VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Detection of Prestige fuel layers buried on the beaches by
means of ground-penetrating radar
Henrique LORENZO*, Pedro ARIAS, Manuel PEREIRA and Fernando RIAL
Department of Natural Resources & Environmental Engineering. University of Vigo
EUET Forestal. Campus A Xunqueira s/n. 36005 - Pontevedra. Spain
* Contact: hlorenzo@uvigo.es
ABSTRACT After the Prestige accident more than 30.000 tons of oil were leaked and arrived on the Galician coast.
Some months later layers of fuel contamination still appear at different depths in the sand of the beaches. GPR could be
used in some inland areas, which are under the influence of high winter tides but remain out of their influence during
spring and summer time. Some in-situ tests were carried out on the beaches with different antennas. Because of the
good results obtained, a cart with wide wheels was adapted to survey the beaches. In this study some examples of a
study case carried out in the beach of Carnota (Galicia) are shown, where it was possible to detect a layer of fuel (15-20
cm thick) of about 40 m2 buried at a depth of 40-50 cm.
1. INTRODUCTION
One important issue in the identification of contamination level in the beaches is to be found
in tidal action. The tidal process is that the first tide brings fuel over the sand but, if it is not
removed, following tides place clean layer of sand on the top of fuel, and the beaches appear to
be clean. Layers of fuel appear at different depths in the sand, from some cm to 1-2 meters.
The lateral extent of the contamination can vary, from 1 cm to more than 1 m.
GPR pulses can not penetrate a sea water saturated sand environment, but Hagrey and
Müller [1] show that surveying could be successful on a brackish water environment, depending
on salinity and water content of the sand. Thus, some contaminated inland areas affected by
high tides could be surveying during low tides in the winter. Also, GPR could be used in the
inland areas which are under the influence of high winter tides but remain out of its influence
during spring and summer. If the signal is not attenuated in this sandy environment, the
prospection success depends on the contast between electromagnetic properties of sand and
fuel. Sauck [2] developed a model for the resistivity of hidrocarbonate plumes on sandy
sediments, but we have not found references about GPR studies of sandy areas contaminated
with this special kind of residual heavy fuel oil.
According to the study cited before, hydrocarbons typically have high apparent resistivity
and a low relative permittivity (before biodegradation). This can also be applied to the Prestige
fuel-oil but, depending on the sea water content, big changes in the primary physical properties
of the mixture can be expected. The fuel water content depends on the sea floating time, but
also on exposure to rain water, especially when it is washed up on the beaches. Fuel also
emulsifies in contact with fresh water, which is a new parameter to be taken into account when
estimating the impedance of the layers and its contrast with the impedance of the sand.
Even so, it is not possible an accurate prediction of how long has been the fuel floating at
sea and neither for how long has been affected by fresh rain water on the beaches or below the
sand. With this complex model of fuel-mixture composition and the difficulty of surveying a
brackish water sandy environment, it was decided to make some in-situ test on the beaches
using 500 & 800 MHz antennas burying prestige fuel layers below sand
2. RESULTS AND DISCUSSION
Some tests were carried out in a winter raining day, in the beaches of Porto do Son
(coordinates 9º 9' W, 42º 37' N) with two antennas of 500 & 800 MHz center frequency. The
presence of fuel below sand was simulated burying some emulsified Prestige fuel taken from
the shoreline with the help of volunteers who were there helping in shoreline and rocks clean-up
(Figure 1).The simulated layer was less than 1m 2, lateral extent 10 cm, and about 30-40 cm
deep, but depth changes slightly in relation with the antenna because the test was made in a
half-inland area, where the sand is not a hard surface.
VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Figure 1: Simulating a buried fuel layer (left) and 800 MHz radargram obtained (right)
Some individual profiles were registered with both antennas over the buried fuel layer, with
better results in the use of the high frequency one because of its better resolution. Figure 5
shows one 800 MHz radargram obtained in the survey, where the reflections from the layer are
clearly distinguished between meters 2 and 3 of the profile.
The good results obtained allowed for a hopeful use of the system to find unknown fuel
layers below sand. A cart with wide wheels was adapted to survey the beaches. A survey wheel
was used to acquire data and a rough laptop with a transflective & sunlight-readable TFT
display was used to make real-time interpretation. Also, a differential GPS was used to obtain
UTM co-ordinates of the position of the profiles and the layers detected.
As an example of some good results obtained, a 10 m long radargram (20 ns time window,
800 MHz) is shown in Fig. 2, together with its interpretation. The presence of the fuel layer was
confirmed in-situ digging a trench in meter 3.5, where it was interpreted as the edge of the layer.
Excavation revealed the presence of the layer at 40 cm depth
Figure 2: GPR+GPS applied together in Carnota beach (left) and radargram showing a fuel
layer buried at 40 cm depth (right)
It is possible to conclude the GPR is a useful tool in the detection of Prestige fuel-oil layers
below sand under some particular beach conditions. This type of survey can be carried out in
the inland areas. A laptop with special transflective display was necessary for real-time
interpretation. In order to be efficient the use of a cart is advisable. The use of a survey wheel
also allows to stop the cart at any place, dig a trench, and start prospection again. The use of a
GPR+GPS system gives a precise location of those areas, and fuel can be removed using its
GPS coordinates without going back to the beach with GPR
REFERENCES
Hagrey S.A. and Müller C. (2000) GPR study of pore water content and salinity in sand.
Geophysical Prospecting, 48: 63-85.
Sauck W.A. (2000) A model for the resistivity structure of LNAPL plumes and their environs in
sandy sediments. Journal of Applied Geophysics, 44:151-165.