A well casing is insonified with a first acoustic wave in a first mode
that may be any mode of a set including: extensional mode, thickness
mode, flexural mode. A first echo is received at a first acoustic
transducer for receiving, a first signal is produced and a first
measurement is extracted from the first signal. The casing is then
insonified with a second acoustic wave in a second mode that may be any
mode of the set of modes but is distinct from the first mode. A second
echo is received at a selected second acoustic transducer for receiving
and a second signal is produced. A second measurement is extracted from
the second signal. The zone behind the casing of the well is evaluated
from a combination of the first measurement and the second measurement.