherent moisture conditions explained 62.2 % of variation from the control (0 m extension) cable. The extension cable causes a decrease in the returning electromagnetic-wave energy; leading to a decline in the slope used in automatic end-point determination. Calibration for each probe installation when the soil is saturated, and at small water contents is recommended.

The ability of time-domain reflectometry, frequency-domain and neutron moderation techniques in measuring soil water content in a Brown Chromosol is examined. An in situ calibration, across a limited range of water contents, for the neutron moderation method is more sensitive to changing soil water content than the factory supplied "universal" calibration. Comparison of the EnviroSCAN® frequency-domain system and the NMM count ratio indicates the frequency-domain technique is more sensitive to change in soil water conditions. The EnviroSCAN® system is well suited to continuous profile-based measurement of soil water content. Results with the time-domain reflectometry technique were disappointing, indicating the limited applicability of time-domain reflectometry in profile based soil water content measurement in heavy-textured soil, or soil with a large electrical conductivity. The method of auguring to a known depth and placement of the time-domain reflectometry probe into undisturbed soil is not recommended.

A time-domain reflectometry system is adapted for in situ measurement of water in an iron ore stockpile. The laboratory calibration for water content of the processed iron ore compares favourably to a field calibration. In the field study, the 28 m extension cable used to connect the probes to the time-domain reflectometry affected the end-point determination of the timedomain reflectometry system. To account for this, 0.197 should be subtracted from the reported apparent dielectric before calculation of volumetric moisture content.

Abstract	i
Acknowledgements	ii
List of symbols	iii
Table of contents	iv
List of figures	vii
List of tables	xi
General introduction	xiii
Aims	xv
Chapter one A review of the literature
1.1. MEASURING MOISTURE STATUS OF POROUS MATERIALS	1
1.1.1 Wetness	2
1.1.2 Energy - potential	3
1.1.3 Volumetric moisture content	5

