ARMP – field plots, part 2

Basic injection voltage (V1) and current (I) between C1C2 and resultant differential voltage (V2) between P1P2 were covered in part 1, all using standard archaeological geophysics preferred 137Hz square-wave excitation.

Part 2 continues the test results, using exactly the same setup in part 1, and covers control (test load, background measurements), and detected signal range using square-wave excitation. Part 3 will then start to look at various stimuli – varying the square wave frequency, and then using alternative waveforms.

Control

For reference, the test signal generation/injection chain was operated on the bench to check the injection performance.

Waveforms were recorded directly from the Wavetek 275 arbitrary signal generator, and the outputs of the HP 6826A bipolar amplifier - both open-circuit and into a purely-resistive (at these frequencies) 4k7 dummy load (note a). The effect of the survey leads used to connect to the probes was also checked.

Fig 2.1 – Sample injection performance into dummy load with 137Hz square wave excitation
V1(upper/green), I (lower/purple) – averaged in scope

Waveforms were recorded as above, but don’t merit inclusion here other than the example above, as they all show the chosen waveforms being reproduced faithfully and no discernible ringing or overshoot effects, whether into the dummy load or with the C1C2 injection path open-circuit.

As well as connecting the dummy load directly to the injection amplifier, the load was connected via various sets of survey leads to check they didn’t introduce any problems. The survey leads used for these tests were a set of existing survey leads, as used for most twin-probe archaeological resistivity work (flexible unscreened twin-core mains cable as used for, say, an electric lawnmower or hedge-trimmer), and (ii) a lead made from Belden 8760.

Analysis of these leads using a Peak Electronics ‘Atlas LCR-40LCR-40’ passive component analyser showed that for 50m lengths of lead, the normal survey lead had resistance 3 ohms and an inductance per core of app 40 μH, and inter-core capacitance app 10nF; the Belden had resistance app 2 ohms and inductance per core of app 15 μH, and inter-core capacitance app 6nF.

No artefacts could be discerned as a result of introducing the survey leads twixt injection amplifier and dummy load when all was connected as it would be in a survey. The only artefact noticed was if the probes C1 and C2 completed an injection path, and hence injection current was passing in the survey lead into the load, but one of P1 or P2 was not in the ground (or making contact) then a small coupled edge was spotted on the P circuit – but that was only if the far end of the P lead was left floating.

External interference

These tests were almost all acquired using a Wavetek signal generator, HP amplifier, and Fluke 4-channel DSO; all these bench instruments used for these outdoor tests were powered from a small 240v inverter, which was a potential noise source – either radiated or conducted. The main noise from the inverter, which was present throughout all the tests in both part 1 and here in part 2 and 3, was of the order of tens of millivolts at several kHz.

It was known that this might cause a problem, as might 50Hz mains in a nearby building, and a mains-powered electric fencing energiser, so the immediately following background noise plots were acquired with a battery-powered rig (two-channel PictoTech 3205 and laptop) as control, the two sensing soil probes P1 and P2 (at their max 50m separation at either end of the test traverse) were connected to ch1 and ch2 of the PicoScope.

In all except the immediately following control plots (figs 2.2 and 2.3) the 50Hz mains supply in the nearby property (whose earthing / ground return point for the mains supply was some 25m from the test traverse at the closest point) was isolated, but was switched on just for these interference-measurement tests. Similarly, the electric fencing energiser for the nearby farm was also powered-off, except during these interference-measurement sweeps. The grounding-point for the electric fencing energiser return was approximately 50m from the P1 location and 95m from P2. The background sweeps (figs 2.2 and 2.3) were chosen to include a capture when an electric fence energiser pulsed.

Fig 2.2 – Typical single-ended background P1 and P2 waveforms at X=50m – single sweep – with NO injection
and nearby 50Hz electricity supply ON, and electric fence energiser ground pulse, and inverter noise

The individual waveforms for P1 and P2 so closely track each other on this plot as to be indistinguishable except for when the electric fence energiser pulsed.

