Electroseismic testing.  Need help filtering 60hz noise - Page 1
Forum section: electronics / projects

author: viper
author_rank: Regular Contributor
timestamp: August 09, 2016, 06:53:13 pm
content: I will explain what I am doing so hopefully someone can direct me better. I have a large property and need to drill for water. I have learned that electroseismic methods are being employed to find highly permeable zones based on the principals of a dipole moment in water. Basically as a seismic P wave is sent through the soil, this will cause a disruption of electrons in the water, and generate an electric charge that can be detected at the surface with a dipole antenna. The methods I am using are based on other commercial systems that are fed back through software to 3D map areas.Basically a dipole antenna is created with two steel rods placed X distance apart embedded in the ground. A seismic wave is generated by a large hammer.I have attempted this by embedding two electrodes and connecting them to one channel on a scope. I connected the other channel to a piezo transducer as the trigger so I could trap the moment when the wave was generated. So far, I am unable to gather meaningful data because there is so much 60hz noise on the antenna. I need help filtering this. All I have been able to find is the Matlab software and scopes with some of this built in. However, my scope does not do much filtering (Owon) so I may need to filter externally. It seems that digital filtering may be the best way to remove this?There is some possibility that I will need to amp the antenna signal which will make the noise even worse but hoping I can see it once I get the noise out.Help would be much appreciated!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2016, 08:10:13 pm
content: Unless the 60 Hz background is really too large (so much to limit the ADC resolution), I agree that digital filtering of the 60 Hz is likely the best method.One problem could be the scope as a source for the signal. So I would prefer a battery powered instrument. It could be really difficult to keep 60 Hz signal small with a mains powered instrument. Battery powered should be more practical in the field anyway. So you would need an small instrument to record data at something like a 1 kHz sampling rate and reasonable resolution (e.g. 16 Bit). A normal soundcard might not work because much of the signal might be in the < 10 Hz range.

author: viper
author_rank: Regular Contributor
timestamp: August 09, 2016, 08:39:28 pm
content: Thanks. I am using a battery powered scope. This model can sample at 1ghz/s.What is really baffling is a similar commercial product is using nothing but a smart phone for this data collection but I think they are employing a sharp technique by using the earphone amp to push the signal and using the mic input for the signal source. They are using digital filtering employed within the phone. However, I cannot find even one simple device I can buy as an external digital filter. All I can find are scopes with filters built in, or software that works more in the DAQ realm.can you offer any advice to easily kill the 60hz noise?

author: edavid
author_rank: Super Contributor
timestamp: August 09, 2016, 10:51:04 pm
content: You can build a good hardware notch filter using the traditional twin-T circuit, or a switched capacitor filter IC.However, even if this water detection method actually works, you're not going to get anywhere without processing the signal in software, so you might as well use a software notch filter as well.

author: viper
author_rank: Regular Contributor
timestamp: August 09, 2016, 11:18:24 pm
content: Couple things. Right now, I am mostly just focused on getting the noise out and picking up a recordable response from the ground. However, I really did not intend on a software based system right now and planned to simply capture the snapshots of the responses from the scope and graphing in Excel. If my understanding are correct, I should easily be able to see target zones through rise time and amplitude responses as well as time differences to estimate depth.Also, in considering the filters, I am wondering if by chance, my Behringer Feedback destroyer might function well as a filter for this? It is a rather slink rig I use for audio and I can target a bunch of frequencies, add the DB of cancellation, and slope. I should be able to dial right in on 60hz and kill it as well as another filter for any harmonics. Just wondering if this thing would just become another noise generator at these sensitivity levels?

author: edavid
author_rank: Super Contributor
timestamp: August 10, 2016, 12:07:44 am
content: Give it a try. It's probably not any more noisy than your oscilloscope.

author: viper
author_rank: Regular Contributor
timestamp: August 10, 2016, 12:41:48 am
content: Actually I am quite surprised! Took all of the 48db of attenuation I had but getting much closer!Basically I can dial in up to 12 different frequencies and push them up or down 48db. I am going to test without the EQ in circuit to see if I can detect any additional noise from the EQ. I think I will have problems with all the cables and such inducing their own noise. Just laying the scope on the ground picks up noise. Not sure how I will manage seeing less than a mV but we shall try.

