Joint use of GPR and ERI to image the subsoil structure in a Sandy costal environment Revista- Journal of Coastal Research

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Journal of Coastal Research, Special Issue 56, 2009
Journal of Coastal Research SI 56 956 - 960 ICS2009 (Proceedings) Portugal ISSN 0749-0258
Joint Use of GPR and ERI to Image the Subsoil Structure in a Sandy
Coastal Environment
D. Gomez-Ortiz
†,
M. Pereira
‡,
T. Martin-Crespo
†
, F.I. Rial
‡,
A. Novo, H.
Lorenzo
‡
and J.R. Vidal
∞
†Area de Geologia
University Rey Juan
Carlos, Mostoles
28933, SPAIN
david.gomez@urjc.es
‡ EUET Forestal
University of Vigo, Pontevedra
36005, SPAIN
firv@uvigo.es
∞ Instituto Universitario de
Xeoloxía
University of A Coruña, A Coruña
15071, SPAIN
xemoncho@udc.es
ABSTRACT
G
OMEZ
-O
RTIZ
, D., P
EREIRA
, M., M
ARTIN
-C
RESPO
, T., R
IAL
, F.I., N
OVO
, A., L
ORENZO
, H. and V
IDAL
, J.R., 2009.
Joint use of GPR and ERI to image the subsoil structure in a sandy coastal environment. Journal of Coastal
Research, SI 56 (Proceedings of the 10th International Coastal Symposium), 956 – 960. Lisbon, Portugal, ISSN
0749-0258.
In the present work we show a study carried out in a coastal sandy environment in NW Spain with the main aim
of imaging the shallow subsoil structure. The goals of the study included the detection of the water table and
bedrock, but also a description of the aeolian dunes and progradating sand layers. With these objectives two
independent but complementary geophysical techniques were jointly used: ground-penetrating radar (GPR) and
electrical resistivity imaging (ERI). Regarding GPR, shielded antennas of 200 & 250 MHz from two different
manufacturers were used in the study; it also allowed us to make a comparative study between them. To obtain a
more detailed image of the sand layers structure it was also used a shielded 500 MHz antenna. 100 & 200 MHz
unshielded antennas were also used to apply the CMP method in order to obtain a precise estimation of the
velocity of the electromagnetic waves into the subsoil. ERI was carried out in order to obtain independent
estimations of both water table and bedrock depths, as well as the resistivity values of the groundwater water that
are useful to determine the occurrence -or not- of fresh water on marine water. Although the resolution of ERI is
low comparing with GPR, the penetration depth is greater. The results show that the bedrock is clearly identified
in ERI profiles at about 8-9 m depth, whereas the water table is located at 5-6 m depth in good agreement with
the GPR profiles. Resistivity values in the most external ERI profiles show that the water table is constituted by
fresh water. In addition to this, the high resolution of both 200-250 and 500 Mhz antennae has allowed to obtain
a clear image of the internal structure of the sand deposits. This information has been used to infer the dynamic
of progradation of the coastal system.
ADITIONAL INDEX WORDS: bedrock, water table, progradation
INTRODUCTION
In this work a study of the coastal sandy environment of the port
of Bares in the north of Galicia (Spain) is presented. Coastal
environments are the ones of the most active and sensitive
sedimentary depositional mediums. They typically consist of
different dune systems prograding over a coastal plain.
Characteristics of dune systems are strongly conditioned by the
sedimentary dynamics and this is mostly determined by
characteristics of the wind field and sediment properties
(Hernandez et al, 2002). For this reason it is necessary to know the
internal structure of dunes, in order to establish an evolution
model, to approach the future of the dune system. The GPR
technique provides a unique insight into the internal structure of
dunes which is not achieved by any other non-destructive
geophysical technique. It has been used to examine the internal
structure of aeolian sedimentary deposits such as ancient sand
dunes (Harari, 1996) and more recently, Holocene dunes and
dunefields (Bristow et al, 2000 and 2005; Bristow and Pucillo,
2006; Pedersen and Clemmensen, 2005; Costas et al., 2006,
among others). The main objective of the survey is to image the
shallow subsoil structure of the area using two different
geophysical techniques: ground-penetrating radar (GPR) and
electrical resistivity imaging (ERI). Combining different
geophysical techniques, the main characteristics of the coastal
system (thickness of the sand deposit, internal dune structure,
depth to water table) has been determined and the direction of
progradation has been defined.
