The document discusses using ground penetrating radar (GPR) to map subsurface features that influence soil drainage in two research plots. GPR was used to collect radar data along transects in the plots, which was processed to produce depth-slice images showing reflector intensities at specified depths. Soil cores and augering provided ground-truthing. Depth slices revealed features like an infilled river channel and undulating sand and silt layers. GPR allowed rapid determination of depth to gravel and potentially the groundwater table. The next steps are to produce depth-to-gravel maps and assess if GPR can differentiate soil moisture conditions at different times. In summary, GPR provides a means to measure soil drainage parameters that can help improve irrigation practices.

1. Angela L Lane 1*, Paul G Peterson 2, Carolyn B Hedley 2, Samuel T McColl 1, Ian C Fuller 1
1 Institute of Agriculture and Environment, Massey University, Palmerston North 4442, New Zealand.
2 Landcare Research Limited, Palmerston North 4442, New Zealand
GPR & GPS
• During September 2015 a Sensors & Software GPR with 200-MHz antennae was
used to collect radargrams transects in the two 0.4 ha plots. The radar transects
were spaced to achieve a 2×2 m grid pattern, and were spatially and
topographically corrected by measuring grid coordinates at 10-m intervals using a
Trimble® R8 RTK-dGPS. 100-MHz antennae were used to collect two radargrams
in the pasture plot, to penetrate to the inferred depth-to-groundwater.
POST PROCESSING
• Processing (rubber-banding & velocity correction) of radargrams was conducted
using the EKKO View Deluxe, GFP Edit 4 and EKKO Project 3 software package.
The arable and pasture plot grids were used produce depth-slice images, which
show radar-reflector intensities at specified depths below the grid surface.
SOIL CORES, AUGERING & PIEZOMETER MEASUREMENTS
• Soil cores and augering provided ground-truthing of the radargrams. The
locations for these were chosen after a preliminary review of the radargrams to
target features of interest. Soil cores were collected using a Giddings rig corer,
diameter 67 mm. A hydraulic auger was deployed as several locations across both
plots to determine depth to gravel. Piezometer measurements were taken at the
pasture plot to give an indication of depth to groundwater.
• Soil cores were analysed for bulk density, soil moisture and particle size.
Predicting soil drainage is vital if new precision irrigation technologies are to be utilised
effectively. Predicting drainage requires knowledge of the spatially varying subsurface features
of the soil, e.g. soil-thickness, flow pathways, depth to gravel and depth to groundwater table.
The ability to map these features rapidly and non-invasively would make soil drainage
assessments more affordable and less damaging to the soil itself. One approach to this could
be the use of geophysical techniques, such as ground penetrating radar (GPR) 1,2,3., but their
usefulness in these applications has not been fully explored.
THE AIM OF THIS RESEARCH is to assess the applicability of GPR for identifying subsurface
features that are relevant for controlling soil drainage in alluvial soils.
ACKNOWLEDGEMENTS John Dando, Landcare Research for collecting the soil cores and augering; Eric Breard , Massey University for assistance with laser particle analysis; Brian Aspin Scholarship, George Mason Sustainable Land Use Scholarship, Colin Holmes Dairy Scholarship & Horizons Advanced
Sustainable Land Use Scholarship for assistance with funding Angela Lane’s Masters study.
Ground penetrating radar has proven to be useful for mapping some sub-surface features in recent alluvium. For example, it has allowed rapid determination of the depth to gravels below fine-
grained alluvium, and potentially can image the groundwater table. The use of depth-slices helped with the identification and mapping of sub-surface structures, such as infilled channels, which
can help with interpreting high-resolution topographic data such as that derived from LiDAR, as well as identifying features not expressed at the surface. The next step in this project is to use the
radar depth slices to produce a contour map of depth-to-gravels for each plot. Beyond that, we intend to compare radargrams between the data collected in September 2015 to subsequent surveys
proposed for March 2016 when soils are expected to be drier. This will allow us to assess whether GPR can also be used to differentiate soil moisture conditions. In summary, GPR can provide a
means for measuring soil drainage parameters, which ultimately may assist with improvements to irrigation practice.
• Two 0.4 ha (40 × 100 m) research plots: Plot 1 – pasture & Plot 2 - arable
• Located on terraces formed by the Manawatu River, in Palmerston North
• Soil types - Plot 1: Rangitikei silt loam over sand & Manawatu fine sandy loam; Plot 2:
Manawatu silt loam over sand & Manawatu sandy loam
Above left: Depth Slice at 0.78 – 0.91 m depth below ground showing a sharp diagonal transition at 30
– 50 m across the plot. This strong boundary is interpreted to be a former river channel bank.
Above right: Radargram section (from red line on the depth slice on left), showing the same sharp
transition (at 45 m), and suggesting there is an infilled-channel on the right. River gravels are
interpreted to occur at ~0.8 m below ground on the left, to ~2 m below ground in the former channel.
Above left: Depth Slice at 0.52 – 0.65 m depth below ground, with strong patchy reflectors across the
plot at this depth suggesting undulating layers of material, confirmed by coring to be sands and silts.
Above right: Radargram section (from red line on the depth slice on left), showing the strong
undulating reflectors indicated in the depth slice. The depth to gravels is interpreted to be the
base of the strong lower reflectors at about 1.5-2.0 metres below ground.
REFERENCES : 1. Adamchuk, V.I., et al., On-the-go soil sensors for precision agriculture. Computers and Electronics in Agriculture, 2004. 44(1): p. 71-91. ; 2. Viscarra Rossel, R.A., A.B. McBratney, and B. Minasny, eds. Proximal soil sensing. Progress in soil science. 2010, Springer: Dordrecht London. xxiv, 446.; 3.
Davis, J.L. and A.P. Annan, Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophysical Prospecting, 1989. 37(5): p. 531-551.
Left: GPR set up on a PVC
trolley;
Right: collecting soil cores
with a Giddings rig corer
ARABLE RADARGRAMS (200 MHz)
RESULTS
PASTURE RADARGRAMS (200 MHz)
LIDAR DATASOIL TYPES
METHODS
CONCLUSION
PREDICTING SOIL DRAINAGE
STUDY SITE
The previously mapped soil types (above left) correlate well with the topography seen in the 1-m LiDAR-
derived Digital Terrain Model (coloured areas) and hillshade (grey areas) in the map on the right. These soil
and topographic boundaries match well with the radar data and coring.
PASTURE RADARGRAMS (100 MHz)
Pasture plot radargram section at
approximately same location as section
shown for 200 MHz antennae. The radar
penetrates further into the gravels, and
appears to attenuate at the water table
(also identified at that depth with the
piezometer in the plot).
Pasture Plot Arable Plot
Using Ground Penetrating Radar to map soil drainage
patterns to improve irrigation efficiency
*Email: angelalane.nz@gmail.com