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.

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).

Michael,

If it is desirable to support USB stick or Micro SD (with FAT support) there are a few options that will make the job a bit easier.

1. There is this CH376 chip that handles the FAT structure and lets you write to USB Flash drives or SD cards (not too easy to find, as it does not seem to be handled by any common distributors - seems to only be available direct from the manufacturer).   There is a module available that is intended to interface with an Arduino that could be used ($12.95) from Marlin P. Jones Associates (https://www.mpja.com/USB-to-FAT-File-System-Control-Module-CH376-for-Arduino/productinfo/31813%20MP/)
2. I have used something similar to this (Vinculum) to read/write USB drives (with FAT), the VINC2 parts can support SD cards).  It sure made it a lot easier to handle the task of file system support (and we were in a huge ARM part with tons of RAM).

Good luck,

Gene

Never seen those before !

Not sure about reliability of supply and the data sheet is a bit lacking. Worth thinking about.

The FTDI chips (Vinculum) might be be OK - I'll need to look again - I use their parts for USB serial ports on things quite a lot.

MK

fmilburn  wrote:

 

...

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.
...

I'd let the ones that do the work choose the tools, in a community effort.

fmilburn  wrote:

 

...

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.
...

I'd let the ones that do the work choose the tools, in a community effort.

documented with up to date project description, diagrams

Did someone say PowerPoint? : )

No? Oh well, now it's mentioned : ) I'm happy to put the block diagram so far into that, so that anyone can modify it, since I've got reasonable idea from the thread so far, but it will need refining since it's just ideas currently. It's easy to copy the PowerPoint slide into the same deck, and modify it to express other possible implementations by anyone.

Otherwise I can use Visio although I don't like it as much, and I'm much quicker with powerpoint. Anyway it's easy to copy-paste from Powerpoint or Visio into any other application like Word etc, or generate .png graphics for open documents, and so on. Any good documentation or organising or any other contribution would be great if you don't mind helping fmilburn or Jan or anyone (and collectively we might eventually know about as much archaeology as that kid Short Round in Indiana Jones), and hope your home moving-in is going well Frank!!

I'll post my ideas so far as a hand drawn and scanned block diagram - but not until tomorrow since I have no scanner or squared paper here.

MK

Thanks Shabaz and Michael,

Readers looking at this for the first time and mechanical engineers without a clue everywhere will thank you for the block diagram. PowerPoint is the universal tool of corporate drones and I know it well :-)