time-lapse geophysics
TRANSCRIPT
’Tunde Adekoya
Supervisors:Assc./Prof. Jeffrey ShraggeAsst./Prof. Matthias LeopoldAsst./Prof. Gavan McGrath
Time-lapse Geophysical Monitoring of the Subsurface Hydrology at
Kings Park, South Western Australia
Introductory Statement
This research work was prompted by reported cases of decline in some native vegetation in the Kings Park remnant bushland.
Some researchers believe the limiting factor responsible for the decline is water, yet none had applied geophysics to understand the availability and variations in water at the Park.
Deceased Banksia tree (Proteaceae)
Aims
Provide improved understanding of the hydrology of Kings Park
- assist management in making long term decisions on sustainability
Delineate water table and underground water sources
- understand where and how vegetation could access water
Deceased Banksia tree (Proteaceae)
Study area
Study area within Kings Park showing the transects at Low and High sites (Google Earth)
Regional Geology
Generalised map of Perth Basin (after Crostella and Backhouse (2000): In Leyland 2011)
Geophysical methods:
-Time-lapse electrical resistivity tomography (TL ERT)
-Time-lapse ground penetrating radar (TL GPR)
Hydrological tests:
- Grainsize analysis
- Soil water retention test and Archie’s relations
- Soil moisture content
Methodology
Styles, 2012
ERT measurements
Loke, 2004
Assoc. of Central Oklahoma Government
Multi-channel EarthImager resistivity
meter
GPR
Hydrological tests were carried out to better understand the subsurface physical properties of the soils in the study area and also to serve as ‘ground truth’ for the surface geophysics.
Hydrological tests
Results and discussion
0-2020-40
40-6060-80
80-100
100-120
120-140
140-160
160-180
180-200
200-220
220-240
240-260
260-280
280-3000
20
40
60
80
Fine grained sands
Lowhigh
Depth (cm)
Indi
vidu
al %
reta
ined
0-20
20-40
40-60
60-80
80-100
100-
120
120-
140
140-
160
160-
180
180-
200
200-
220
220-
240
240-
260
260-
280
280-
300
05
101520253035
Medium grained sands
Lowhigh
Depth (cm)
Indi
vidu
al %
reta
ined
0-20
20-40
40-60
60-80
80-100
100-
120
120-
140
140-
160
160-
180
180-
200
200-
220
220-
240
240-
260
260-
280
280-
300
00.10.20.30.40.50.6
Coarse grained sands
Lowhigh
Depth (cm)
Indi
vidu
al %
reta
ined
0-20
20-40
40-60
60-80
80-100
100-
120
120-
140
140-
160
160-
180
180-
200
200-
220
220-
240
240-
260
260-
280
280-
300
00.5
11.5
22.5
3
Silts/Clays
Lowhigh
Depth (cm)
Indi
vidu
al %
reta
ined
Grainsize distributions for Low and High sites
Results and discussion
1 10 1000.1
1
10
100
1000
10000
Retention curve
replica 1replica 2replica 3
Volumetric water content, VWC (%)
Pres
sure
, kPa
0 100 200 300 400 500 600 700 80005
1015202530354045
f(x) = 365.201466556238 x -̂0.688753759027738R² = 0.837006589954422
Archie's Relations
20-40cm280-300cm
Apparent Resistivity (Ωm)
Volu
met
ric
wat
er c
onte
nt (%
)
3 5 7 9 11 13 150
50
100
150
200
250
300
350
400
450
Moisture variation with depth
Low siteHigh sitePR2_LowPR2_High
Volumetric moisture content (%)
Dept
h (c
m)
Relationship between soil moisture content and resistivity
Results and discussionWater table
Results and discussion
TL ERT Section (May to June)
TL ERT Section (June to July)
TL ERT Section (July to August)
13/04-12/05
13/05-12/06
13/06-12/07
13/07-12/08
13/08-12/09
0
20
40
60
80
100
120
140
160 Rainfall pattern during ERT
Rainfall Period
Amou
nt o
f Rai
nfal
l (m
m)
Red colour scale indicates high resistivity (low water content)
Blue colour scale indicates low resistivity (high water content)
Results and discussion
Vadose zone moisture level in July(Low site)
Vadose zone moisture level in August (Low site)
Vadose zone moisture level in June (Low site)
Vadose zone moisture level in May (Low site)
Water content reflections
01/05-31/05
01/06-30/06
01/07-31/07
01/08-31/08
020406080
100120140160180 Rainfall Pattern during GPR
Rainfall Period
Amou
nt o
f Rai
nfal
l(mm
)
Soil water and declining tree species
During dry summer months deep rooted Banksia (Proteaceae) trees rely on groundwater
A greater proportion of mortality of Banksia trees was observed in the high site where soil water availability is low and trees are unlikely to be accessing the water table
Trees growing in the low site may have access to the water table and therefore can maintain physiological functioning through drought periods
With predictions of further rainfall declines in SW Australia (IPCC 2013) Banksia tree mortality will likely increase in the future
The TL ERT reveals monthly spatial variations in moisture content in the studied sites
The TL GPR was successfully used to monitor variations in vadose zone water content
Geophysical investigations indicated that the seasonal wetting front propagates to at least 10 m below the surface
The hydrological tests indicated the properties of the subsurface lithologies and confirmed the responses of the resistivity measurements
Soils at both sites are not significantly different (mainly sands) with low water retention capacity
Water retention capacity appears to increase with depth from about 3.5 m due to increase in silt/clay content
Water is likely the main limiting factor responsible for the decline in Banksia trees
Conclusions
THANK YOU
References
Crostella, A. and Backhouse, J. (2000), Geology and petroleum exploration of the central and southern Perth Basin. Geological Survey of Western Australia Report, 57. Davidson, W. A. (1995), Hydrogeology and groundwater resources of the Perth Region, Western Australia: Western Australia. Western Australia Geological Survey Bulletin 142.
IPCC (2013) Climate change 2013: the physical science basis. In: Stocker TF, Qin D, Plattner G, Tignor MMB, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM (eds) Working group 1 contribution to the fifth assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge and New York, pp 1-1535
Leyland, L. (2011), Hydrogeology of Leederville Aquifer, Central Perth Basin, Western Australia.PhD Thesis, University of Western Australia. Loke, M. H. (2004), Tutorial: 2-D and 3-D electrical imaging surveys. McPherson, A. and Jones, A. (2004), Appendix D: Perth basin geology review and site class assessment. Geoscience Australia. Reid, L. B., Bloomfield, G., Ricard, L. P., Botman, C. and Wilkes, P. (2012), Shallow geothermal regime in the Perth Metropolitan Area. Australian Journal of Earth Sciences 59, 1033-1048. Turner, S., Bean, L. B., Dettman, M., McKeller, J. L.,McLoughlin, S. and Thulborn T. (2010), Australian Jurassic sedimentary and fossil successions: current work and future prospects for marine and non-marine correlation. GFF: Journal of the Geological Society of Sweden 131 (1), 49–70. Truss, S., Grasmueck, M., Vega, S. and Viggiano, D. A. (2007), Imaging rainfall drainage within the Miami oolitic limestone using high-resolution time-lapse ground-penetrating radar, Water Resour. Res., 43.