1 2 3 Previous 193 Replies Latest reply on Jul 9, 2020 10:14 AM by kltm

    Archaeology Resistivity Meter

    kltm

      Hi I'm looking for ideas on an update to a resistivity meter for archaeology. The only published designs for diy were in 2 magazines. One was published in 1997 and the other in 2003. I have copies of both articles available. The reason behind this is the current high cost of available equipment, usually well beyond the reach of most archaeological groups. I've attached a basic block diagram. In the first magazine article the meter is very basic. It relied on the operators to write down the reading given as the survey was taken. Given that a normal survey grid is 20m x 20m and 1 reading is taken on every sq mtr there would be 400 readings to write down and then input into a program used to interpret the results. The later article is really an update to the first where a PIC has been added to record the readings. This again is prone to error, because eadings are taken manually by pressing a button.

      I'm sure given the advances in electronics there must be better ways. 

       

       

       

      Basic block diagram.

        • 1. Re: Archaeology Resistivity Meter
          michaelkellett

          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

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          • 2. Re: Archaeology Resistivity Meter
            shabaz

            Hi Michael,

             

            I was wondering, since the design doesn't call for impedance, maybe just two channels of an audio interface could be re-purposed.. using one channel to record the received signal, and the other channel to record a timecode (for getting the phase, and for getting position).

            Then, the Pi/Laptop could post-process, since I'm guessing there's no need to build up the picture in real-time (or is there kltm ?). I suppose it could be almost real-time, since computing power is good anyway.

            But then it got me wondering why the 1997 design didn't do this.. since ok-ish sound cards were available at that time too surely. So maybe I'm missing something.

            Also, maybe the DDS could do a radar-like sweep, or steps, from 20-200.. Were you thinking of using a microcontroller for the DDS, or FPGA?

            ----------------------

            (EDIT: This thread is long, so here's a link to jump to approx. comment number 135.

            2 of 2 people found this helpful
            • 3. Re: Archaeology Resistivity Meter
              shabaz

              Automated afterward could be fun : ) Can imagine archaeology groups hacking robot mowers : )

              https://www.youtube.com/watch?v=zwpwHs9P6gA

              • 4. Re: Archaeology Resistivity Meter
                kltm

                Hi Michael This all sounds very interesting and encouraging. I see you have found the original article, the update is also on slideshare somewhere. I haven’t really thought much about cost, but as you can see on the parts list, they give a ballpark of £48, but that was 1997. I think I would go with a Pi and 10” pi touchscreen. I don’t think programming in Python is that difficult. I did think of having a visual display where a grid was drawn on screen and filled in as work progressed, a bit like Fifty shades of grey. The moving probe bit has been done by R M Frobisher for their TAR-3 meter, but their offering costs £2986 currently. I’m a bit of an amateur when it comes to designing electronics, but I’ve built plenty of electronic bits in the past, mostly from magazine articles like this one. I also have a good bit of experience with the Raspberry Pi.
                Thank you for your ideas, all food for thought. Now to design.

                Ken

                • 5. Re: Archaeology Resistivity Meter
                  kltm

                  Brilliant, although R M Frobisher do offer a wheeled probe for their meter. I prefer the walk, although it’s not so good when you do 8 -  10 grids in a day. A long way to walk.

                  • 6. Re: Archaeology Resistivity Meter
                    kltm

                    The screen read out real time would be very useful. There is an open source programme called snuffler, which everyone uses to post process the data. It’s normally stored on a micro sd card and post processed. Trouble is the waiting to see what you’ve got. It also means that any areas of interest can be redone at 1 reading every half a metre (4 per sq mtr) for better resolution. Doing the whole grid at this resolution is very time consuming (1600 reading per 20m x 20m.).

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                    • 7. Re: Archaeology Resistivity Meter
                      shabaz

                      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!

                      2 of 2 people found this helpful
                      • 8. Re: Archaeology Resistivity Meter
                        michaelkellett

                        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

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                        • 9. Re: Archaeology Resistivity Meter
                          kltm

                          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

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                          • 10. Re: Archaeology Resistivity Meter
                            shabaz

                            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?

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                            • 11. Re: Archaeology Resistivity Meter
                              kltm

                              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

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                              • 12. Re: Archaeology Resistivity Meter
                                fmilburn

                                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

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                                • 13. Re: Archaeology Resistivity Meter
                                  kltm

                                  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

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                                  • 14. Re: Archaeology Resistivity Meter
                                    fmilburn

                                    Thanks, that is helpful.  Actually a lot of things are even cheaper today.  What about surface mount parts, FPGAs, and PCB design.  Are you comfortable with those?  I like the simple diagram that Shabaz provided.  Are you comfortable programming microcontrollers in C/C++?  I think you said at one point you were comfortable with Python.  I would think a Pi or laptop could handle the user interface and post capture data analysis in essentially real time and display results as well as give instruction and take user input.  There are good mapping and plotting libraries for Python as an example.  But personally I would do the data capture on a microcontroller as Shabaz outlined.

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