Archaeology Resistivity Meter

Interesting ideas so far.  I meant to chime in earlier, but things have been pretty busy for me lately (building a deck and entertaining my Grandchildren again).

Several years ago, I was asked to sit in with some friends of my sister that work at a geotech company.  They were looking to build impedance measurement devices for soil surveys.  I came across this amazing looking chip from Analog Devices that looked like a great way to measure impedance (including a complex component).

Here is a snippet from the specification:

The AD5934 is a high precision impedance converter system solution that combines an on-board frequency generator with a 12-bit, 250 kSPS, analog-to-digital converter (ADC). The frequency generator allows an external complex impedance to be excited with a known frequency. The response signal from the impedance is sampled by the on-board ADC and a discrete Fourier transform (DFT) is processed by an on-board DSP engine. The DFT algorithm returns a real (R) and imaginary (I) data-word at each output frequency.Once calibrated, the magnitude of the impedance and relative phase of the impedance at each frequency point along the sweep is easily calculated using the following two equations:Magnitude = 22IR+Phase = tan−1(I/R) A similar device, available from Analog Devices, Inc., is the AD5933, which is a 2.7 V to 5.5 V, 1 MSPS, 12-bit impedance converter, with an internal temperature sensor, available in a  16-lead SSOP.

This might be of some help in your planning.

Good luck and let me know if you need any help on this project.

Hi Gene,

It's a super-interesting chip, I was keen to use it a few years ago for plant soil purposes, and for hydroponics - to try to see if the soil or liquid has nutrients. The idea being to have a signature of known good soil or water by sweeping through the spectrum. I never got to try it though sadly, the project moved on to something else.

It was felt that it could have had a lot of merit because then you could publish the signature, so others could try to replicate a yield (it wasn't going to be for farmers, more for home use), and to not waste nutrient. But, I have no idea in practice if the result would have been usable, or too inconsistent/variable.

I wish I'd done some work on it at the time, since it could have been useful for other purposes too.

The proposed design so far is one half of the impedance measuring system, but with digital processing. In theory it could be converted to an impedance measuring system with no additional hardware change, just a software upgrade, since the frequency will be know, and there will be some sync pulse from the FPGA, we just need to internally multiply with a 90 degree out of phase signal from that sync pulse too.

Referencing others is a good thing, I am not going to hide it if I have examined something - it is in the open and anyone who has a legal or moral issue can point to specifics. No-one has peeked at any code.

If Stallman read this thread (or if Turing could) I believe it is far-fetched to think they would have an issue with this design but I can't speak for them.

I repeat - this thread is full of references to the work of others and not original suggestions. What would Alan Turing say about having a peek inside? (Other than the Swiss made Enigma

 

Paul, if weren't such a mild mannered person I might get quite cross.

I've posted two completely original circuits and simulations in this thread and a completely original block diagram.

I've also participated in exchanges with others to refine my suggestions and theirs.

The most trenchant criticism of my design suggestions has been that they are too unlike what every one else does.

Of course, like any decent engineer, I have researched the subject and read some papers on it.

Other participants have made novel and helpful contributions.

The implication that the essence of this thread is rip off other peoples' work is risible.

MK

Its not the design, or the technique, which is specified by EH in the official guidelines that I have razed concerns with.

It's the "This unit does it this way" and "That unit does it that way" comments that run through this thread. What does that matter if its YOUR work?

As I have said why copy someone else's work: it may not be the best, or even good. Those who have experience in wet fields should be contributing original ideas. If you only receive instructions on how to copy others work how can YOU produce new, and exciting, equipment?

Paul D

Dear MK,

I have not said that your circuit designs are not yours or they copies of anyone's work. It never occurred to me that you would waste your time copying.

My original post was more nuanced. Why try to 'reproduce' when you have the skill to 'advance'. My concern was that you were being directed by users of existing equipment to copy/reproduce with modifications rather than being given advice and guidance on the next generation of equipment.

Paul D

Hi Shabaz, re:

shabaz  wrote:

...

There was the question "why not do it simpler and use a square wave like the EPE design" and partially the answer is that yes you can if you program the internal computer to do so. It is flexible. But the intent is to use sine waves. The end computed result will be the same (it has to be, to be compatible with existing software), but done differently. The new implementation calculates impedance, then derives resistance and then throws away the impedance measurement if it is not needed. ...

