Small-scale soil moisture determination with ground-penetrating radar (GPR)

Transcription

1 Small-scale soil moisture determination with ground-penetrating radar (GPR) Jan Igel & Holger Preetz Leibniz Institute for Applied Geophysics, Hannover, Germany 04/05/2010 EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 1 /12

2 Moisture distribution in the topsoil Depends on: Weather conditions Vegetation and rooting Texture Humus content Bulk density Aggregates Cultivation Example: Moisture distribution in a sandy topsoil In general, soil-moisture distribution is not homogeneous The actual distribution is important for all non-linear processes (evapotranspiration, water flow, heat storage... ) small-scale variability is needed for numerical simulations EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 2 /12

3 Moisture distribution in the topsoil Depends on: Weather conditions Vegetation and rooting Texture Humus content Bulk density Aggregates Cultivation Example: Moisture distribution in a sandy topsoil In general, soil-moisture distribution is not homogeneous The actual distribution is important for all non-linear processes (evapotranspiration, water flow, heat storage... ) small-scale variability is needed for numerical simulations EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 2 /12

4 Moisture distribution in the topsoil Depends on: Weather conditions Vegetation and rooting Texture Humus content Bulk density Aggregates Cultivation Example: Moisture distribution in a sandy topsoil In general, soil-moisture distribution is not homogeneous The actual distribution is important for all non-linear processes (evapotranspiration, water flow, heat storage... ) small-scale variability is needed for numerical simulations EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 2 /12

5 Methods to determine soil moisture spatially Point measurements Soil sampling and gravimetric moisture determination TDR (time-domain reflectometry) + simple and accurate invasive and time consuming Field and regional scale Remote sensing + fast and suitable for large areas limited spatial resolution limited penetration in soil influence of vegetation There is a need for soil-moisture measurements on larger areas with high spatial and temporal resolution Geophysical techniques: ERT, EMI, (MRT), GPR... GPR is a promising method and has been successfully used for soil-moisture determination since several years Challenge: Optimise technique regarding high spatial resolution and measuring progress EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 3 /12

6 Methods to determine soil moisture spatially Point measurements Soil sampling and gravimetric moisture determination TDR (time-domain reflectometry) + simple and accurate invasive and time consuming Gap Field and regional scale Remote sensing + fast and suitable for large areas limited spatial resolution limited penetration in soil influence of vegetation There is a need for soil-moisture measurements on larger areas with high spatial and temporal resolution Geophysical techniques: ERT, EMI, (MRT), GPR... GPR is a promising method and has been successfully used for soil-moisture determination since several years Challenge: Optimise technique regarding high spatial resolution and measuring progress EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 3 /12

7 Methods to determine soil moisture spatially Point measurements Soil sampling and gravimetric moisture determination TDR (time-domain reflectometry) + simple and accurate invasive and time consuming Gap Field and regional scale Remote sensing + fast and suitable for large areas limited spatial resolution limited penetration in soil influence of vegetation There is a need for soil-moisture measurements on larger areas with high spatial and temporal resolution Geophysical techniques: ERT, EMI, (MRT), GPR... GPR is a promising method and has been successfully used for soil-moisture determination since several years Challenge: Optimise technique regarding high spatial resolution and measuring progress EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 3 /12

8 Soil moisture determination by GPR Dielectric permittivity is correlated to soil moisture: ε = f(θ V ) soil matrix 4 ε r 9 free water ε r 80 air ε r = 1 bound water ε r «80 EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 4 /12

9 Soil moisture determination by GPR Dielectric permittivity is correlated to soil moisture: ε = f(θ V ) soil matrix 4 ε r 9 free water ε r 80 air ε r = 1 bound water ε r «80 Mixing models Volume dependent: Dobson, CRIM... Structure dependent: DeLoor, Maxwell-Garnet... Empirical: Topp or site specific EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 4 /12

