Archaeology Resistivity Meter

Hi Ken,

This is a very interesting project with great comments from Michael and Shabaz  I did not realize how low the soil resistance typically is.  Wikipedia gives the usual values to be from 10 up to 1000 (Ω-m) though sometimes higher.

https://en.wikipedia.org/wiki/Soil_resistivity

I am jumping ahead but assuming the typical user would be an enthusiast building this themselves as a diy project, open source.  If so, that would influence the choice of components, complexity and cost. Can you describe in a bit more detail your thoughts on that?

Frank

Hi Frank

yes as you have guessed I am an enthusiast building this for myself. I belong to an amateur archaeology group and we do have access to an RM Frobisher unit, although this is somewhat restricted. We quite often carry out surveys for other groups who don’t have access to equipment, mainly because of cost. The RM Frobisher unit is in the region of £3000 currently, and that’s about the going price for this sort of equipment. To my mind it’s heavily overpriced for what it is. You will see that the ball park cost figure put in on the article by Robert Beck is about £50.00, but that was 1997. I reckon on today’s prices I could put that at maybe £150 to £200. In use the RM Frobisher unit is fairly automatic. Place the probes at C1 R1 and it takes a reading and beeps to let you know, then lift the probes and place them in C1 R2 and so on up to 20. At the end of the row there is a double beep and you would swing round and put the probe in C2 R20 and come back down. At the end of the grid there is long beep, the unit then saves all the readings to sd card and shuts down. There is facility in the software to mark trees and other obstructions as dummy readings. The one criticism I have is the menus in the software are poorly written.
The biggest shortfall in the current offerings is not having a live view. A simple on screen grid that would be populated as readings are taken. We currently have to post process usually that evening so you have no idea what you have until then. Sometimes on a grid you will have an area of interest. With these we would go back over that section, but increase the number of readings to every half a meter to improve the resolution. Easier to do when your all set up. Anyway this should hopefully give you a flavour of what’s required. The majority requirement will be in the software I guess, but I can play at that. I don’t think the electronics are that sophisticated, but they could certainly do with being updated.
Ken

Hi Michael Not sure about the frequencies in use. Is this the frequency of the square wave to generate pseudo AC. If so the original article talks about 137Hz  being the ideal, because of possible interference from 50/60Hz mains hum, and the need to filter out these frequencies later.
Not sure if i’m right or talking rubbish. Slap my wrists if I am.
I’ve also seen talk of LTSpice for simulation. I have used this, but quite a while ago. I’ll download it again and try to follow what you are doing.

All very interesting and once again thank you.

Ken

This is the test signal but it really should not be a square wave but a sine wave - the design I'm suggesting uses sine waves and decodes the measured signal by multiplying by sin and cos rather than +/- 1.

(I know that the DIY design uses a square wave and possibly some commercial devices do but it is a very bad idea. - with modern processors and electronics there really is no justification for not using a sine wave.

A square wave generator and rectangular de- modulator is sensitive to odd harmonics of the operating frequency - which makes it much harder to pick a good operating frequency.)

There is no perfect frequency -  the best to use depends on local interference conditions so I'm suggesting be able to set the frequency in the range 20Hz to 200Hz which more than covers the range that most commercial devices offer.

Re. the FPGA - it may not be necessary

MK

Thanks Michael, good explanation. I get it now.  Simpler than I thought.

EDIT:  that is, simpler except for the FPGA :-). I’ve never used FPGAs.

I'm hoping this application can avoid an FPGA, the ADC can talk directly to the processor - it'll need an interrupt every 1ms but the 120MHz STM32l4xxxx can cope with that.

But happy to help put an FPGA in just for the fun of it

MK

Hehe my vote is stick the FPGA in : )

It could be nice to offload that portion, gets people more interested in FPGA, plus the processor code will be slightly simplified and more portable for future people to use different or less performance devices or experiment/modify. In theory since then there's very little timing concern for the processor, some people could rip out the processor and use a Pi and a process on Linux (not that I'm suggesting it is a good idea, but technically possible) to prototype code enhancements too.

A nice little Lattice FPGA would be OK, not expensive, they only need about 100k of flash in the processor to hold the map, so no extra flash chips required.

The tools are much easier than Xilinx.

MK

I downloaded the Lattice software last night, and tested putting the default blinky LED bitstream onto the ICEstick board, now planning to run through a tutorial to get some practice with the tools : )

Oh well... another shiny object to distract me

I’m just trying hard to keep up.

I think the best FPGA would be the ICE40UP5K - 48 pin QFN package so home solderable. £4.80 from Mouser , supported by Radiant software, has RAM blocks, 1Mbit SRAM and multipliers on chip.

And even better I've used them before

Are enough people interested in this to get together and buy some boards ?

MK

I think the best FPGA would be the ICE40UP5K - 48 pin QFN package so home solderable. £4.80 from Mouser , supported by Radiant software, has RAM blocks, 1Mbit SRAM and multipliers on chip.

And even better I've used them before

Are enough people interested in this to get together and buy some boards ?

MK

I'd happily do so for a set of boards, and help in areas where I can contribute. The technical challenge is interesting..

I am in - sounds like a good learning opportunity and I will help where I can.  I just did a search for the ICE40HX1K-STICK-EVNICE40HX1K-STICK-EVN and can’t find it in stock in the usual places - Newark, Digikey, Mouser.  I was thinking to order the STM NUCLEO-L4R5ZI as well so as to have dev boards to play with.

I’ll certainly like to try with the boards, I’ve got a lot of learning to do.