[archaeological] geophysical techniques

following MAKE magazine: http://www.make-digital.com/make/vol08/?pg=47#pg47

and Shawn Carlson in Amateur Scientist January 1999.

with a few changes (cassette case, some tech changes):

1] Separate out tiny polypropylene strands from a thicker cord (bought at OBI)

2] Drill holes (0.8mm) in exact centre top and bottom of clear cassette case. Thread tiny strand into tiny needle and thread this through bottom hole and then top. Gaffer tape top thread and place cassette box so that lower part of thread hangs in free space. Make the thread taut by taping several coins to this lower part, and glue both ends of the strand with epoxy.

3] Glue a small mirror to one tiny magnet (4x1mm neodym). With tweezers sandwich the thread between mirrored magnet with another small magnet.

4] Hotglue a small piece of copper behind the magnet to damp oscillations (inducing eddy currents in the copper when the magnet moves and thus an opposing magnetic field).

5] Construct a set of nulling magnets by hotgluing four ferrite ring magnets (23x13x5) to another cassette box. Polarity should be observed so the magnets shoyuld repel each other and will need to be held in place while the hotglue dries! These should be centred to align with the magnetometer mirror.

6] See nulling location notes below (question: how critical is alignment of nulling magnets vertically with centre of mirror?)

• location of components/nulling magnets
  

  

  from: http://xtrsystems.com/magnetometer/layout.html

  Actually, I find that the balance, laser, and detector can be oriented in just about any direction but the nulling magnet assembly is often best located on the North-South line relative to the balance. The nulling magnet assembly can be translated along this line and can be rotated to provide suitable alignment. Alignment is achieved when the bright red spot of the laser diode falls upon the center of the detector and the lengthening of the period of oscillation occurs as described in the article. However, the nulling magnet assembly does not have to be located to the North of the balance; it can be located to the South but it will have to be rotated by 180° for proper operation.

  It is worth repeating that the nulling magnet assembly must be located on the North-South line relative to the balance. If you find that the assembly's optimum position lies elsewhere then it is likely that there is a significant perturbation in the earth's magnetic field due to some local ferrous material. Moving the system to another location is advisable under these circumstances as there is likely to be considerable interference with the real terrestrial signal.

http://www.regulusastro.com/regulus/papers/magnetometer/index.html

http://www.britastro.org/aurora/jamjar.htm

http://www.eaas.co.uk/news/magnetometer.html

http://www.backyardastronomy.net/compass_magnetometer.html

http://www.carnicom.com/mag2.htm

Compass Detector (compass, led, phototransistor and coil): http://www.dcs.lancs.ac.uk/iono/aurorawatch/detectors/compass.html

MAKE: http://www.make-digital.com/make/vol08/?pg=47#pg47

Torsion balance and coils: http://xtrsystems.com/magnetometer/layout.html

Travel magnetometer in a cassette box:

http://www.astro.uni-bonn.de/~kbagschi/magnet-e.html

Links: http://www.sas.org/E-Bulletin/2004-04-02/wanderings/body.html

http://arxiv.org/html/physics/0207062

Data sheet: http://www.speakesensors.com/PDF/detail.pdf

Pins (reading downwards from top) from back with all pins closest to right): 0 F/B, 1 GND, 2 OUT, 3 +5V

Which also demonstrates use of counter and INT0 interrupt on pin 25 of the ATmega128 [pin18 on scrying connector].

