Appendix M

Resistivity  and  I P  Survey                                              

 

A survey was conducted with closely spaced observation points using MiniRes serial number 14 (an early “standard” design MiniRes). It illustrates the value of induced polarization. The MiniRes allows the observer of resistivity to take an IP reading with a minimum investment in time; usually an additional 10 or 20 seconds. Many small targets that do not yield a significant resistivity anomaly do yield a noticeable IP anomaly. This advantage of IP is particularly helpful in shallow surveys of archeological or engineering sites. See Appendix X.

Figure A below is a graph of resistivity and IP conducted over a grass covered athletic field. The geology consisted of weathered limestone covered with about one foot of residual soil. The Wenner “A” spacing of the electrodes was 1.283 foot (0.391 meter). The orientation of the array is parallel to the direction of the survey line. Variations in the resistivity are believed to be due mostly to variation in the thickness of the residual soil.

On the field was a ten-foot by ten-foot (3-meter by 3-meter) reinforced concrete slab. The survey line passed along one side of that slab at a distance of one-half foot. The slab extended from Station 32.2 to 42.2. Note the minimal effect on the resistivitiy readings but the major effect of the slab and its steel on the IP observations.

There was also a vertical steel pole adjacent to the line at about station 28. Again, the IP signal is clear and there is little or no resistivity anomaly.

Resistivity in is Ohm-Meters and has been calculated by multiplying the resistance by 2 * p * A = 2.46. The IP is in degrees of phase. It has been calculated by multiplying the IP reading by 180/Pi and dividing by the resistance.

Figure B below is a short survey line that shows the major IP anomaly caused by the slab. It also illustrates how the IP anomaly decreases rapidly as the line moves away from the slab. The electrode array was parallel to the side of the slab.

Figure C displays the resistivity along the same survey line as on Page 3. There is a small decrease in resistivity as the line approaches the slab and its reinforcing steel.

Clearly the IP signal is much greater for certain small targets and can be of great advantage in most archeological and engineering investigation, especially considering how little time is required to obtain the IP reading with the MiniRes.

 


Figure A

 

 
 

Figure B

 

 
 

Figure C

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