Archaeology Resistivity Meter

Certainly possible to do much better than the design in the Everyday Practical Electronics design by using modern components.

https://www.slideshare.net/jplateado/earth-resistivity-meter-part-1a-robert-beck

The block diagram is just about legible in the link, but the article gives a good background.

We could do it now by making a DDS oscillator which can work at any frequency between 20 and 200 Hz (more range than commercial boxes offer). Then you need an amplifier able to manage +/- 25V out (not hard), a low noise diff amp and a low noise 24 bit ADC.

A micro to drive it all and either a serial interface to a laptop or maybe a Pi if you prefer. (Laptops solve all the boring battery/screen/keyboard/box issues for you).

No point in using wireless - because you have to have wires any way to make the system work..

Sounds interesting, walking the dog yesterday I'd got as far as designing a system (in my head) with an autonomous moving probe but perhaps we should walk before running .........

How much should the electronic box (excluding the laptop) cost ?

MK

Hi Michael,

Just thinking about it some more, I wonder if there's a case for BLE expansion maybe too (could be an add-on module later I guess).

The reason is, then if the microcontroller sent the measurement via BLE, it would be possible for a phone or tablet with GPS to automatically position the measured pixel in the right place. It could all be done in a browser app, since there's nowadays both BLE and GPS capability directly accessible from the browser (the entire code can be in a local html file, no need for a cloud service if it's an area with poor cellular coverage).

The processing to extract the measurement would have to be done in the microcontroller though using DSP techniques.. because I don't have the knowledge/skill to do that in a browser!

I'm a bit hampered in my thinking on this because I've never seen this kind of work done in real life.

I think the normal thing is that you have a fixed station and a moving station, each has a measuring probe and a current injection probe so you are making  4 wire resistance measurements.

There has to be a wire connection between the fixed and moving stations to complete the measuring loop.

The basic measuring head needs a processor and/or FPGA to do the signal processing but it only produces 1 number (resistance) for each point so the data rate is very low.

You could Bluetooth it to a phone easily enough.

The moving station could send data back to a a non moving PC or processing thing, or the moving station could contain all the works required - but if that were a display and a Pi it would be

quite power hungry. The fixed station could send power along the cable.

Some display is needed at the moving end for operator feedback. Only tiny amounts of data are involved so there is no need to resort to SD cards and the like - soldered in NV ram will be fine.

If you did 20 of 20x20 squares at 0.5m resolution per day it would only be 32k data points =  approx 64k x 8 = 512k data (I'm allowing 8 bytes each for resistance and sample ID).

So you could have everything, serial connection by wire. local storage on moving station and Bluetooth to a phone as well.

I'll need to  take a look at snuffler to see if it could run on a micro.

Several rather random thoughts there - may be helpful.

MK

Hi Michael you are correct. There are 2 remote probes C1 and P1. Depending on how many grids. You were doing they would be positioned at 15 metres outside the grid area and at right angles centrally along the Y axis of the grid. We generally do more grids than this so we use up to 100mtrs of cable from the remote to the movable frame with C2 and P2 on it. I guess it could be a box of some sort mounted on the movable frame and connecting via Bluetooth to a small tablet mounted facing the operator. All settings and recordings could be made to the tablet. Most accept SD cards. Again most tablets are not that power hungry.

Ken

I didn't entirely understand the process, I initially thought the probes were like on 4 wheels, so all four moved together. I should read the article rather than scanning! : (

The article has this monochrome image, I'm guessing people start at the green circle, and go up and down until they reach the red circle..

I wonder what the reasoning is behind having the probes set up in this way, and how accurately the probes need to be kept in the same angle.

C1 and C2 are the source probes, and P1 and P2 are the sense probes.

Here's a sketch of what I think you're roughly envisioning?

Hi the probes don’t have to accurately aligned and they only need to make contact with the soil to take a reading. Some units take up to 20 readings in very quick succession and use average of this. You are correct in the method used to take the readings. The reason for the way it is done is laziness I guess. Most software, snuffler included, default to you starting in the south west corner of the grid and following the path you have drawn. Take readings 1 to 20 in row 1, then 20 to 1 in row 2 and so on.

Ken

Hi Shabaz This looks interesting, I’ll have to do some reading up on FPGA’s as I have no experience of them at all. I think I can understand the rest of it though. Thank you for your time and effort, it’s very much appreciated.
Ken

Hi Ken,

A low-cost training board may be the ICE40HX1K-STICK-EVNICE40HX1K-STICK-EVN which contains the FPGA, memory for the bitstream, USB programming interface, a 12MHz clock source, a few LEDs and not much else.

I've purchased that to self-learn that particular FPGA model too. It has solderable pins, and a socket, with easily enough pins for ADC, DAC and microcontroller interfacing.

The only annoying thing about it is that it is quite a large board, and will snap off if hanging out the side of a PC/laptop, so a USB type A plug-to-socket cable could be useful too.

I think the modified portion of the diagram ends up looking like this based on michaelkellett suggestions. The FPGA<->Microcontroller interface is only vaguely drawn, it needs designing. Ideally the FPGA needs to clock everything, so the microcontroller can sit waiting on an interrupt and read the ADC data. Some sync line would be used to indicate to the microcontroller that the DAC is about to start generating a new cycle of sine wave. The config lines would need to set up the rate at which the DAC and ADC operate, and start/stop the operation.

Hi Ken,

A low-cost training board may be the ICE40HX1K-STICK-EVNICE40HX1K-STICK-EVN which contains the FPGA, memory for the bitstream, USB programming interface, a 12MHz clock source, a few LEDs and not much else.

I've purchased that to self-learn that particular FPGA model too. It has solderable pins, and a socket, with easily enough pins for ADC, DAC and microcontroller interfacing.

The only annoying thing about it is that it is quite a large board, and will snap off if hanging out the side of a PC/laptop, so a USB type A plug-to-socket cable could be useful too.

I think the modified portion of the diagram ends up looking like this based on michaelkellett suggestions. The FPGA<->Microcontroller interface is only vaguely drawn, it needs designing. Ideally the FPGA needs to clock everything, so the microcontroller can sit waiting on an interrupt and read the ADC data. Some sync line would be used to indicate to the microcontroller that the DAC is about to start generating a new cycle of sine wave. The config lines would need to set up the rate at which the DAC and ADC operate, and start/stop the operation.

Hi Shabaz

I’ll order one of the FPGA training boards so I can get my mind around them.

Thank you

Ken