1.1.4 Relationship of methodologies	6
1.1.4.1 Bulk density	6
1.1.4.2 Soil moisture characteristic	7
1.2. MEASURING IN SITU MOISTURE CONTENT WITH NEUTRON MOISTURE MODERATION	8
1.2.1 Introduction	8
1.2.2 Basis of the NMM technique	9
1.2.3 NMM calibration	10
1.2.4 Field operation	11
1.2.5 Radiation hazard	12
1.3. DIELECTRIC PROPERTIES OF SOIL	13
1.3.1 Introduction	13
1.3.2 The effect of frequency	14
1.3.3 Temperature effect on dielectric measurement	15
1.4. Time-Domain Reflectometry (TDR)	17
1.4.1 Introduction	17
1.4.2 Measurement principle of time-domain reflectometry	18
1.4.3 Electromagnetic wave reflection - determination of impedance	19
1.4.4 TDR probe geometry, design and sensitivity	20
1.4.5 TDR calibration	23
1.4.6 The refractive index and mixing models	24
1.4.7 Bulk density effect on tdr calibration	27
1.4.8 Bulk soil electrical conductivity ( ̴̶��̷) effect on ̴��̴�� measurement	29
1.4.9 Time measurement errors	30
1.4.10 Extension cables	31
1.4.11 Field operation	32
1.4.12 Current situation with tdr instrument development	33
1.4.12.1 Step-pulse tdr systems based on tektronix cable testers	33
1.4.12.2 Dedicated step pulse TDR instruments	35
1.4.13 TDR summary	38
1.5. FREQUENCY-DOMAIN (CAPACITANCE) TECHNIQUE	39
1.5.1 Introduction	39
1.5.2 FD calibration	42
1.5.3 FD precision	43
1.5.4 Influence of air-gap on measured dielectric	44
1.5.5 Field application of FD technique	44
1.5.6 FD summary	45
1.6. OTHER DIELECTRIC-BASED DEVICES	46
1.7. POTENTIAL	47
1.7.1 Introduction	47
1.7.2 Thermal	47
1.7.3 Tensiometry	49
1.7.4 Electrical resistance blocks	50
1.7.5 Psychrometers	51
1.8 NON-INVASIVE TECHNIQUES	52
1.8.1 Microwave reflectance and attenuation	52
1.8.2 Near infrared reflectance	52
1.8.3 Gamma ray attenuation	53
1.8.4 Nuclear magnetic resonance	53
1.8.5 Ground penetrating radar	54
1.9. SUMMARY	54
Chapter two Calibration of time-domain reflectometry and effect of coaxial extension cable on reported apparent dielectric
2.1 ABSTRACT	57
2.2 INTRODUCTION	58
2.3 MATERIALS AND METHODS	60
2.3.1 Soil	60
2.3.2 Equipment	63
2.3.2.2 Probes and extension cables	63
2.3.2.3 Tension tables	65
2.3.3 Measurements and determination of Ka	67
2.4 RESULTS AND DISCUSSION	68
2.4.1 Comparison of reported ̴��̴�� by TDR instruments to known ̴��̴��	68
2.4.2 Consideration of bulk density and soil dielectric	69
2.4.3 Comparison of ̴��̴�� reported by TRASḘ̴�� TDR and Pyelab TDR systems	71
2.4.4 TDR system conversion from Ka to ̴��̴��	73
2.4.5 Comparison of reported Ka by TDR instruments	73
2.4.6 Calibration of Pyelab and TRASḘ̴�� TDR systems	74
2.4.7 Effect of extension cable on reported Ka	78
2.4.7.1 Combined data for the RG-58 extension cable	78
2.4.7.2 Vertosol as an example	79
2.4.7.3 Measurement with RG-8 extension cable in a Vertosol	90
2.5 CONCLUSIONS	93
Chapter three Instrumental characteristics of neutron moderation and frequency-domain field sensors
3.1. ABSTRACT	95
3.2. NEUTRON MODERATION COUNT RATE AND EFFECT ON ERROR DETERMINATION AND PRECISION	95
3.2.1 Introduction	95
3.2.2 Methods and materials	96
3.2.3 Results and discussion	97
3.3. MEASUREMENT CHARACTERISTICS OF A DOWN-HOLE FREQUENCY-DOMAIN SENSOR	100
3.3.1 Introduction	100
3.3.2 Materials and methods	101
3.3.3 Results and discussion	103
3.4. CONCLUSIONS	105
Chapter four Field measurement of ̴��̴�� with neutron moderation and dielectric techniques
4.1. ABSTRACT	107
4.2. INTRODUCTION	107
4.3. MATERIALS AND METHODS	108
4.3.1 The site	108
4.3.2 Equipment and installation	109
4.3.3 In situ calibration of the NMM Probe	111
4.4. RESULTS AND DISCUSSION	113
4.4.1 Calibration of the NMM probe	113
4.4.2 Comparison with the factory supplied '��universal'�� calibration	116
4.4.3 Measurement of ̴��̴�� during a drying cycle	117
4.4.4 Calibration and operational considerations of the IH1 probe	119
4.4.5 Continual ̴��̴�� measurement	122
4.4.6 Measurement with time-domain reflectometry (TDR)	123
4.5. CONCLUSIONS	126
Chapter five Measuring moisture content of stockpiled iron ore with the time-domain reflectometry technique
5.1. ABSTRACT	127
5.2. INTRODUCTION	128
5.3. LABORATORY CALIBRATION	129
5.3.1 Materials and Methods	129
5.3.2 Results and Discussion	132
5.3.2.1 Calibration of TDR for estimation of iron ore moisture content	132
5.3.2.2 Effect of transmission cable on reported '��K_a	137
5.4. FIELD MEASUREMENT AND CALIBRATION	139
5.4.1 Materials and Methods	139
5.4.1.1 The site	139
5.4.1.2 Experimental design	140
5.4.2 Results and Discussion	142
5.4.2.1 Calibration to ̴��̴��	144
5.4.2.2 Calibration for extension cable length	145
5.4.2.3 Determination of in situ bulk density (̴̶��b)	146
5.4.2.4 Moisture measurement across stockpile	147
5.5. CONCLUSIONS	152
Chapter six General discussion, conclusions and further work
6.1. GENERAL DISCUSSION	155
6.2. CONCLUSIONS	158
6.3. FUTURE DIRECTIONS	159
BIBLIOGRAPHY	161
APPENDICES	177

Comparison of techniques for measuring the water content of soil and other porous media

ABSTRACT

TABLE OF CONTENTS