http://scrying.svn.sourceforge.net/viewvc/scrying/freqcounttest.c?view=markup

PIC Magnetometry Logger. John Becker. EPE. Everyday Practical Electronics. July/August 2004

http://homepage.ntlworld.com/clarcana/proj/magnetometer/index.html

http://www.speakesensors.com/products.htm

http://www.picotech.com/experiments/earths-magnetic-field/

http://www.directivesystems.com/PDF/MAGTALK.PDF

http://www.ursa.fi/ursa/jaostot/revontulet/magnet/enmagnet.html

http://www.prc68.com/I/Sensors.shtml#Fluxgate

http://www.thunting.com/geotech/forums/showthread.php?t=13732

http://ourworld.compuserve.com/homepages/demerson/magnet.htm

http://geotech.thunting.com/cgi-bin/pages/common/index.pl?page=mag&file=projects.dat

thermal dependency of FGM-3: http://www.tuc.nrao.edu/~demerson/cs/Thermplt.htm

Constructed after:

http://www.ilotresor.com/prospection/pro_magn_protons.html

water bottle as pictured (dimensions). 10 layers of 0.65mm wire

rough tests with frequency generator show working filter (to be tested again)

• hum in amplifier when polarizing supply is applied (what is this frequency?)
• noise only (radio) when switched to amplifier

• test outside in countryside
• re-test amplifier
• add resonating cap (2200 Hz - should be 2100 Hz for Berlin) - equation?

Redesigned board to eliminate self oscillations:

See: http://1010.co.uk/proton1.tar.gz for schematic and PCB design based on: http://www.ilotresor.com/prospection/pro_magn_protons.html

• notes on testing in Peenemuende
  

  Lots of harmonics (50Hz ??? 180Hz) but with energy around 2100 Hz indicating filter is working. Should be tested now 1M OFF the GROUND

  • what is humming when not attached? (switching circuit seems to amplify

  or exaggerate mains 50Hz)

  • use distilled water or vodka rather than tap water!
  • use of tuning cap(value? - see below)
• testing all stages of the ilotresor design
  

  Pre-amp -> passband filter -> headphone amp

  Pre-amp and headphone amp (both based on 2n2222) not working for some reason. Pre-amp replaced with op37 based design based on "Practical Guidelines for building a Magnetometer for Hobbyists" (Bayot) and "Construction of a Proton Magnetometer" (Ruhunisiri and Jayananda).

  Pre-amp tested successfully with signal generator (100mV signal through 100K/10R voltage divider) and with filter (as per ilotresor design). Headphone amp replaced with LM386 design. Gain of first two stages for 2100 Hz= 50 dB (10logVout/Vin). Filter also working. Possible self-oscillations could be due to external noise.

• measuring induction of the coil
  

  In order to determine suitable tuning cap which will sit across the front end of the pre-amp (signal/GND).

  Set up circuit with function generator connected by way of 1K resistor to oscilloscope. Known capacitor (100nF) and the coil are connected across the scope input (so between end of 1K and GND).

  Step through frequencies for function generator (100mV pp) say 1-4KHz and look to see where signal peaks. Capacitance must be higher to bring down resonant frequency.

  L (Inductance in Henries) = 1/[C(Farads)*(2PI*F(HZ))^2]
  

  In this case for our coil 22mH.

  Required capacitance C=10^9/(0.03948*2100^2*22) = 261 nF
  

  Where 2100 is our precession frequency and 22 is mH for our measured coil inductance. Successfully tested with scope/generator and 269 nF caps.

• TODO
  
  • test in the field, 1m above ground and with new fluids (distilled

  water, vodka).

• precession frequency Berlin = 2102 Hz

(49,395 nT x 0.04256)

from: PracticalBuildingGuidelines.pdf

The two coils are wound in opposite directions with respect to each other and connected in series. They are mounted parallel to each other so that external magnetic field noise common to both coils is cancelled out. The signal from the sample in one of the coils can then be detected with minimal interference. Both coils are large enough to accomodate a 2.0 ounce (59 ml) bottle containing the sample.

(see: http://www.exstrom.com/magnum/master-Z-H-7.html )

Data sheet: http://www.burle.com/cgi/byteserver.pl/pdf/4526a.pdf

Photomultiplier handbook: http://psec.uchicago.edu/links/Photomultiplier_Handbook.pdf

Data sheet: http://cn.ic-on-line.cn/IOL/datasheet/tsl257_394751.pdf