10 Soil moisture determination by GPR Dielectric permittivity is correlated to soil moisture: ε = f(θ V ) Groundwave Reflection at ground surface T groundwave R T R T groundwave soil v c 0 ε soil r air R 1 ε soil r 1+ soilε soil r soil EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 4 /12

11 GPR groundwave, principle Moveout measurement (MO) Simulated GPR data air T R soil EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 5 /12

12 GPR groundwave, principle Moveout measurement (MO) Simulated GPR data air T R soil EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 5 /12

13 GPR groundwave, principle Moveout measurement (MO) Simulated GPR data air T R soil EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 5 /12

14 GPR groundwave, principle Moveout measurement (MO) Simulated GPR data air airwave T airwave R v = dx/dt groundwave groundwave soil slope groundwave 1/v soil ε soil EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 5 /12

15 part, x > x opt). In this mode, the permittivity distribution along a profile can be deduced rapidly. However, Title can Motivation be a challenge GPR Moisture to identify GPR Groundwave the ground wave GPR Reflection Conclusion in solely a CO measurement especially in laterally and vertically heterogeneous soils where numerous phases will interfere. Groundwave measurements A combination of both methods was proposed by Du (1996) and showed to be the most appropriate to this date. First, a moveout measurement is carried out by separating the transmitter and receiver antenna. The optimal transmitterreceiver offset x opt is determined to the distance where the air and ground wave are separated and do not influence one another or interfere with reflected waves. Then, the profile is mapped with a CO setup as illustrated in Fig This ensures the correct identification of the different phases in the radargram. Conventional method first moveout (MO) measurement then constant-offset (CO) measurement 0 x 1 x opt x 1/c 0 1/v soil aw gw Drawbacks: t moveout constant offset Figure 4.20: Schematic traveltime diagram of a ground wave measurement consisting of a moveout measurement from x 1 to x opt followed by a constant offset measurement at x > x opt (aw: air wave, gw: ground wave). time consuming due to two measuring modes (MO and CO) velocity determination from MO sometimes difficult (heterogeneity) The approach introduced above has some basic disadvantages: lateral resolution limited by optimal T R-distance ˆ Measurements in two modes (MO or CMP and CO) have to be carried out EGU 2010 which J. Igel & require H. Preetz, a modification LIAG Hannover of the layout Small-scale and thus soil are moisture timedetermination consuming. with GPR 6 /12

16 part, x > x opt). In this mode, the permittivity distribution along a profile can be deduced rapidly. However, Title can Motivation be a challenge GPR Moisture to identify GPR Groundwave the ground wave GPR Reflection Conclusion in solely a CO measurement especially in laterally and vertically heterogeneous soils where numerous phases will interfere. Groundwave measurements A combination of both methods was proposed by Du (1996) and showed to be the most appropriate to this date. First, a moveout measurement is carried out by separating the transmitter and receiver antenna. The optimal transmitterreceiver offset x opt is determined to the distance where the air and ground wave are separated and do not influence one another or interfere with reflected waves. Then, the profile is mapped with a CO setup as illustrated in Fig This ensures the correct identification of the different phases in the radargram. Conventional method first moveout (MO) measurement then constant-offset (CO) measurement 0 x 1 x opt x 1/c 0 1/v soil aw gw Drawbacks: t moveout constant offset Figure 4.20: Schematic traveltime diagram of a ground wave measurement consisting of a moveout measurement from x 1 to x opt followed by a constant offset measurement at x > x opt (aw: air wave, gw: ground wave). time consuming due to two measuring modes (MO and CO) velocity determination from MO sometimes difficult (heterogeneity) The approach introduced above has some basic disadvantages: lateral resolution limited by optimal T R-distance ˆ Measurements in two modes (MO or CMP and CO) have to be carried out EGU 2010 which J. Igel & require H. Preetz, a modification LIAG Hannover of the layout Small-scale and thus soil are moisture timedetermination consuming. with GPR 6 /12