METHODOLOGY
The Ground Penetrating Radar (GPR) is a close-range remote
sensing technique with radar based on the emission/reception of
short electromagnetic impulses (1-20ns) in the VHF-UHF
frequency range. It has evolved in the last two decades from a
specific geophysical technique to a survey tool with a lot of
applications (civil engineering, forensic studies, archaeology, etc).
GPR equipment usually consists of a laptop, a central unit and a
pair of antennas. The transmitter antenna emits the
electromagnetic pulses into the ground and the receiver antenna
receives the portion of the signal energy that is reflected due to the
electromagnetic properties changes of the crossed materials. The
greater the contrast is, the stronger the reflections will be. In this
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Journal of Coastal Research, Special Issue 56, 2009
Joint use of GPR and ERI to image the subsoil structure
way, a space-time record (radargram) is generated with all these
reflections by moving the antennas over the surface. To perform a
suitable data acquisition it is essential to select the appropriate
radar antennas for each particular survey depending on the target
depth and the required resolution. Commercially available GPR
antennas typically range from 10 MHz to 4 GHz. In general, as the
frequency of the antenna increases so does the resolution, but the
penetration capacity of the signal decreases (Annan 2003). The
combination of GPR with other geophysical survey techniques
such as ERI, provides a powerful tool for imaging the subsoil
structure.
ERI is a geophysical prospecting technique designed for the
investigation of areas of complex geology; this involves
measuring a series of resistivity profiles, using a computer to
control measurements between selected sets of an electrode array.
Since increasing separation between electrodes provides
information from increasingly greater depths, the measured
apparent resistivity values can be processed to provide an image of
true resistivity against depth.
The principal applications of this technique include (e.g Telford
et al., 1990) definition of aquifer boundary units, such as
aquitards, bedrock, faults and fractures, the detection of voids in
karstic regions, the mapping of saltwater intrusions into coastal
aquifers, the identification of contaminated groundwater plumes,
the detection of mineralized zones, and the exploration of sand
and gravel resources among others.
Several standard electrode arrays are available, with different
horizontal and vertical resolution, penetration depth and signal-to-
noise ratio. The Wenner array is usually applied for a good
vertical resolution, but may also provide a reasonable horizontal
resolution (e.g. Sasaki, 1992). This method has greater signal-to-
noise ratio than the others because the potential electrodes are
placed between the two current electrodes. Thus, the Wenner array
has been used here in order to compare the results with those from
GPR.
RESULTS
A total of 52 measurements have been recorded in the area
under study using both techniques, GPR and ERI. The final survey
distance was about 3000 meters. In Figure 1, we can see the GPS
positioning of all the selected profiles. Regarding GPR data, two
different systems synchronized with a GPS navigator, were used.