author: bobaruni
author_rank: Regular Contributor
timestamp: August 10, 2016, 03:32:33 am
content: Can't you use a differential input circuit to cancel out the common mode line noise?

author: StillTrying
author_rank: Super Contributor
timestamp: August 10, 2016, 09:42:30 pm
content: Putting the scope,s 20MHz BW limit on, and averaging at least X16 would help to see what's there.

author: cvanc
author_rank: Frequent Contributor
timestamp: August 10, 2016, 10:50:14 pm
content: Quote from: viper on August 09, 2016, 06:53:13 pmPlease clarify. Exactlyhowdo you have your scope input(s) wired?Are both rods connected to the BNC center pin, and the outside of the BNC grounded?Are both rods connected to the BNC center pin, and the outside of the BNC not connected to anything?Is one rod on the BNC center pin and one rod on the outside of the BNC?Is this all on one single input BNC, or spread across two inputs?Some other answer?It might help to post a picture or two of your setup.

author: viper
author_rank: Regular Contributor
timestamp: August 11, 2016, 04:00:21 pm
content: Thanks for the reply!Right now, I am testing with only the center pin connected to one electrode, and the ground connected to the other, both on the same channel. I am using other channel with the piezo connected to it to use as a trigger and time constant.I have determined that with only the BNC lead on the scope and disconnected, the scope stays pretty constant at about 1.8mV of noise.I have some evidence that I could see responses as high as 2mV but it would be difficult to pinpoint through the 1.8mv noise. I am also looking into ways to improve the design or layout of the dipole antenna to better receive the response. We already know the frequency received would fall in the 10-200hz range but there can be no finite expectation of response frequency.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2016, 04:42:41 pm
content: The Scope is not that good to take the data, as it is high speed but usually low resolution (e.g. 8 Bit). It you are lucky the scope offers some kind of averaging / filtering as a high resolution mode or similar. Reducing the bandwidth also reduces the noise quite a lot.In principle something like an audio ADC would be more suitable - but the low frequency limit might be tricky. The good thing is that with 16 Bit resolution one can accept quite a high 60 Hz background and still get the signal after filtering in software. The data wanted for later processing is more like 16 Bits and 1-10 kHz sampling rate.Also the type of antenna / electrodes can be important. It also influences how to look at the signal. The antenna also influences the 60 Hz Pickup.You might need extra amplification, as the 1 M input impedance of the scope might be too low, and the scope input might not be really low noise in the range of interest. A twin T -filter can work against 60 Hz pickup, but this would also have quite an influence on the 50-70 Hz range, even in the active version with higher Q. So if possible I would try to get away without it.

author: viper
author_rank: Regular Contributor
timestamp: August 11, 2016, 05:10:28 pm
content: Thank you. Would there be a way to reconfigure the antenna to optimize it for this application? I know there are other configurations being employed by basically using 4 electrodes instead of 2 in which the 2 electrodes on the left would be wired in parallel or connected at their central point to one side of a scope channel, and the right hand side the same way connected to the other side of the scope channel.As I mentioned before, there is a similar process being used commercially using nothing but a cell phone to collect the data and I would expect that resolution and performance to be much less than any digital scope. They are, however, connecting the mic input to an audio output, effectively using the internal amp to boost the signal through a feedback loop with a couple series resistors.

author: C
author_rank: Super Contributor
timestamp: August 11, 2016, 06:20:03 pm
content: Think of how you are currently testing, one antenna.If you have two antennas you get two samples of your 60Hz. A difference removes most of this.With out your hammer and two antennas, Any signal in one could appear in the other antenna as a reduced and delayed copy.When your hammer hits and a short time after this changes.With digital recording starting pre-hammer and continuing past hammer echos, you can process to give best results for active time.Note that you will need 3 or more antennas to get 3D and would be good to have same time samples. Four in a square could make this easer.

author: viper
author_rank: Regular Contributor
timestamp: August 11, 2016, 06:57:18 pm
content: I did not quite understand your response. Are you envisioning multiple dipoles on separate channels? What I was referring to was effectively adding additional dipoles, all paralleled together to effectively boost the response.I am not super concerned about removing "all" noise, I simply need to be able to see the signal in that chaos.I am trying to figure out the best array of electrodes to yield the best possible response while limiting the noise picked up.