Differential measurement on a scale which can show the electric fence pulse:

Fig 2.3 – Typical background differential P1P2 (V2) waveform at X=50m – single sweep – with NO injection
and nearby 50Hz electricity supply ON, and electric fence energiser ground pulse, and inverter noise

The mains supply nearby was then isolated, and the electric fencing energiser powered-off again, before continuing with the tests below. (There was still some small 50Hz noise though, even with the inverter switched off.)

Fig 2.4 – Typical background differential P1P2 (V2) waveform at X=50m – single sweep – with NO injection
and nearby 50Hz electricity supply OFF, and inverter noise

It should be noted that the effect of the stray/return mains current can cause significant asymmetry in the injection:

Fig 2.5 – Circa 5mA injection in presence of nearby 50Hz electricity supply,
as measured across 100R in series with C1 injection lead

But if the resistance calculations are based on instantaneously measured injection current I then reasonable resistance results can still be achieved by suitable windowing (see part 3).

Signal Range

With standard square-wave excitation, spikes and ringing in the V2 (P1P2) signal can occur at pulse reversal time, significantly in excess of the proportions exhibited by the injection waveform (V1, I). The V2 peaks can be up to an order of magnitude in excess of signal.

For example at X=15m, in ‘good’ ground, even though the total range between stable +ve and stable –ve on P1P2 was only 9.0v; the range between peak +ve and peak –ve was 32.4v

Fig 2.6 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=15m – single sweep
Note V2 peaks at reversal

At X=30m, when only just over 5mA could be injected (by V1 = +/-60v) on a small stony patch, even though the total V2 range between stable +ve and stable –ve was only 0.7v the range between peak +ve and peak –ve was 60.2v :

Fig 2.7 – Injected current (I) and differential P1P2 (V2) waveform at X=30m (stony ground) – single sweep
Note V2 peaks at reversal

Nevertheless, gating so sampling occurs – for 137Hz square wave excitation for window between say 1ms and 3ms from reversal keeps clear of erroneous readings, but the spikes can put stress on the input circuitry.

Analysis and comments on injection waveforms to follow in part 3.

Notes

(a)    Dummy load - the purely-resistive dummy load totalled 4k7, chosen as representative of the resistance found on the injection path C1C2 on this traverse. The load was made of 2k [tap] 200R [tap] 2k5; the C1C2 injection was applied across the full 4k7, but when needed the P1 and P2 probes were applied across the two taps so they only measured the differential voltage across 200R as typical for the actual resistance measured P1P2 on this traverse.

Continued in part 3

DM 23 Aug 2020

Attachments:

ARMP plots 2 v06.pdf

ARMP – field plots, part 3

Basic injection voltage (V1) and current (I) between C1C2 and resultant differential voltage (V2) between P1P2 were covered in part 1, all using standard archaeological geophysics preferred 137Hz square-wave excitation. Part 2 addressed some controls, background interference, and detected signal range using square-wave excitation.

Part 3 now documents various stimuli – varying the square wave frequency, and then using alternative waveforms. Analysis of those, and implications thereof, will follow in part 4.

Method

Archaeological geophysics commonly uses 137Hz square-wave excitation, with options to reduce the frequency. To inform discussion on excitation strategies, the following stimuli were tested:

• Square wave 137Hz
• Square wave 80Hz
• Square wave 32Hz
• Sine wave 137Hz
• Triangle / symmetric sawtooth wave 137Hz
• Trapezoid wave 137Hz

In each of the alternative excitation scenarios, a recording was made of the injection results between C1 and C2 (V1and I), the resultant differential potential between P1 and P2 (V2), and the derived resistance (V2), across the full 50m of the test traverse. For each of the above stimuli, a control was also conducted using the full length of the survey leads, but connected to a dummy load of instead of the probes, to check for any artefacts.