This has allowed us to make a comparative study between the
results obtained with each system. Most of the measurements (32
radargrams, 1950 meters) have been carried out with the RAMAC
equipment of MalaGeoScience. This general purpose system is
versatile, modular and user configurable for single or multi-
channel operation. The radargrams have been processed with the
same filter configuration (dewow, bandpass, gain function and
trace average removal) in order to enhance the desired targets and
to reduce clutter. As we can see in Figure 1, all the profiles (25
profiles) were made with the 250MHz shielded antenna and 4 of
these profiles were recorded again with the 500MHz antenna to
obtain a more detailed image of the sand layers structure. An
additional study to apply the Common mid-point (CMP) method
were performed with the 100MHz (2 profiles) and 200MHz (1
profile) unshielded antennas. This radar data processing permits to
extract a precise estimation of the velocity of the electromagnetic
waves into the subsoil, using a move out correction where the
constant velocity can be applied (Daniels, 2004). The most
interesting profiles (14 profiles, 699 meters) were recorded again
with the SIR-3000 from Geophysical Survey Systems with the 200
MHz shielded antenna. This is another one of the best general
purpose GPR systems that can be found on the market. Unlike the
RAMAC, the GSSI antennas are directly connected to a special
device that operates as control unit and display. Regarding ERI
data, 6 profiles have been made covering a total distance of 345
meters of study. A Syscal Kid Switch 24 of Iris Instruments was
employed. This tomography electric system is reduced in size and
weight and very versatile for shallow prospecting (until 8m
depth).The situation of these profiles also coincides with some of
those made with the GPR systems as we can see in Figure 1.
These most representative profiles where data were acquired with
different sources permit us to be more accurate in the analysis
process, facilitating the interpretation of the results to be able to
compare different information from the same target.
Velocity estimations
Common mid-point (CMP) soundings were used to measure
velocity and hence translate regular GPR time observations into
reliable depth estimates. The results of the two studies made with
the 100 MHz and 200 MHz unshielded antennas were satisfactory
and show similar conclusions using semblance analysis. In Figure
2, the example with the 200 MHz antenna is displayed. The cross
section on the left shows the semblance analysis for the data on
the right. Significant peaks occur at different travel times between
30 and 90 ns. RMS velocities are picked to be between 0.105 and
0.12 (m/ns) reason why a velocity of 0.11(m/ns) was selected for
Figure 1. Location of the GPR and ERI profiles carried out at the study area.
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Journal of Coastal Research, Special Issue 56, 2009
Gomez-Ortiz et al.
depth estimation.
GPR Results
Figure 3 shows the radargrams obtained using both Mala-
Geoscience 500 Mhz antenna and GSSI 200 Mhz one (R1 in
Figure 1). The profiles have a nearly N-S orientation and they
show strong reflectors in the first 3 m depth that image the internal
structure of the dune system. They consist of dipping reflectors
exhibiting truncations and/or interruptions allowing us to define
several radar boundaries and facies. The 200 Mhz antenna profile
as been chosen to interpret the internal geometry of the different
dune bodies identified and it will be described in more detail in
next section. In addition, a clear reflector located at about 3 m
mean depth and slightly dipping towards the beginning of the
profiles (towards the N) has been identified as the location of the
water table. A strong attenuation of the signal is produced under
this reflector. A new discontinuous sub horizontal reflection,
located at 8 m mean depth, would correspond to the location of
the bedrock, that in the study area corresponds to granitic rocks.
Figure 4 shows again the comparison of two radargrams
obtained using both Mala-Geoscience 500 Mhz antenna and
GSSI 200 Mhz one (R2 in Figure 1). In this case, the profiles
have a W-E orientation and they mainly show two strong
reflections. The first one, very continuous and slightly convex
upwards, is located at 3 m mean depth and it would correspond to
the location of the water table. The deeper one, located at about
6-7 m depth, is sub horizontal and slightly more discontinuous,
and would correspond to the location of the bedrock. A strong
attenuation of the signal is observed under this depth.
Figure 5A shows the first 70 m of the Mala-Geoscience 250
Mhz antenna radargram obtained along the beach following a
SW-NE orientation (R3 in Figure 1). The profile is defined by
several continuous sub horizontal reflections exhibiting a strong
attenuation under a depth of about 2.5 m. No dipping reflections
or structures have been observed. The strong attenuation
corresponds to the presence of saline water that would be located
at 2-3 m depth.
