Figure 2 - uploaded by Aaron Davis
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3: Schematic describing the fundamental operating principle of nuclear magnetic resonance as applied to nuclei with a non-zero nuclear magnetic dipole moment as a result of charge and spin angular momentum. Pulse transmission in 2) aligns the magnetic moments at right angles to the external magnetic fields and the resulting decay back to equilibrium is measured in 3) (after (Legchenko et al., 2002))
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http://shodhganga.inflibnet.ac.in/handle/10603/8176
Citations
In this report we analyse the results of a ground-based time-domain electromagnetic survey
along the beds of two rivers in the town of Liquiçá, Timor-Leste. The first survey, which consists
of 25 stations at 40 m intervals, follows the course of the Gularloa River for a distance of 1200 m
from the opening of the river into the Strait of Ombai southward toward the hills. The second
survey follows the Laclo'o River for 640 m, from the river opening to the south, over 17 stations.
The purpose of this report is to obtain information about the extent and depth of the saltwater
intrusion from the Ombai Strait under the river gravels, and to determine the thickness and
extent of the freshwater aquifers beneath the river beds. From our results, we see that the
saltwater interface and mixing zones are visible for about 400 m inland, deepening at a rate of
about 1 m in every 4 m, to a maximal depth of detection of about 100 m. In the Laclo'o River,
the saltwater wedge appears to go further inland at a depth of about 80 m. The reason for this
is not known, although it may be due to a lower peak flow in the river during flood.
We have also detected what we interpret to be a shallow freshwater aquifer directly under
the Gularloa River. This aquifer, which is about 5 m to 10 m beneath the surface of the river
bed and about 40 m thick at its deepest extent, persists inland for the entire extent of the
survey. It appears to be underlain by a more conductive layer that could indicate the presence
of saturated clays acting as an aquitard. It is unknown what lies beneath this aquifer layer,
although the conductivity results indicate that this could be a secondary aquifer, somewhat
connected to the saltwater intrusion. For the Laclo'o River, we have detected a thin freshwater
aquifer that is in the very near surface and about 20 m thick at its thickest extent. There is
a slightly more conductive layer underneath this aquifer, but its composition is unknown. In
both surveys, we have detected an anomalous reading (at 320 m from the mouth on the Laclo'o
River; and about 400 m away from the opening in the Gularloa). This is most probably a buried
pipeline that was installed at some time in the past.
For future work in this area, we recommend the placement of monitoring wells at about 400 m
to 500 m inland from the opening of the Gularloa river. This will allow for monitoring of the
vulnerability of the first aquifer to future development of the water resource, but also to determine
the presence of a secondary aquifer at about 80 m depth. The Laclo'o River appears
to be more vulnerable to freshwater extraction. Our recommendation is to use exploration
drilling at about 500 m inland to monitor the extent of seawater infiltrations and to determine
the presence of a clay aquitard underneath the primary surficial aquifer.
