11 - 7
GDS
HEGGY ET AL.: MARTIAN GEOELECTRICAL MODELS
Table 3. Geoelectrical Model for the Fluidized Crater Ejecta Depositsa
e0
e00
s 10À6, S/m
m
2 MHz
20 MHz
2 MHz
20 MHz
2 MHz
20 MHz
2 MHz
20 MHz
Dust layer
3
2.7
0.25
0.22
28
240
1.5
1.1
Ejecta deposit
4
3.7
0.5
0.45
14
500
1
1
Regolith
14
12
0.7
0.6
77
670
1.2
1
Eroded basalt
9
8.1
0.2
0.2
22
220
1
1
Wet basalt
36
32
12
10.5
1344
11724
1
1
Basalt + ground ice
11
10
1
0.7
224
780
1
1
a
Similar material can have different dielectric properties as the geophysical conditions ( porosity, temperature, grain size) in each layer are different.
case the important reflection that occurs between the ground
hardly identify the location of each interface. Only the
ice and the water-saturated layer, due to the strong dielectric
basalt basement can be distinguished. Such a geological
contrast between the two materials. This is also visible in
model, even if it does not contain a water-saturated layer,
the corresponding reflection of the electric field plot, and
suggests that low losses due to the low permittivity of the
corresponds to an attenuation of À50 dB that is in the
first subsurface layers could reflect the presence of carbo-
detection range of the Netlander GPR and the MARSIS
nated material (in a geological context presenting adequate
instrument too.
evidences of past hydrological sedimentary processes).
[24] For the outwash plains shown in the right upper part
of Figure 5 (denoted by 2.2), we have a different situation
5. Discussion
where the interface between the dry sediments and the
[27] Numerical simulation of a 2 MHz electromagnetic
compact lava can be identified on the attenuation and
wave propagating in the described geological models shows
electric field graphs, while even in the presence of a high
the variation in the radar ability to detect and distinguish the
dielectric contrast, it is difficult to distinguish between the
presence of a water saturated layer in terrains where we
compact lava layer and the water saturated basalt, because
expect discontinuities in the ground ice thermal properties
of the low dynamic at this depth (Figure 5, top right). An
attenuation of À90 dB is still in the range of the Netlander
that might lead to the presence of liquid water in the first
few hundred of meters of the Martian subsurface. We can
GPR, but is behind the detectability limit for the MARSIS
mainly distinguish three cases:
orbital experiment. After the water saturated layer, we can
[28] The first case corresponds to a volcanic context in
observe the decay in the radar signal as the wave travels in
which the radar pulse penetrates down to the water saturated
the ground ice and reaches a low dynamic point which is
layer, but due to the near subsurface stratigraphy, we cannot
below the instrument limit.
[25] Results for the ejecta deposits site are presented in
distinguish reflection on different geological interfaces and
the bottom left part of Figure 5 (denoted by 2.3). The peaks
the one arising from the water-ground ice interface. This is
on the attenuation (in dB) curve identify each geological
the case of the radar echo simulation representing the ejecta
interface. The thin water saturated layer does not show a
deposits site. This is due to the presence of an important
strong signal as in the shallow aquifer case (Figure 5, top
dielectric contrast between the other dry volcanic layers,
left), although being around the À60 dB level. This is due to
which contains different amounts of iron oxides under
a larger number of upper geological layers, causing strong
different compaction levels (decreasing the porosity
multiple reflections, and then leaving less energy available
increases the dielectric constant and the conductivity).
[29] The second case is represented by the shallow
at level of the water saturated layer. An important fact to be
aquifer associated with local geothermalism and the out-
noted from these three previous simulations is the presence
wash plains. We observe here an exponential attenuation of
of the broadened region in the attenuation curves, which
the radar wave when propagating into the first subsurface
characterize the presence of water.
[26] The last case presented in the bottom right part of
layers, without any sharp reflection at the interfaces of the
Figure 5 (denoted by 2.4) corresponds to the layered
geological layers since they show low dielectric contrast. A
deposits terrain. This case presents geoelectrical properties
stronger reflection can then be observed on the water-rich
very favorable for radar penetration, since materials con-
layer, producing a broadened region in the attenuation
stituting the first three layers contain no iron oxides. We
curves. It constitutes a kind of ideal case to detect and
have then there low dielectric losses and no magnetic losses.
probably identify subsurface water, if the signal is not too
The first three layers are thin compared to the wavelength,
attenuated by the first geological layers, as it is the case for
and as they present no important dielectric contrast, we can
outwash plains.
Table 4. Geoelectrical Model for the Layered Deposits Terraina
e0
e00
s 10À6, S/m
m
2 MHz
20 MHz
2 MHz
20 MHz
2 MHz
20 MHz
2 MHz
20 MHz
$1.1
Dry mudstone
3.2
2.8
0.2
0.1
24
120
1.2
Gypsum
4
3.8
0.6
0.5
67
560
1
1
Carbonates (aragonite)
6
5.7
0.1
0.1
12
110
1
1
Basalt basement
8
7.6
0.5
0.4
56
450
1
1
a
Similar material can have different dielectric properties as the geophysical conditions ( porosity, temperature, grain size) in each layer are different.