time-lapse geophysics

’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

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Fine grained sands

Lowhigh

Depth (cm)

Indi

vidu

al %

reta

ined

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101520253035

Medium grained sands

Lowhigh

Depth (cm)

Indi

vidu

al %

reta

ined

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00.10.20.30.40.50.6

Coarse grained sands

Lowhigh

Depth (cm)

Indi

vidu

al %

reta

ined

0-20

20-40

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00.5

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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

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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

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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

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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.