ERI Results
Figure 5B shows the resistivity section obtained along the beach
following the same orientation that the radargram of Figure 5A
(E3 in Figure 1). Three sub horizontal layers with different
resistivity values can be observed. The shallower one extends
from surface up to 2-3 m depth and the resistivity values are
higher than 20-30 ohm·m The intermediate one extends form 2-3
m up to 8-9 m depth and is characterised by resistivity values
lower than 0.5 ohm·m. The deeper one extends from 8-9 m and
Figure 2. Common mid-point (CMP) study. a) Radargram with the
200Mhz unshielded antenna. b) Semblance analysis results.
Figure 3. Comparison of GPR profiles (R1) carried out at with 200 Mhz GSSI equipment (a) and 500 Mhz Mala-Geoscience (b).The
internal structure of the dune bodies and the location of the water table can be observed.
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Journal of Coastal Research, Special Issue 56, 2009
Joint use of GPR and ERI to image the subsoil structure
Figure 4. Comparison of GPR profiles (R2) carried out at with 200 Mhz GSSI equipment (a) and 250 Mhz Mala-Geoscience (b).Two
strong reflectors define both the location of the water table and the bedrock. The uppermost part of the profiles also images the internal
structure of the dunes.
Figure 5. Comparison of GPR profile (R3) carried out at with 250 Mhz Mala-Geoscience equipment (a) and resistivity profile E3.
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Journal of Coastal Research, Special Issue 56, 2009
Gomez-Ortiz et al.
the resistivity values are 5-10 ohm·m. Taking into account the
resistivity values, the first layer corresponds to non-saturated
sandy materials, the intermediate one to salt water saturated sandy
materials, and the third one to the granitic bed rock.
Figure 6 corresponds to the resistivity section that coincides
with the radargrams of figure 3 (E1 in Figure 1). Two different
sub horizontal layers can be distinguished: the first one extends
from surface up to 3-7 m depth, being deeper at the beginning of
the profile and shallower at the end. The resistivity values are
higher than 1000 ohm·m with a small area of lower resistivity
values (about 200 ohm·m) located at 20 m in the horizontal
distance. Below this layer, a second one with lower resistivity
values (< 20 ohm·m) extends up to the deepest part of the profile.
The resistivity of the shallower layer typically corresponds to dry
sands whereas the resistivity values of the lower one are in good
agreement with fresh water saturated sands. Thus, the boundary
between both layers has been interpreted as the location of the
water table.
DISCUSSION AND CONCLUSIONS
GPR and ERI results have provided useful information about
the internal structure of the dune system, the depth to the water
table and bedrock, and the occurrence of salt water saturated
sandy materials. Regarding the GPR technique, the use of
antennas with different frequencies has allowed to accurately
image the internal structure of the sand dunes. The interpretation
shown in figure 3 reveals the existence of several sand bodies
defined by truncations or interruptions or reflectors, as well as
onlap relationships at the interior of the sand bodies. The general
dip of the reflections towards the S indicates a general northwards
migration direction of the dune system. The occurrence of some
dune bodies with a sub horizontal geometry and massive internal
structure would correspond to periods of lower wind energy
defining stability conditions without dune migration.
ERI technique has no enough resolution to accurately image the
internal structure of the dunes but it has greater penetration depth
than GPR and has allowed to obtain the depth to the water table
and bedrock. Comparing the results of both GPR and ERI, we can
conclude that the depth to the water table obtained strongly agrees
confirming that the water table is located at a mean depth of 3 m,
progressively deepening up to 7 m towards the south of the study
area. The obtained resistivity values have confirmed that only
fresh water is present in the area, except for the profile carried out
along the beach, where the strong attenuation of the GPR signal
and the lower resistivity values (< 0.5 ohm·m) confirm the
occurrence of salt water at about 2-3 m depth.
The depth to the basement has been determined using both
techniques and a good agreement has also been obtained. It can be
said that the bedrock, constituted by granitic materials, is located
at a mean depth of 8 m, ranging from 6 to 9 m.
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