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.

Hi Michael,

I had a stab at finding a solution to the power supplies.. I wasn't sure what source voltage to target, but I took an assumption of 6V, which could be two LiPo in series (e.g.  two 18650, or better still, a single camera battery, since they are low-cost, for hopefully at least a full day of use).

I spent ages trying to find a transformer (and trying to shoe-horn stuff like audio transformers into the role unsuccessfully), but there's the 6-in-one coils, (all 1:1 ratio to each other) and two of those devices could provide enough windings to cover all the analog supply rails:

Here the orange boxed windings are all one transformer, and the red boxed windings are the second transformer. The voltage regulator parts in the diagram are just placeholders, they don't represent any particular device.

I'm wondering if something like LT3439 could drive them. Its also got a oscillator sync pin which could be useful.

At 200 kHz I think VPH1-1400-RVPH1-1400-R (datasheet PDF) might be a suitable transformer. Regarding 5V, 3.3V and 1.2V, these could be from some DC-DC converter directly from the battery maybe. If you think this is the way to go, I can try to prototype up the analog supplies, since i'm not sure how effective I could simulate this.

I just packed my house to move to Oregon yesterday and all my lab with it. So for the next few weeks I will mostly be working with computer, paper, and pencil only.

michaelkellett - I am going to try to maintain LTspice by myself but will ask for your model if I get in trouble. There are a few things I don’t understand but let me try to work through them first.

Regarding the following - I used to be a project manager. Forgive me

I think this string of comments will potentially become difficult to follow and it would be nice to see it well documented with up to date project description, diagrams, schematics, drawings, design files, etc. in one place. Is that best done on GitHub?  Anyway, I volunteer but request guidance. Do you have an opinion on using GitHub for project documentation other than just code Jan Cumps?  I am seeing this done quite a bit now.

We should update the block diagram from shabaz early on. In my previous life the block diagram was fixed as well as key project objectives early in the project and only changed after much snarling and gnashing of teeth.  I propose the following be clarified now:

* block diagram - key hardware like uC can be pencilled in for now, major things like whether to use a FPGA and architecture need to be fixed early

* cost - 50£, 100£, 200£?  There should be some idea of single board cost.
* component size and packages

* the level of expertise required to build one - this will determine how many of these actually get built and used by DIYers.
* Michael laid out a good summary of performance characteristics
* I also like Michael’s comment that this could a general impedance analyzer. That is the way I would use it. The use cases need to be defined. Any others so as to minimize scope creep?

Regarding the above I am happy to make something up to get it started but others may already know or have better thoughts.

kltm do you have thoughts?  It would be great if other archaeologists found this useful and built one.
Frank

Thanks for ideas.

I like the idea of a standard dual lithium cell - can you point at any camera batteries ?

I'm not thinking that hard about PSU yet, there's still a big Elephant in the shape of the display to resolve and it might use more power than everything else.

However I would normally use some kind of switcher for the +/-15V supplies, LT8471 would do, LT3463 might do but is at the limit of power.

The nice thing about using LT switchers is that they have models and example circuits in LTSpice.

Some of the supplies will use very little current but must be low noise and I was thinking of using a quad op amp to tap down from the +/- 15 V supplies - not staggeringly efficient but low noise and simple.

MK

Good luck with moving !

Totally agree that we will need to be  (a lot) more formal to turn this thread into a thing that gets built.

kltm , hope you don't feel I've/we've muscled in on your scheme, it is well intentioned.

Is resistance surveying allowed under social distancing rules - it ought to be OK (outdoors and in fields)

Do you survey in any particular area or all over - I'm thinking about when it gets to testing this thing I'd like to see what happens.

(Are dogs allowed ?)

MK

Hi Frank

I am in a group on Facebook called Geophysical Prospecting in Archaeology. I will post it on there. Should I add that a GitHub project will be set up yet?

I’m very excited to see the progress so far, but I’m afraid some of the concepts are beyond me. Still great to see. I am sure there are a number of groups that will welcome a DIY update to the current (expensive) kit available.

I did build the original 2003 Resistivity Logger and although it works, the RS232 interface is very hit and miss, so you can not rely on being able to download the data. I’ve emailed you a pic of my efforts.

Ken

Hi Michael

No problem ‘muscling in’ on this. I’m quite excited in seeing the progress so far. Your experiences are streets ahead of mine, so I’m looking forward to learning a lot. I did build the original 2003 Resistivity Logger a few years ago and although it works ok the RS232 interface is very hit and miss, so you can’t reliably download the readings. As backup we would write the readings down as we go, obviously very time consuming. I can send you pics of the unit I made if you’d like to see it.
We do a lot of our geophys here in the U.K. at a local National Trust stately home. It is possible to stay 2 metres apart when surveying. One person would operate the unit and the other would hold and guide the cable to the fixed probes. Two others would move tasked with moving the lines. Oh and yes dogs are allowed. We can easily get permission to survey at anytime as well.
Thank you for time and efforts so far

Ken

Hi Michael,

The LT3439 is supposedly a low-noise driver which I'd thought I could prototype rapidly since I believed I had a dev-board for it and could have ripped out the transformer there, but turns out I have a different, LT3575 dev-board : ( I could knock up a test card on my next China PCB order and send one to you too if it's worthwhile just for general experimentation for any project, since we can't simulate that method well compared to the switcher designs.

Sony NP-F series batteries seem quite good (original or some decent-mnfr third-party replacements), for instance this Amazon pack of two batteries and a charger is £30. The batteries come in different heights/capacities, all with the same footprint and attachment. I have a huge Ansmann brand 6.6Ah NP-F970 which fits my non-Sony camera monitor, so the range covers extremes and seems popular.

