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An interdisciplinary
geophysical approach to detect cavities in a karst region**

Baradello Luca, Bratus Antonio, Nieto Yabar Daniel, Paganini
Paolo, Palmieri Francesco

(7th International Congress of the Brazilian Geophysical Society Salvador de Bahia, 2001)

(7th International Congress of the Brazilian Geophysical Society Salvador de Bahia, 2001)

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The aim of this research carried out by Istituto Nazionale di Oceanografia e di
Geofisica Sperimentale-OGS Trieste, was to apply several geophysical
methodologies such as Earth Resistivity Tomography, Ground
Penetrating
Radar,
and microgravity survey to detect cavities in karst area. A well-known cavity
system was chosen as the test site, in order to verify geophysical data with a
real situation. The results was really satisfactory, since the methods had a
very good power of investigation and they has been complementary which other.
The surveying lines location was carried out in collaboration with the “Civico
Museo di Storia Naturale” of Trieste.

The aim of this
research was to integrate several geophysical methodologies to detect the
presence of cavities in the karst area. The investigation was carried out in a
site test which particular conditions allowed to test very well the
methodologies employed.
Unfortunately, due to local environmental conditions wasn’t possible applied
homogeneously the proposed methods. The selected area was above the Doria cave
(n° 3875-3876), which is located in the Carso Triestino.
The choice of this
site is related to particular conditions such as:

·
the cavity system is well-know and was available a complete bibliography and a
detailed topography survey

·
the cave shows a rather regular geometry and it is placed along a horizontal
axis, which direction is continuous and defined on the surface

·
the cavity is divided into two branch, of which the easterly is interrupted

The comparison of data highlight the potentiality of several methodologies and
it has showed a good similarity of the results.

The data so collected have been processed following the usual steps:

·
tidal effects using the harmonic developments

·
drift correction considering drift linear with time

·
latitude correction assigning a reference latitude to the reference station
(lower elevation) and computing for the other stations the correction term,
according to the latitude gravity gradient

·
Faye correction considering also the square term of height

·
Bouguer correction taking into account also the curvature correction

·
terrain correction has been automatically computed using the right prism method

After geometry, the steps of processing were:

1) Time-zero drift correction;

2) Background removal algorithm with a mean of traces without strong targets;

3) Amplitude analysis and gain function (Spherical divergence correction and AGC);

4) Band-pass filter;

5)
*f-k* migration;

6)
Building of cuboid and his interpretation

To estimate the average velocity of the investigated site we used wide-angle reflection and refraction (WARR) method. We collected a few WARR and then we calculated georadar velocity with semblance. The velocity range of reflections checked was stable between 9 and 10 cm/ns, a typical characteristic velocity of limestone. In this case the resolution obtained is 50-60 cm. Four adjacent migrated sections are displayed into cuboid. The interesting target is 1.5-2 m width and 7 m deep. We can see that signal goes to the bottom and moves from south to north through section (a) to (d). Probably it is a cave that connects the two cavities 3875 and 3876. This signal is not visible on all lines and therefore we can stand that either there is a third cavity or it’s a tunnel that connected the two cavities and now partly collapsed.

Loading all georadar data collected into a cuboid allows us to do a good
interpretation: so we can follow one reflector using several sections (inline,
crossline, arbitrary plane and time slice).

The same profile (a) before and (b) after f-k migration. The algorithm focuses
the top of the cavity and a dipping continuous reflector.

A
cuboid composed by four migrated sections. The strong central signal shows the
cavity.