17 Groundwave measurements: Optimised layout Result of FD-simulation and EGU 2010 J. Igel & H. Preetz, LIAG Hannover experiment Small-scale soil moisture determination with GPR 7 /12 Layout with 2 receivers Benefit of new layout only time differences have to be determined zero crossings can be picked instead of first arrivals easy data processing only constant-offset measuring mode is needed fast measuring progress small distance between both receivers possible high spatial resolution

18 Groundwave measurements: Optimised layout Result of FD-simulation and EGU 2010 J. Igel & H. Preetz, LIAG Hannover experiment Small-scale soil moisture determination with GPR 7 /12 Layout with 2 receivers Benefit of new layout only time differences have to be determined zero crossings can be picked instead of first arrivals easy data processing only constant-offset measuring mode is needed fast measuring progress small distance between both receivers possible high spatial resolution

19 Groundwave measurements: Optimised layout Result of FD-simulation and EGU 2010 J. Igel & H. Preetz, LIAG Hannover experiment Small-scale soil moisture determination with GPR 7 /12 Layout with 2 receivers Benefit of new layout only time differences have to be determined zero crossings can be picked instead of first arrivals easy data processing only constant-offset measuring mode is needed fast measuring progress small distance between both receivers possible high spatial resolution

20 Groundwave measurements: Optimised layout Result of FD-simulation and EGU 2010 J. Igel & H. Preetz, LIAG Hannover experiment Small-scale soil moisture determination with GPR 7 /12 Layout with 2 receivers Benefit of new layout only time differences have to be determined zero crossings can be picked instead of first arrivals easy data processing only constant-offset measuring mode is needed fast measuring progress small distance between both receivers possible high spatial resolution

21 Groundwave measurements: Optimised layout Layout with 2 receivers FD-simulation (CO) R1 R2 v = R1R2/ t ε r = c 2 0 /v 2 EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 7 /12

22 Groundwave measurements: Optimised layout FD-model: lateral resolution? Background: dry sand (ε r = 3) Anomalies: moist sand (ε r = 9, separation = 10 cm) FD-simulation: GPR analysis input model ε r [ ] model 1 receiver x [m] Result of FD-simulation and experiment Conventional layout lower spatial resolution bad fit of absolute values for small structures Optimised layout (2 receivers) high spatial resolution good fit of absolute values EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 7 /12

23 Groundwave measurements: Optimised layout FD-model: lateral resolution? Background: dry sand (ε r = 3) Anomalies: moist sand (ε r = 9, separation = 10 cm) FD-simulation: GPR analysis input model ε r [ ] model 1 receiver 2 receivers x [m] Result of FD-simulation and experiment Conventional layout lower spatial resolution bad fit of absolute values for small structures Optimised layout (2 receivers) high spatial resolution good fit of absolute values EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 7 /12

24 Groundwave measurements: Optimised layout Sandbox setup Background: dry sand (ε r = 3) Anomaly: moist sand (ε r = 5.8, width = 15 cm) Sandbox experiment: GPR analysis TDR data ε r [ ] in situ 1 receiver x [m] Result of FD-simulation and experiment Conventional layout lower spatial resolution bad fit of absolute values for small structures Optimised layout (2 receivers) high spatial resolution good fit of absolute values EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 7 /12

25 Groundwave measurements: Optimised layout Sandbox setup Background: dry sand (ε r = 3) Anomaly: moist sand (ε r = 5.8, width = 15 cm) Sandbox experiment: GPR analysis TDR data ε r [ ] in situ 1 receiver 2 receivers x [m] Result of FD-simulation and experiment Conventional layout lower spatial resolution bad fit of absolute values for small structures Optimised layout (2 receivers) high spatial resolution good fit of absolute values EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 7 /12

26 Moisture by groundwave in a sandy soil (grassland) Site 1 10 Θ V [%] 8 15 y [m] x [m] 0 EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 8 /12