There are NP-F holders on Aliexpress. Some chargers have adapter plates which are sold separately, so they are sometimes lower-cost.

For a project recently, I used a cheaper adapter plate (about £10), and a smaller camera battery (not NP-F, I can't recall the model), and for that one I permanently fitted the adapter plate and battery inside the enclosure, because I put a charger circuit inside the box too. However for this use-case I guess that's not essential, and it could be an external mounted slide-on holder. Hehe I hope we're not speculating too much about how this project gets used! Still, it's more lightweight than lead-acid or whatever that the older systems may have run on : )

Hello Shabaz,

I've not spotted the LT3439 before - there would be a big trade in efficiency against psu noise  - not sure yet that it's necessary for this project.

Golly, that battery is £180 from Sony, I wonder what's inside the Amazon ones (ah - I see they only have half the capacity.)

Plug in batteries are nice, or you could buy a 4 x 18650 pack with protection from RS, 7.4V @ 5.2 Ah about £30. But you wouldn't be able to swap charged up batteries in an emergency.

If this box is to have a graphics display we need to scope its power requirement before we can really home in on a battery.

Which leads to some questions I hope Ken can answer:

Should the instrument have a graphics display ?

Would the graphics only be useful if there was a full port of Snuffler or something like it ?

I'm assuming it must have 10 hours endurance from a single charge - is that about right ?

Is it worth the extra hassle (considerable in software) of being able to plug in either a USB stick or Micro SD card memory ?

Some background information:

A nice display (800 x 480, TFT colour capacitive touch) will add about £80 to the cost of the bits.

USB card or SD card rather pushes you towards using an RTOS for the software and makes the software much more complicated.

Running a big bit of code like Snuffler could be quite a lot of work.

You can achieve these things by plugging into a laptop (has to be Windows for Snuffler so no RPi).

MK

Ken,

Your build of the original 2003 logger is very nice - I am impressed.  You should post it here so others can see it. The Improved electronics that Michael and Shabaz have outlined should give improved resolution so it will be interesting to do a comparison.

Regarding the Facebook group and input in general.  We are close to being able to create a specification and start the design documentation as I outlined above.  I am not that knowledgeable about  GitHub and have only used basic features in the past. It would be a public repository so no problem for others to access it. When the draft is complete it would be good to post that in the group and solicit feedback.  Is there a particularly knowledgeable professional you work with that could give feedback?

Frank

Hi Michael,

I tried out the Snuffler software, and I feel it's definitely too much effort to replicate it, since it's not open source (but is free-to-use) and seems to be Windows-only. It does three main things:

(1) accept data (either in real-time via COM port or from a flat file such as CSV), and the format is straightforward for at least one of the devices it works with,

(2) plots the data,

(3) graphically processes the data or manipulates it (for stuff like edge detection, interpolation and so on).

For an on-board display, just speculating, perhaps only (1) and (2) could be needed, along with fewer basic graphic manipulation options, and as you say just use laptop if the full capabilities of (3) are needed. For on-board display to work I guess it does need to be a pretty nice display, since seeing the data is so core to it. I can try to investigate UART connected via USB to Android as an option (it is supported in Android Studio based apps apparently).. but depends if that's a usable practical scenario all day or if the built-in display is preferable.

Also, incredible the price Sony is charging for what is still a consumer battery...

The Ansmann 6.6 Ah battery is £35, it's a high-quality consumer brand too, so trustworthy I think. I have one of those batteries for about half a year, using this dual fast charger (£24). I don't have any Sony-manufactured gear that uses NP-F, but have ended up with a couple of items that can use the battery (a camera monitor, and a camera light both from different large brands (Atomos and Manfrotto) which optionally can have an adapter plate attached for NP-F, but the plate for the light is a separate purchased item which I don't own).

Hi Michael,

I tried out the Snuffler software, and I feel it's definitely too much effort to replicate it, since it's not open source (but is free-to-use) and seems to be Windows-only. It does three main things:

(1) accept data (either in real-time via COM port or from a flat file such as CSV), and the format is straightforward for at least one of the devices it works with,

(2) plots the data,

(3) graphically processes the data or manipulates it (for stuff like edge detection, interpolation and so on).

For an on-board display, just speculating, perhaps only (1) and (2) could be needed, along with fewer basic graphic manipulation options, and as you say just use laptop if the full capabilities of (3) are needed. For on-board display to work I guess it does need to be a pretty nice display, since seeing the data is so core to it. I can try to investigate UART connected via USB to Android as an option (it is supported in Android Studio based apps apparently).. but depends if that's a usable practical scenario all day or if the built-in display is preferable.

Also, incredible the price Sony is charging for what is still a consumer battery...

The Ansmann 6.6 Ah battery is £35, it's a high-quality consumer brand too, so trustworthy I think. I have one of those batteries for about half a year, using this dual fast charger (£24). I don't have any Sony-manufactured gear that uses NP-F, but have ended up with a couple of items that can use the battery (a camera monitor, and a camera light both from different large brands (Atomos and Manfrotto) which optionally can have an adapter plate attached for NP-F, but the plate for the light is a separate purchased item which I don't own).

Hi Shabaz,

(1) and (2) in the Snuffler capabilities would seem easy to implement on a relatively low resolution screen In the field as you suggest. For a 20x20 or even 40x40 grid a simple graphic like heatmap in MATLAB could be displayed easily enough with a uC. That would allow an operator to see fliers or anomalies as they are appear.