author: C
author_rank: Super Contributor
timestamp: August 11, 2016, 07:09:21 pm
content: Start your testing with two steel rods in the ground.Connect one to channel one of scope.Connect second to channel two of scope.Put scope in A+B, scope mode where channel one is added to channel two.Invert channel two.You now see the difference of two steel rods.How much of your 60Hz still exists?What is the signal you want?The difference of the two or the sum of the two?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2016, 07:10:53 pm
content: The mic input of the cell phone could be better suited (e.g. 16 Bit 40 kHz) compared to the scope (e.g. 1 GHz, 8 Bit).For a real measurement you likely want many channels, e.g. not just 1 or 2, but more like 30 or more. The "Hammer" tends to be relatively expensive compared to the recording electronics. Also looking at all the data is not that simple. There are a lot of possible effects in the ground. So this really is something for a professional who knows what to expect from the sensors and how to interpret the results.I don't see much sense in just adding more electrodes for more signal strength. A different configuration will also show different results. Also ready 2 electrodes give a kind of gradient. With 3 electrodes one might get the next derivative. So this will give different data, not just more signal. Signal strength is more a question of the size / depth of electrodes and amplification. So two electrodes close by might replace a larger / deeper one.Anyway today the recording is relatively cheap - so I would tend to record the data separate, as I don't know which combination is more useful. If need software can add afterwards. So I am more like particle physics: record everything and check for effects later.

author: viper
author_rank: Regular Contributor
timestamp: August 11, 2016, 08:33:10 pm
content: C, here is a snap of the A+B setup. It effectively cleaned up the signal but still a very present 60hz exists, but we might be getting closer.I am curious, not being experienced with the FFT modes of a scope, I have hamming, hanning, blackman, and rectangle modes which I am wondering might be able to average the signals for me a non inverted A+B mode to double the signal? Or is that out of the question?I should note that in testing, I have about 2mv of noise with leads on but not connected, but if I remove just the clipping tips and ground jumpers, I get about .6mv of noise. Once I connect to the ground electrodes, I get a pretty consistent 36-38mv of noise. However, my actual test area will be very remote so I am sure this noise would be reduced in that area. I am currently testing within 50ft of underground and OH low voltage electric lines. Probably not ideal. I may shoot out and test in a more remote location.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 11, 2016, 08:47:47 pm
content: The hamming, hanning, blackman FFT windows determine how much weight is given to data near the edges fo the FFT window. The rectangular window gives the least noise, but also the most artefacts due to the limited window length and the ends. Except for the rectangular window the difference is usually not that big.The FFT is OK to check for noise, but not very helpful for the later signal.Much of the noise still looks like higher frequency, like 100 kHz range from SMPS, lights or similar.

author: StillTrying
author_rank: Super Contributor
timestamp: August 11, 2016, 09:15:37 pm
content: Quote from: viper on August 11, 2016, 08:33:10 pmIf you used A-B the 60Hz would disappear. Average over 64 samples to reduce some of the other noise.

author: C
author_rank: Super Contributor
timestamp: August 11, 2016, 09:22:32 pm
content: If the 60Hz is still there then you have an amplitude difference or a phase difference or both.On your scopeIf you can get rid of the 60Hz by changing the volts per div setting then you have a 0 phase difference. Use the variable volts per div control.If some remains then you have a Phase difference.If you put the two steel rods in the ground on an east west line, think of what you get.To get a 0 phase difference, the 60 Hz must be equal distance from the rods. A disk shape north south.The phase will change to a max when on the east west line of your steel rods.This is assuming that the speed of 60Hz does not change between source and the two rods.Sound is a good example here.https://en.wikipedia.org/wiki/Speed_of_soundIf the speed of the signal you want changes like this then Phase & time delay is a problemFinding your water is not simple. To do this from just two steel rods is very difficult. Water 100 feed down is almost the same as 100 feet north or south.Your hammer is creating a expanding Sphere to create an echo.Time could tell you how big the sphere is if your signal does not act like sound. Dry dirt vs Wet dirt would make sphere expansion unequal based on what it passes through.

author: StillTrying
author_rank: Super Contributor
timestamp: August 11, 2016, 09:38:04 pm
content: Quote from: C on August 11, 2016, 09:22:32 pmNot an option on that scope.