I'm sorry but I don't believe that is the case that you could get the same result. The EPE uses the same or similar technique to other working, proven instruments - it applies a DC signal, waits until a steady state is achieved, and then takes a reading. The DC is then reversed (to avoid polarisation accumulating) and the reading is repeated. Some primitive instruments may just apply the DC and wait for a fixed time before taking the measurement, that may mean taking it before a steady state is achieved, or risk polarisation, or just waste time. The 'device under test' which is being measured is not just a simple resistance. The sample is often many tens of metres long* and inevitably has some capacitance. DC is applied - either constant voltage or constant current between one set of probes, and once the injected current has settled and the resultant measuring probes return steady readings, then that is one good reading. I know, from fieldwork, that sometimes the ~3ms from 137Hz reversing frequency sometimes doesn't leave long enough for the DC conditions to settle, hence why sometimes a lower reversal rate is used to give sufficient times for the reading to settle. Sine wave excitation will not achieve this.

* processing a line of 20x20m grids, you may well be 50m away from the fixed set of probes. Even at the edge of grid nearest to the fixed pegs, when in twin-dipole mode - the most common mode - the fixed pegs must be at least 30x the distance between the two travelling probes, so 0.5m separation on frame => fixed probes can be no closer than 15m to the edge of the grid. Fixed probes then leapfrog every three or four grids.

I am curious why it is important to get the same results.  The results change according to the moisture in the ground, where the stakes are placed, all sorts of stuff.  The important thing is to map the underground which this more sensitive circuit with the ability to change frequency and waveform should do better.

Besides if it is so different that should get rid of all the complaints about copying ;-)

Paul, Mike, Shaz et al:

paul_d_arch  wrote:

...The problem is - as you have said - is you have no experience of using this equipment in a wet field and those who have (may not have any electronics experience)  are telling you about the commercial equipment they are using....

just to explain where I'm coming from - Physics first degree, then microelectronics. Career in IT then rugged embedded electronics, including designing carrier-phase GPS, inertial nav, high-vibration-deployed kit, etc. Long term archaeologist, arch geophys for at least 20 years.

Dave

Frank,

Copying may have been was the wrong word - duplicating/replicating  may have been better. You have a valid point, why not take advantage of new technology to move forward.

The 137Hz is, I suspect, a leftover artefact from the1920's and early hand wound equipment. There is a mathematical reason for 137 but the need for it may have long gone or other frequencies may be better suited to modern equipment.

Paul D

137Hz reversal is, I believe, chosen to be the lowest frequency that is clear of 50Hz and 60Hz mains and their harmonics, that gives just long enough in most cases to take a reading. Higher reversal rate negligible or no benefit for most use cases.

Hi Dave,

It can be demonstrated with (say) MATLAB. I'm tied up today, but it's possible to show that when switched at the source frequency, this is a mixing or multiplying action, and you end up with a steady fixed value measurement, regardless of sine or square excitation. The only difference is that there is more non-DC content to filter off with square excitation.

Although it seems to be a DC measurement, it isn't, it is a very slow AC measurement that can be (sometimes) better understood as DC, but when looking at it from how it is measured, it can be seen it really is AC.

I have a test instrument that uses square wave excitation. However I have another test instrument that uses a sine wave. Both provide similar results (almost near-identical, because they are both well-designed instruments). Both use averaging.

In any case, it can be tested for real too. The proposed design can handle the square wave and synchronised measurement method, i.e. there's no hardware difference to do either method.

The sine-wave method can be tested to see if it makes any difference, good or bad.

Hi Dave,

It can be demonstrated with (say) MATLAB. I'm tied up today, but it's possible to show that when switched at the source frequency, this is a mixing or multiplying action, and you end up with a steady fixed value measurement, regardless of sine or square excitation. The only difference is that there is more non-DC content to filter off with square excitation.

Although it seems to be a DC measurement, it isn't, it is a very slow AC measurement that can be (sometimes) better understood as DC, but when looking at it from how it is measured, it can be seen it really is AC.

I have a test instrument that uses square wave excitation. However I have another test instrument that uses a sine wave. Both provide similar results (almost near-identical, because they are both well-designed instruments). Both use averaging.

In any case, it can be tested for real too. The proposed design can handle the square wave and synchronised measurement method, i.e. there's no hardware difference to do either method.

The sine-wave method can be tested to see if it makes any difference, good or bad.