a horizon depth as ecological indicator for grassland: mapping approach by using geophysical methods...
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A horizon depth as ecological indicator for grassland: Mapping approach by using geophysical methods
GRELLIER Séraphine(1), FLORSCH Nicolas(2), JANEAU Jean-Louis(1), LORENTZ Simon(3), PODWOJEWSKI Pascal(1)
References: Wilcox B.P. (2002). Shrub control and streamflow on rangelands: A process based viewpoint. Journal of Range Management 55: 31 8-326.Hibbard K.A., Archer S., Schimel D.S., Valentine D.W. (2001). Biogeochemical changes accompanying woody plant encroachment in a subtropical savanna. Ecology 82: 1999–2011.
1. INTRODUCTION
2. METHODSStudy area:Potshini catchment (near Bergville), representative of the KwaZulu-Natal Drakensberg foothills - 28 48' 37" S; 29 21' 19" E.
In the Slingram EM38 apparatus, one coil serves as a transmitter and produces an alternative magnetic field in the ground.
It induces an electric field
( electric field and magnetic induction). The later leads to a density currentwhere σ is the conductivity. These currents produce a secondary magnetic field which is measured by using the receiving coil. Hence the secondary field reflect the conductivity.
1 IRD c/o School of Bioresources Engineering and Environmental Hydrology (BEEH), Rabie Saunders Building, UKZN, Box X01, Scottsville, 3209, South Africa.2 UMMISCO/IRD, 32, avenue Henri Varagnat, 93143 Bondy Cedex, France; UPMC, Paris; Dept of Mathematics and Applied Mathematics, UCT, South Africa.3 School of Bioresources Engineering and Environmental Hydrology (BEEH), Rabie Saunders Building, UKZN, Box X01, Scottsville, 3209, South Africa.
Ecological and soil survey:Trees mapping and topography have been realized with DGPS (Leica).Grain size fractions (pipette method) were measured every 5cm until 65cm depth (always after reaching the B horizon).
3. RESULTS and DISCUSSION
4. CONCLUSION
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Interface depth
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Vertical Dipole Mode (grounded)
app.Horizontal Dipole Mode (grounded)
app.Vertical Dipole Mode(0.5 m above the ground)
app.
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Fielddata
Reconstructedsynthetic data
Invertedparameters
Inversion of EM38 data to retrieve the conductivities of A and B horizons and the interface depth
Figure 1
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Interface depth
Validation by using measurements on a gully face and close electrical sounding. Clay amount and electrical resistivity were both measured on an up-dated face.
App
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istiv
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Ohm
.m)
Dep
th (
m)
Rem ote effectof the gu lly face
Synthetic soundingderived from the resistiv ity log
R esistiv ity log m easured onthe gully face
In terpreta tion of the near gu lly sounding. Experim ental Sclum berger
sounding 4 m eters apartfrom the gully
Clay amount (%)
Lower lim it ofthe A horizon
S im ulation ofthe gu lly face
The deeper layer is fic tive and used totake into account the gu lly face 4 m apart to m odel the deep apparent resistiv ity
0.1 1.0 10 10 100 1000True resistiv ity (O hm .m )Spacing (m )
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Electrical sounding
Resistivity log
Bayesian method:The Bayesian inverse computation is based on:
While data are Gaussian-like, one makes use of:
Relative sensibility of the 3 modesas a function of depth (layered medium)Red: VDMBlue: HDM (normalized)Brown: VDM at 50 cm height (normalized)
Slingram method:
The ground integrating probe response depends whether the dipoles are handled vertically (VDM) or horizontally (HDM): this provide two independent measurements. A third one is obtained by hanging the device 50cm above the ground. But using three three measures, one can retrieve the three parameters of a two layer shallow sub-surface: the two conductivities and the depth of the interface.
Principle of the EM38
EM38 survey, vertical position on the ground (VDM).
a priori in form ationon data
a priori in form ationon param eters
a posteriori informationon parameters
physica l law betw een dataand paream eters [d=G (m )]
from this pdf, computation of marginal pdf, means and moments for all parameters(conductivity of the two layersand the first layer thickness)
Figure 2
control name EM38 Sounding
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point 2 31 42point 3 49 43
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Depth (cm) of transition of A and B horizons on 4 control points.All controls are well validated except point 2, which can be explained by a non-two layers structure at this point: heterogeneity of the grassland appears here.
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tivity
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Dep
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R em ote e ffecto f the gu lly face
Synthetic soundingderived from the resis tiv ity log
R esis tiv ity log m easured onthe gu lly face
In terpretation of the near gully sounding. Experim enta l Sc lum berger
sounding 4 m eters apartfrom the gu lly
Clay am ount (% )
Lower lim it ofthe A horizon
S im ula tion o fthe gu lly face
The deeper layer is fictive and used totake in to account the gu lly face 4 m apart to m odel the deep apparent resistiv ity
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True res istiv ity (O hm .m )S pacing (m )
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Resistivity log
Validation by using m easurem ents on a gully faceand close electrical sounding. Clay am ount and electrical resistivity were both m easured on an up-dated face.
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Fielddata
Invertedparam eters
Inversion of EM38 data to retrieve the conductivities of A and B horizons and the interface depth
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Trees location (dot) and topography (color)
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Trees density per onehundred square meters
num ber/are
Left figure: corre lations betw een trees density and conductiv ity. Topography (right) and density of trees ( center). O nly on the ha lf upper right part the trees have grow n.
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seedlings
big trees
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Figure 1
Figure 2
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H igh conductiv ity(m ore c layey) ==>low tree dens ity
Low conductiv ity(less c layey) ==>high tree dens ity
2
There is no h igh tree densityw here h igh c lay am ount occurs
Very low conductivity ==> low tree density
There is no high tree density where high and very low clay amount occurs
a priori in form ationon data
a priori in form ationon param eters
a posteriori informationon parameters
physical law between dataand paream eters [d=G (m )]
from this pdf, computation of marginal pdf, means and moments for all parameters(conductivity of the two layersand the first layer thickness)
The expanding grasslands in Southern Africa and all around contribute to agro-pastoral activities and to the evolution of the ecological quality of soils. In this context, grasslands are sometimes invaded by trees, which have eventually a strong impact on the ecosystemss (Hibbard et.al., 2001, Wilcox et al. 2002). The mapping of the A horizon, which is involved in the resistance to erosion, could be a relevant indicator of the determinisms and interactions contributing to asses soil quality and soil evolution in the landscape.
Objectives here: to understand the potential involving role of the A and B horizons in the presence of invading trees (Acacia Sieberiana) by using geophysical methods (Slingram).
The transition between the A and the B horizon (depth < 0.6 m) appears stiff in term of conductivity contrast (A being rather resistive while B is clayey and conductive). As a very robust inversion procedure, the Bayesian approach is efficient to map the A horizon thickness and both layer conductivities, and reveals large clay variations in the B horizon.