author: viper
author_rank: Regular Contributor
timestamp: August 11, 2016, 10:18:12 pm
content: C, I don't quite follow on the V/div tuning but I obvious have the simple V/div knobs but only as certain divisions. I cannot select exactly what I want.My electrodes right now are N-S but I am headed out to change them. I will note that one commercial unit specifically specifies that these rods be placed E-W and I suspect it is to pronounce the noise so it can be filtered.I see exactly what you guys are saying about the phasing. I will head out and do more tests to determine if we can get the phase relationship to offset so I can cancel them. I will also go do testing in a remote area so I can see what stands out.As I might have mentioned, this testing is not evasive in trying to determine "what" exactly is below, or even exactly "where". The responses for now will be completely subjective in that I will only compare results of my test data with other sites in the same area. The only thing I can lean on right now is minimal responses likely means less defined layers and probably minimal water retention. More wave activity returned would mean we have more potential as sedimentary layers are most ideal. A sharp rise time means a defined response in that area.The speed of the P wave will have to be estimated based on the drill records in the area of the geology but I already know if we have a hitter, it will be between 30-80ft. In my controlled testing, I already know the geology and depth to water so I am hopeful that I can correlate my responses with the ground conditions to tune my methods. Obviously with a scope, I can easily see these timed responses with enough resolution.If I can lets say "comb" out much of the noise for now, is there an optimal way to use FFT or additive channels to improve the responses? I am concerned that if I do this wrong, I might filter out my responses. They will not be repetitive as a defined frequency, just a random potential.Also, I am wondering about triggering options and timing if I burn both channels on the antenna responses? I have never used the EXT trigger on a scope and not sure what it will handle? I am again using the piezo element so a quick sine wave and I have simply been tuning my trigger level based on that. The scope will not allow be to set a trigger higher than 600mV and I don't want to harm the scope. That piezo element is throwing a nasty spike that I put a schottky on it in parallel to unload it. It might work better with series resistors and just a cap.

author: C
author_rank: Super Contributor
timestamp: August 12, 2016, 12:10:12 am
content: Your scopeWould guess that is does not have the functions to do much.With out the variable gain control(variable volts/div) on a channel you can not balance out amplitude.You would need variable time delay between channels to do phase.What you have seen on your scope is that there are active ways of removing what you don't want from a signal.A good amp will Increase the signal wanted more then the noise it adds while keeping total signal less than some limit.Think of a bunch of people all talking in a room.What you are trying to do is to hear all the new talkers in a short time span.Find loudest one talking, remove from both inputs.From the result above find next loudest and remove.If you think about this, you have no idea what someone will say.Result with two inputs isTalker One has this aptitude difference and this phase difference.Each talker would be different.The more inputs you have the easer this can get but takes more processing power.On the digital side, you could have an output for each talker.To do this you are using what you know to assist finding more answers.You know that each talker can not be in same spot.For speed of sound you know it changes but might be able to compute good/better guess for the room from all the talkers.You know there is no P wave before you make it happen.A recording of before, during and after will give best chance.You can then guess how to remove the 60Hz during by looking at the before and after.Cross-correlationhttps://en.wikipedia.org/wiki/Cross-correlationThe more inputs you have, the more ways you can look to get answers.You need the extra bits of the sound card for aptitude difference.You need the extra samples of the scope for phase difference.What is the frequency range of your P wave?Might think of software to turn sound card into a scope.

author: viper
author_rank: Regular Contributor
timestamp: August 12, 2016, 01:27:03 am
content: I tested in a remote location with electrodes E-W. I have no detectable 60hz noise. Maybe some harmonics but all seem to be way up the range. My primary noise is 18mV p-p at 1 cycle per uV so 1Mhz, which I thought was an odd frequency being in the middle of nothing but.....Can an RC filter take care of this problem pretty easy? If I can trim some of the higher frequencies, I might be able to get somewhere.We have talked about an amp but no one has recommended one? Seems most just make them but I figure of there are some premades on proto boards or something, that might be just as easy. ? Or are we talking much more than an op amp?

author: C
author_rank: Super Contributor
timestamp: August 12, 2016, 06:12:18 pm
content: Did a little looking for youLooks like the signal could be in 170-900 Hz range.What I found the amplitude is in the PC microphone range.For this frequency range good sound recorders are better then scope.So record a signal then process the signal with something like Audacity on a PC.

