You don't even have to do that..

A car driving by a coil, distorts the earth's field by a surprising amount.
With a simple differential amplifier (three op-amps and some 1%
resistors) you can see traffic going by on the third floor of an
apartment building, 10-20 feet from the street below, using a
"telephone pickup" coil.  Been there, done that.

With a larger coil, especially one with a large area, and closer in,
the signal is much larger.

A loop of wire in the road is good too, but you don't need to go to
that extreme.

The "metal detector" approach works, but you'll be better off, if you
must go that route, to use the balanced field type, that won't be
sensitive to heat, cold, rain etc.
A pair of coils are used, with the larger one in a shape that's hard
to describe in text, but basically such that the smaller coil can be
placed right over it in a spot where it gets no, or almost no signal
when the large coil is driven with a fairly large signal. External
metal, ferrous or not, distorts that field, and imbalances the system
resulting in a larger signal in the small coil.  Boxcar integrator or
other synchronus detection can be done, to reject noise, and the
amplitude and phase of the signal will tell you something about the
target.
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