27 Moisture by groundwave in a sandy soil (grassland) Site 1 Density function y [m] Θ V [%] p data fit Θ V [%] normal distr. Θ V = 9 ± 2% 94 Dielectric permittivity ɛ Variograms x direction y direction x [m] anisotropy: caused by former cultivation (grassland formerly used as tillage) 0 γ [ ] h [m] γ [ ] a max = 1.5 m, a min = 0.3 m h [m] Figure 4.30: Statistical analysis of the permittivity distribution determined with the ground wave at location 2. The directional variogram is calculated in x- and y-direction and an exponential model is fitted to the curves. The range of the fitted models is: ax = 1.0 m and ay = 0.3 m. EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 8 /12

28 Moisture by groundwave in a sandy soil (grassland) Site 1 Site 2 10 Θ V [%] 10 Θ V [%] y [m] y [m] x [m] x [m] 0 anisotropy: caused by former cultivation (grassland formerly used as tillage) isotropic pattern: area formerly not used as tillage natural variability, a = 0.35 m EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 8 /12

29 Reflection at soil surface Measuring layout Reflection at metal, soil and soil covered by grass EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 9 /12

30 Reflection at soil surface Measuring layout Reflection at metal, soil and soil 4.3 Determining the permittivity by the coefficient of reflection covered by grass 0 normalised amplitude [ ] metal soil soil + vegetation t [ns] 8 Figure 4.8: Radar trace of a 1 GHz horn antenna showing a wave re at a metal plate, a soil surface without vegetation and a soil surfac EGU 2010 J. Igel & H. Preetz, LIAGvegetation. Hannover The Small-scale amplitude soil moisture normalised determination to with thegpr maximum amplitude 9 /

31 Field measurement Reflection, site 1 Title Motivation GPR Moisture GPR Groundwave GPR Reflection Conclusion Groundwave, site Θ V [%] 15 y [m] x [m] 0 same pattern caused by cultivation small differences of absolute values and variability due to different sampling depth and lateral resolution ( 25 cm vs. 13 cm) EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 10 /12

32 Field measurement Reflection, site 1 Title Motivation GPR Moisture GPR Groundwave GPR Reflection Conclusion Groundwave, site 1 10 Θ V [%] 10 Θ V [%] y [m] y [m] x [m] x [m] 0 same pattern caused by cultivation small differences of absolute values and variability due to different sampling depth and lateral resolution ( 25 cm vs. 13 cm) EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 10 /12

33 Conclusion Title Motivation GPR Moisture GPR Groundwave GPR Reflection Conclusion GPR groundwave technique has been optimised regarding lateral resolution ( 10 cm) and measuring progress. Analysing reflections at the ground surface shows similar results. GPR can be used for fast, non-invasive, high-resolution soil-moisture mapping and provide important input for realistic numerical simulations. Field measuremets on different grassland-sites demonstrate that soil moisture shows high variability with correlation length of a few decimetres soil moisture distribution is influenced by the former cultivation and soil may preserve this effect for a longer time. EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 11 /12

34 Conclusion Title Motivation GPR Moisture GPR Groundwave GPR Reflection Conclusion GPR groundwave technique has been optimised regarding lateral resolution ( 10 cm) and measuring progress. Analysing reflections at the ground surface shows similar results. GPR can be used for fast, non-invasive, high-resolution soil-moisture mapping and provide important input for realistic numerical simulations. Field measuremets on different grassland-sites demonstrate that soil moisture shows high variability with correlation length of a few decimetres soil moisture distribution is influenced by the former cultivation and soil may preserve this effect for a longer time. EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 11 /12

35 Open questions and outlook Vegetation has an impact on measuring results. Depth of investigation of the groundwave is still an object of research z = fct (frequency, antenna separation, ε-distribution) inversion might provide information on the moisture distribution with depth and further enhance lateral resolution. EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 12 /12

36 Open questions and outlook Vegetation has an impact on measuring results. Depth of investigation of the groundwave is still an object of research z = fct (frequency, antenna separation, ε-distribution) inversion might provide information on the moisture distribution with depth and further enhance lateral resolution. EGU 2010 J. Igel & H. Preetz, LIAG Hannover Small-scale soil moisture determination with GPR 12 /12