author: jitter
author_rank: Frequent Contributor
timestamp: August 13, 2016, 05:36:38 am
content: QuoteMy daily work is in testing and calibrating a specific instrument that has to deal with the same challenges you are facing. Because of an NDA, I cannot divulge any details, but there is no harm in a global desciption that might point you in the right direction.The instrument operates in the DC-1 MHz range and can supply and sink currents while measuring the response. It can function as a voltage or current source and generate any signal shape, and is fully under the control of a PC with proprietary software running it. It can also just "listen", like a scope or DMM.The interesting part for you is its front end. It's a differential amplifier with two electrodes with an incredibly high input impedance. It's common mode rejection is very good, and I think that's what you should be looking at building/buying. Basically each electrode is buffered by a very high input impedance opamp (e.g. AD ADA4627) and the differential signal is amplified and filtered by a very low noise opamp (e.g. AD797).Quote from: viper on August 12, 2016, 01:27:03 amYes, but the scope itself will have multiple SMPS sections inside, maybe you're looking at a harmonic from them...QuoteIf it's coming from within? I don't know, I think you should experiment.Perhaps Dave can chime in? I seem to remember him having been working in a similar field... albeit on oceans...

author: KE5FX
author_rank: Super Contributor
timestamp: August 13, 2016, 07:05:25 am
content: This all sounds extremely woo-ish. Like any other form of dowsing, it probably only "works" because it's hard to prove that it doesn't.Even if the technique works, it will yield signals in the nanovolt to microvolt range. Your scope is useful in the millivolts to tens of volts range. You are going to need lock-in detection with a repetitive stimulus signal if you're going to pick up anything at all, I suspect. Talk to some physics profs and grad students about it, if possible, and be prepared to show peer-reviewed literature before they take you seriously.

author: htassell
author_rank: Unknown
timestamp: August 13, 2016, 07:47:30 am
content: Hi,As a geophysicist I've had a good deal of experience with most geophysical methods. Up front, I would say that electrokinetic method - which you are discussing, whilst does work in some circumstances, is certainly not the most reliable nor effective techniques at detecting the presence of groundwater.The principle of the technique is a little bit different from what you describe - the dipole array isnotan antenna it is a pair of galvanic electrodes that are measuring a change in potential (voltage) in the ground as a result of the compressional wave from the seismic source (typically a sledgehammer) causing a disruption to a phenomena known as Streaming Potential. Streaming Potential is a phenomena whereby the flow of groundwater with dissolved ionic species in solution through certain geological formations causes localized charge distributions which in turn generates a measurable 'self-potential' or voltage.The theory of eletrokinetic sounding techniques is that a sledgehammer or explosive source induces a displacement or movement of groundwater through the formation, with accompanying localized zones of charge build-up. If you stick your electrodes in the ground, you can measure a voltage or potential across this zone. This is a galvanic phenomena - you are not measuring a B-field or H-field. Equipment used to measure the phenomena is pretty similar to a multimeter - just a high input impedance AFE with an ADC. Typical potentials measured are in the 1 to 100's of mV.The main problem is that the ability to generate a measurable electrokinetic voltage is highly dependent upon the geology, the chemistry of the groundwater and whether there is any net of movement of the groundwater naturally. These factors tend to be orders of magnitude higher in the potentials they generate than the EKS response someone can generate from the surface with a sledgehammer.My advice in terms of trying to find groundwater would be to simply carry out a self potential survey - that is, just measure the self potential generated by groundwater movement naturally. The technique is simple and will give you an indication of where to look. Just stick one electrode in the ground and then with a roving electrode, measure voltage every x metres in an orthogonal grid. Then grid/contour the measurements into a map of self potential (it will be positive and negative). Where you have significant potential, this can provide an indication of the presence of water flow in the near surface.Ultimately if you want to directly measure the insitu % water content of the earth, NMR or nuclear magnetic resonance surveys are the best approach (provided there is no significant abundance of magnetite in your geology). Vista Clara make some of the best NMR systems.

author: dmills
author_rank: Super Contributor
timestamp: August 13, 2016, 10:44:57 pm
content: IIRC the proton NMR resonance in the earths magnetic field is usually about a kHz or so or so, depends on how far from the equator you are.Personally I would hook the electrodes to a hifi amp via a highpass filter and listen.