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Geophysics in the Park: A Volunteer’s View

Written by The Reader, 19th March 2015

Last week the first physical part of the Big Dig got underway with the geophysical survey, led by the Museum of Liverpool.

We’re still awaiting the final report on the results before we post them up here, but below is a chance to hear a review of the day by one of our volunteers, Arthur, in his own words:

At last, Thursday; and geophysics in the Park.  There were seven of us assembled for instruction at the Mansion House and Liz introduced us to Mark Adams, our archaeologist and geophysicist for the day.  We had been given instructions: no metal at all, no zips, no pop closures, no eyelets, no rings or jewellery.  It was a cold wind out there and I ended up with long johns over pyjama bottoms, two sweaters and wellies.  Mark told us a bit about the magnetometer that measures very tiny variations in the magnetic field on the ground where it was pointed.  He measured us in anoraks with poppers – three times the magnetic ground level, off with the anorak.

Then off to the lawn in front of the Mansion House, a possible place for a new home for the Calder Stones.  With measuring tapes, lots of cane sticks and rope, Mark set up a 20m square marking a 3m, 4m, 5m triangle to get a right angle laid out. I didn’t know that Pythagoras was a geophysicist!  We then marked out lots of lanes on the square, setting sticks and using a rope marked in metres that we had to follow.

The magnetometer was like a huge TV aerial; a horizontal pole with a LED screen and buttons to press was hung from straps over your shoulder. The pole on either side had big vertical tubes which were pointed at the ground.  I wondered if you pressed the wrong button that red smoke might blast out of the tubes and send you flying through the air, but no, it was quite easy.  You walked along the measured rope and pressed a button at half metre intervals and the machine went ‘pip’.  It did a double ‘pip’ when you reached 20m then you did a U turn, moved the rope to the next stick and came back doing the same ‘step, button’ sequence.  Quite easy once you got into it.  There were forty lanes to measure and we all had a go, successfully.  Richard had to concentrate as his glasses had metal hinges and without these he was as blind as a bat and couldn’t see where he was going.  The rest of us were on dog-watch duty, chasing off dogs with metal studs in their collars that might interfere with the magnetometer.

We were really measuring low and high spots of magnetism.  Where a ditch had been filled with soil would be low magnetism and where there were stones or, particularly stones that had been heated in a hearth, would lead to higher magnetism.  The results came through the next day after Mark had crunched the numbers.  There was a series of low level ‘holes’ in a curve, but it was probably nothing more than a fence at one time.

I had to leave at lunch but the team repeated the process on a site behind the Mansion House and Liz and Mark showed them some theodolite work measuring horizontal and vertical angles to get an accurate map measurement relative to the house for the sites we had studied.

A really interesting day.  Archaeology and curiosity. You can’t beat it.

Thanks Arthur for such a great account of the day.

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