Alternative stimuli (1) - different frequency square waves

The main results in part 1 were all collected using square-wave excitation at 137Hz, but to demonstrate settling and the effect of polarisation, comparisons were also run at 80Hz and 32Hz.

At 137 Hz

Fig 3.1 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m at 137Hz
(single sweep)

Fig 3.2 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m at 137Hz
(averaged in scope)

Fig 3.3 – Absolute injected current (I), differential P1P2 (V2), and derived soil resistance (R2) waveform at X=50m
with 137Hz square wave stimulus (based on data averaged in scope)

At 80Hz

Fig 3.4 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m at 80Hz
(single sweep)

Fig 3.5 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m at 80Hz
(averaged in scope)

Fig 3.6 – Absolute injected current (I), differential P1P2 (V2), and derived soil resistance (R2) waveform at X=50m with 80Hz square wave stimulus (based on data averaged in scope)

At 32Hz

Fig 3.7 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m at 32Hz
(single sweep)

Fig 3.8 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m at 32Hz
(averaged in scope)

Fig 3.9 – Absolute injected current (I), differential P1P2 (V2), and derived soil resistance (R2) waveform at X=50m with 32Hz square wave stimulus (based on data averaged in scope)

Alternative stimuli (2) – 137Hz sine wave

Fig 3.10 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m
with 137Hz sine wave stimulus (single sweep)

Fig 3.11 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m
with 137Hz sine wave stimulus (averaged in scope)

Fig 3.12 – Absolute injected current (I), differential P1P2 (V2), and derived soil resistance (R2) waveform at X=50m with 137Hz sine wave stimulus (based on data averaged in scope)

Alternative stimuli (3) – 137Hz triangular/sawtooth waveform (equal rise and fall times)

Fig 3.13 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m
with 137Hz triangle wave stimulus (single sweep)

Fig 3.14 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m
with 137Hz triangle wave stimulus (averaged in scope)

Fig 3.15 – Absolute injected current (I), differential P1P2 (V2), and derived soil resistance (R2) waveform
at X=50m with 137Hz triangle wave stimulus (based on data averaged in scope)

Alternative stimuli (4) – trapezoid waveform

Final test in this batch was using trapezoid stimulus, base frequency 137Hz, using Wavetek 275 arbitrary waveform function (signal definition +/- 2,047 points in Y, 2048 points in X, per cycle => 3.56 μs/step).

Proportions chosen for Rise | High | Fall | Low were 1:4:1:4 so rise and fall times both 10% of total cycle time and steady high and low states 40% each. For 137Hz, cycle time 7.3ms, rise and fall times 0.73ms, theoretical stimulus steady states 2.92 ms.

Fig 3.16 – Control - Injection waveform (V1 and I) waveform with dummy load (4k7)
between sets of probes instead of soil (full length of survey leads used), 137Hz trapezoid wave

Fig 3.17 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m
with 137Hz trapezoid stimulus (single sweep)

Fig 3.18 – Injected current (I) and voltage (V1) and differential P1P2 (V2) waveform at X=50m
with 137Hz trapezoid stimulus (averaged in scope)

Fig 3.19 – Absolute injected current (I), differential P1P2 (V2), and derived soil resistance (R2) waveform at X=50m with 137Hz trapezoid wave stimulus (based on data averaged in scope)

Analysis of these excitation strategies, and implications thereof, will follow in part 4.

Continued in part 4

DM 24 Aug 2020

Attachments:

ARMP plots 3 v02.pdf

Nice to see some more data, I'll be interested to see your final part.

It would be nice if you were to include some results with more significant noise in the measured voltage.

In your previous post there was a plot of noise from an earthing system but this was largely removed from the

measurements because it was effectively common mode current noise.

There will be cases where differential mode noise is present and this will appear on the voltage but not the

current measurement and so can only be removed by filtering.

Did your measurements preserve numerical data that could be run through a series of different processing methods ?

Thanks.

MK