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Re: [gwmodel] negative values in resistivity survewy

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  • Rpete1220@aol.com
    Vijay, It is difficult to say for sure what is causing the negative values, but the problem is most likely arising from a combination of the highly
    Message 1 of 9 , Feb 27, 2006
      Vijay,

      It is difficult to say for sure what is causing the negative values, but the
      problem is most likely arising from a combination of the highly conductive
      coastal environment and the geometry of your electrode array. I don't have a
      lot of experience in coastal areas, but I know that highly conductive
      subsurface materials in other environments can cause negative resistivities. For
      instance, you can get negative readings if your potential electrodes are
      fairly close toghether. which array(s) are you using? If you are using
      schlumgerger array, what are your current and potential electrode spacings? Do you
      get negative values through the entire curve, or only in the deeper data?

      Ron Petersen



      [Non-text portions of this message have been removed]
    • vijay kumar
      dear sir, thanks for your comments. i used schlumberger array. for first few readings i didnt find any problem, but when i exceed 12 mts(outer electrode
      Message 2 of 9 , Feb 28, 2006
        dear sir,

        thanks for your comments. i used schlumberger array. for first few readings i didnt find any problem, but when i exceed 12 mts(outer electrode distance from the center) i get the negative values. i also changed the direction of the profile from east-west to north-south direction and i got the same neagtive values at approximately same depth. i'm using 1.75 mts electrode (both current and potential) as i could not send the current with standard electrodes. the reason i can see is the thick pilke of sand.

        i have a doubt whether the neagtive values could be the reason of salt water incursion.. any comments on this ... please

        As per your idea of discharging the polarisation, how can i do it? i also have an electrode at the cetre point which is earthed, so now how to nullify the polarisation effect?

        vijay


        Ayman Ahmed <ayman_ahmed02@...> wrote: I guess that negative values are refering to two things:

        1- Polarization problem, where you have to discharge the polarization before taking that reading.
        2- The potential direction is reversed at that certain point, so you have to exchange the position of the potential electrodes.

        I saw that situation in my surveys in areas far away from coast, in fresh water areas.

        Hope that helps
        Ayman


        Rpete1220@... wrote:

        Vijay,

        It is difficult to say for sure what is causing the negative values, but the
        problem is most likely arising from the highly conductive coastal
        environment. I don't have a lot of experience in coastal areas, but I know that
        highly conductive subsurface materials in other environments can cause negative
        resistives. I suspect a lot of the problem could be due to your electrode
        array geometry. which array(s) are you using? If you are using schlumgerger
        array, what are your current and potential electrode spacings? Do you get
        negative values through the entire curve, or only in the deeper data?

        Ron Petersen
      • Ayman Ahmed
        Dear Vijay, Try to reverse the potential electrodes if you got negative values (not the direction of profile). To discharge the polarization connect the
        Message 3 of 9 , Mar 1, 2006
          Dear Vijay,

          Try to reverse the potential electrodes if you got negative values (not the direction of profile). To discharge the polarization connect the electrodes without the current, you could do that manually if you imagine that you are inserting two electrodes and connecting them by a wire without any circuits or current. You should do that for C & P electrodes.
          I am surprised that you use 1.75 mrs electrodes, what kind of power supply you use, what's the output voltage from your instrument, do you use any kind of solution around the C electrodes?

          Please try and let me know what you got.

          Ayman
          vijay kumar <geovijay2005@...> wrote:
          dear sir,

          thanks for your comments. i used schlumberger array. for first few readings i didnt find any problem, but when i exceed 12 mts(outer electrode distance from the center) i get the negative values. i also changed the direction of the profile from east-west to north-south direction and i got the same neagtive values at approximately same depth. i'm using 1.75 mts electrode (both current and potential) as i could not send the current with standard electrodes. the reason i can see is the thick pilke of sand.

          i have a doubt whether the neagtive values could be the reason of salt water incursion.. any comments on this ... please

          As per your idea of discharging the polarisation, how can i do it? i also have an electrode at the cetre point which is earthed, so now how to nullify the polarisation effect?

          vijay


          Ayman Ahmed <ayman_ahmed02@...> wrote: I guess that negative values are refering to two things:

          1- Polarization problem, where you have to discharge the polarization before taking that reading.
          2- The potential direction is reversed at that certain point, so you have to exchange the position of the potential electrodes.

          I saw that situation in my surveys in areas far away from coast, in fresh water areas.

