VLF interpretations from real and imaginary plots

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deepamk1989
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VLF interpretations from real and imaginary plots

Post by deepamk1989 »

Hej all,

I am confused with interpreting graphs of VLF. From what i read high values for real part and high value for imaginary (either on the same side or opposite side) shows it to be a good conductor. Is there any value range for it to be a natural anomaly? If the site is contaminated with industrial waste, will this anomaly be also shown as a good conductor. If so what would be the range? Let me know if somebody can guide me with this as i got exam in two days :(

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geophix
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Re: VLF interpretations from real and imaginary plots

Post by geophix »

Can you show some plots you are trying to interpret? Your question is not clear.

deepamk1989
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Re: VLF interpretations from real and imaginary plots

Post by deepamk1989 »

The site is contaminated with industrial wastes. The original soils consist of till (1‐2 m) on top of hard crystalline bedrock, but the soils nowadays are mainly coarse filling material (sandy‐gravelly composition). The water supply in the village is today based on a drilled well, located west of the village. There is a significant risk for pollution of the existing well since it is located in a fracture zone which stretches towards the industrial site.
1) In order to select location for a new well drilling site VLF‐measurements were made in east of the village. The profile stretches in orientation east‐west, crossing one minor valley. The resulting VLF‐graph is shown below.
Mark the anomalies that you can see in the graph and discuss them with relation to:
‐ what geological structures can result in such anomalies
‐ the strength of the conductors
‐ the depths and the dip of the conductors
Attachments
old industrial site, which probably has contaminated the soils with phenols and heavy metals
old industrial site, which probably has contaminated the soils with phenols and heavy metals

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geophix
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Re: VLF interpretations from real and imaginary plots

Post by geophix »

Since the data is filtered at 30m depth, the anomalies shown in both In-phase and Quadrature (in the left portion of the plot) would indicate relatively deep hence natural conductors such as fracture zones filled with water. The big anomaly shown in In-phase but not in Quadrature (to the right of the plot) probably indicates a conductor shallower than 30 m; so it might be caused by wastes. That's my guess anyway. Here is the document which may help you to interpret data: http://www.guidelinegeo.com/wp-content/ ... -guide.pdf

deepamk1989
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Re: VLF interpretations from real and imaginary plots

Post by deepamk1989 »

Thanks a lot for the reply and the guide. As per the guide the left one is a sure indication of good conductor since the inphase and quadrature are almost equal and of opposite signs. The right one with big anomaly also looks promising to me as you said. I also feel right has to be from conductor less than 30m seeing the anomaly height. Is it possible for very good conductors like the industrial waste with heavy metals to have quadrature anomalies near to zero as in the right one (big anomaly)?

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geophix
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Re: VLF interpretations from real and imaginary plots

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I don't know whether you have the raw data before filtering. I guess the quadrature amplitude of the raw data should be relatively significant for the big anomaly. It's only after the filtering at 30m it becomes almost flat. If you have the raw data and filter it at a smaller spacing, the quadrature amplitude could be significantly different from the values filtered at 30m spacing.

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Re: VLF interpretations from real and imaginary plots

Post by deepamk1989 »

Sorry no raw data available. Thanks a lot for your help :)

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Re: VLF interpretations from real and imaginary plots

Post by deepamk1989 »

Hello again, In an EM survey what are could be the reasons for getting a strong anomaly in quadrature field but not in phase? Instrument has multiple receivers and it showed response only in first two receivers. My guess "it is because the conductors are shallow which made it undetectable for the last receiver". But why just it showed showed quadrature anomaly not in phase?
Thanks :)

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geophix
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Re: VLF interpretations from real and imaginary plots

Post by geophix »

In EM (ground conductivity meter) survey, in-phase data are much less sensitive to variations of conductivities than quadrature data. Normally the in-phase data is almost flat unless there are metallic objects present.

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