WEST GERMAN geophysicists have found a large freshwater aquifer beneath
the Thar desert in Pakistan. The aquifer, which is between 30 and 100 metres
below the ground, has a volume of 10 cubic kilometres – enough, say the
researchers, to supply the water needs of Hamburg (a city of 1.6 million)
for 115 years.

Klaus-Peter Sengpiel and his colleagues from the Federal Institute for
Geosciences and Natural Resources in Hanover, found the aquifer using a
new and very fast technique, which they call multi-frequency airborne electromagnetic
surveying. They later discovered that, in the same area, there had been
a river called Hakra or Ghaggar, which ran dry in the 16th century.

The technique exploits the phenomenon of electromagnetic induction.
When electromagnetic radiation of the right frequency is directed into the
ground, it drives electric currents, and these in turn radiate electromagnetic
radiation. The properties of these ‘secondary’ waves depend on whether ground
water is present and how saline it is.

In deserts, ground water is usually either salty or located at great
depths. Traditionally, geophysicists have determined whether the groundwater
is salty or fresh by making so-called geoelectric measurements. They put
a pair of electrodes in the ground, connect them to a battery and measure
the electric current that flows between them. The size of current depends
on the electrical resistivity of the ground (a quantity measured in ohm
metres), and the resistivity in turn indicates the salinity of any ground
water between the electrodes. Very salty water conducts a current very well,
brackish water not so well, while freshwater is almost a perfect electrical
insulator.

This conventional method of locating fresh ground water has a disadvantage:
it takes a long time – between one and two hours – to make a measurement
at each pair of electrode points. This means that it can take decades to
explore an extended area of desert. With their airborne technique, Sengpiel
and his colleagues can survey an area covering thousands of square kilometres
in only three weeks.

Sengpiel and his colleagues use three transmitters and three receivers,
fitted into a cigar-shaped probe about 10 metres long. They tow the probe,
known as a Kevlar bird, beneath a helicopter at a height of between 30 and
40 metres above the ground at 140 kilometres per hour.

The three transmitters emit electromagnetic waves at frequencies of
386 hertz (Hz), 35 381 Hz and 328 939 Hz. When these penetrate the desert
ground, they induce electric currents to flow, in the same way that electromagnetic
waves from a radio station induce currents in a receiving antenna. The amplitude
and phase of these currents depends on the distribution of resistivity in
the ground. In deserts, however, all layers contain sand. This means that
the induced currents depend only on the salinity of the ground water in
each layer.

The induced currents generate weak electromagnetic waves. It is the
magnetic component of these secondary fields that Sengpiel and his colleagues
measure with the aid of the Kevlar bird, storing the data on magnetic tape.
The probe can measure at all three frequencies four times every second.

The geophysicists used a computer to calculate the pattern of resistivity
across the ground in the Thar desert (see Figure). They flew the probe in
trajectories that were 400 to 800 metres apart, obtaining vertical resistivity
patterns. Later, they combined the data to create horizontal profiles, each
of which showed how salty water was at a certain depth. They were able to
measure the water quality to a depth up to 100 metres.

The geophysicists also used the combined data to create a three-dimensional
model of the Thar aquifer. The profile (see Figure) shows a ‘side view’
(vertical). The body of water is 98 kilometres long, with an average width
of 15 kilometres and a depth of up to 70 metres.

Aided with their computer maps, the West German researchers, supported
by a team of Pakistan hydrologists, were able to select the most promising
sites for drilling wells with impressive success. They obtained freshwater
from the 20 wells they put down.

According to Sengpiel, the new airborne technique still requires some
basic ground measurements and test drillings in order to calibrate the data
obtained from the probe. Measuring with only three frequencies, it provides
a coarser resolution than the traditional geoelectric method. But its superior
speed is the main advantage, allowing large areas to be surveyed quickly.

Meier found A. trichotis during an expedition to the rainforest of northern
Madagascar in April 1989. However, it was only last month that his university
released any details of the find.

The hairy-eared dwarf lemur is one of the smallest of all primates –
it is only 30 centimetres long, including its tail, and weighs only 80 grams.
Until now, zoologists have known of the species only from five specimens
in museums, four of which were collected in the last century. No one has
ever published observations of the animal before.

Albert Gunther discovered the tiny lemur in 1875. He came across an
individual in a collection of preserved animals sent back to Europe by a
colleague in Madagascar.

Before Meier’s expedition last April, the only people to have seen living
hairy-eared dwarf lemurs were the island’s rainforest-dwellers and Andre
Peyrieras, a French entomologist and friend of Meier who lives in Madagascar.

Peyrieras was given a living specimen of the lemur by Madagascans in
1964. But, knowing little about primates, he did not know what he held in
his hands. He believed the animal to be a ‘strange’ specimen of the lesser
mouse lemur. When it died in captivity some time later, Peyrieras preserved
it in Madagascan rum and sent it to the Museum of National History in Paris.

Meier flew to Madagascar in April 1989. He was not looking for hairy-eared
dwarf lemurs but for the lemurs known as aye-ayes. He was hoping to make
a documentary film about these rare nocturnal creatures, themselves thought
to be extinct for much of this century.

Meier began to explore the rainforests near the town of Mananara on
the northeastern coast of the island. At each village he came to, he asked
the inhabitants to describe and make pencil sketches of all the lemurs they
knew. It was in one such village that an old man told Meier of the ‘tsidi
ala’, which in Malagasy means something like the ‘mouse lemur of the woods’.
Meier realised that the description closely matched that of the hairy-eared
dwarf lemur.

Following the old man’s directions, Meier and his Madagascan guides
struck out into the rainforest. For 12 days, they struggled through the
mountainous terrain, shining their flashlights each night in the hope of
seeing the flash of red eyes. Also, each night, they fixed 24 traps in the
treetops. But Meier was not successful. The only lemurs he and his helpers
caught were the abundant dwarf lemurs and lesser dwarf lemurs. Finally,
they gave up.

On their way back to Mananara, their luck changed dramatically. Suddenly,
in the cone of Meier’s head-lamp, a hairy-eared dwarf lemur appeared for
just a few seconds. But Meier, who was weakened by malaria and the antibiotics
he was taking for a knee injury and blood poisoning in his right foot, could
neither catch the animal nor take its photograph.

Meier’s next expedition was more successful, although at first it did
not seem promising. A dog joined Meier’s team and could not be chased away.
It became a nuisance because it had been trained by villagers to bark whenever
it smelt lemurs, which the villagers eat. But the dog was a quick learner
and soon Meier was able to persuade it to stay quiet.

In the end, the dog was the first to discover a burrow used by hairy-eared
dwarf lemurs. In the burrow were two adults and one almost adult animal.
Meier’s Madagascan translator, Roland Accot, managed to catch the young
animal with his hands. Meier then took photographs, measured it and released
it.

Meier returned to West Germany, leaving his French colleague, Roland
Albigniac, to continue studies. Accot caught three more hairy-eared dwarf
lemurs. Meier and Albigniac will publish their results in the journal Folia
Primatologica.

Meier’s new discovery reinforces the importance of Madagascar in the
biological world. For the past 60 million years or so, the island has been
isolated. This has meant that its plants and animals have evolved independently,
with many ‘primitive’ species surviving. In effect, Madagascar is a giant
experiment in alternative evolution.

No one knows how many hairy-eared dwarf lemurs are living in Madagascar.
But the species is in danger.