          Hope that helps
          Ayman


          Rpete1220@... wrote:

          Vijay,

          It is difficult to say for sure what is causing the negative values, but the
          problem is most likely arising from the highly conductive coastal
          environment. I don't have a lot of experience in coastal areas, but I know that
          highly conductive subsurface materials in other environments can cause negative
          resistives. I suspect a lot of the problem could be due to your electrode
          array geometry. which array(s) are you using? If you are using schlumgerger
          array, what are your current and potential electrode spacings? Do you get
          negative values through the entire curve, or only in the deeper data?

          Ron Petersen
        • Rpete1220@aol.com
          Vijay, it is still difficult to grasp your problem. Have you kept the potential electrodes at the same position all the time that you were expanding your
          Message 4 of 9 , Mar 1, 2006
            Vijay,

            it is still difficult to grasp your problem. Have you kept the potential
            electrodes at the same position all the time that you were expanding your
            current electrodes? What was that distance? What was the meter reading at that
            point? Once you got negative values, did you try to expand your potential
            electrodes at that point? Did you stop taking readings as soon as you got a
            negative value, or did you continue to expand the array to see if the values
            again became positive? It might help if you could send a real data sample,
            including AB/2, MN/2, meter reading.


            [Non-text portions of this message have been removed]
          • KRISHNAIAH CHEVVA
            Dear readers, conceptually, I can t imagine the validity of nagative resistivity. It is either resistive or conductive. Most of the conductive bodies also have
            Message 5 of 9 , Mar 1, 2006
              Dear readers,
              conceptually, I can't imagine the validity of nagative
              resistivity. It is either resistive or conductive.
              Most of the conductive bodies also have resistivity
              value greater than zero thai is in decimals. Most of
              the times ve also get -ve values. Immediately we
              change polarity of the potential electrodes or
              increase or decrease the current values. But, wherever
              we do survey, we should not get -ve values. It is not
              even direction dependent also. Direction dependence
              only shows the inisotropy. I strongly feel that there
              will not be any -ve value for any substance.
              Krishnaiah.

              --- Ayman Ahmed <ayman_ahmed02@...> wrote:

              > Dear Vijay,
              >
              > Try to reverse the potential electrodes if you got
              > negative values (not the direction of profile). To
              > discharge the polarization connect the electrodes
              > without the current, you could do that manually if
              > you imagine that you are inserting two electrodes
              > and connecting them by a wire without any circuits
              > or current. You should do that for C & P electrodes.
              > I am surprised that you use 1.75 mrs electrodes,
              > what kind of power supply you use, what's the output
              > voltage from your instrument, do you use any kind of
              > solution around the C electrodes?
              >
              > Please try and let me know what you got.
              >
              > Ayman
              > vijay kumar <geovijay2005@...> wrote:
              > dear sir,
              >
              > thanks for your comments. i used schlumberger
              > array. for first few readings i didnt find any
              > problem, but when i exceed 12 mts(outer electrode
              > distance from the center) i get the negative values.
              > i also changed the direction of the profile from
              > east-west to north-south direction and i got the
              > same neagtive values at approximately same depth.
              > i'm using 1.75 mts electrode (both current and
              > potential) as i could not send the current with
              > standard electrodes. the reason i can see is the
              > thick pilke of sand.
              >
              > i have a doubt whether the neagtive values could
              > be the reason of salt water incursion.. any comments
              > on this ... please
              >
              > As per your idea of discharging the polarisation,
              > how can i do it? i also have an electrode at the
              > cetre point which is earthed, so now how to nullify
              > the polarisation effect?
              >
              > vijay
              >
              >
              > Ayman Ahmed <ayman_ahmed02@...> wrote: I
              > guess that negative values are refering to two
              > things:
              >
              > 1- Polarization problem, where you have to discharge
              > the polarization before taking that reading.
              > 2- The potential direction is reversed at that
              > certain point, so you have to exchange the position
              > of the potential electrodes.
              >
              > I saw that situation in my surveys in areas far away
              > from coast, in fresh water areas.
              >
              > Hope that helps
              > Ayman
              >
              >
              > Rpete1220@... wrote:
              >
              > Vijay,
              >
              > It is difficult to say for sure what is causing the
              > negative values, but the
              > problem is most likely arising from the highly
              > conductive coastal
              > environment. I don't have a lot of experience in
              > coastal areas, but I know that
              > highly conductive subsurface materials in other
              > environments can cause negative
              > resistives. I suspect a lot of the problem could be
              > due to your electrode
              > array geometry. which array(s) are you using? If you
              > are using schlumgerger
              > array, what are your current and potential electrode
              > spacings? Do you get
              > negative values through the entire curve, or only in
              > the deeper data?
              >
              > Ron Petersen
              >
              >
              >


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