Diurnal & other variations - Compensation required???

[rick]

hi carl-


thanks responce--almost did same (email) to


randy on pi unit--perhaps people want a more


immediate responce to ?/statements/etc than


a forum provides...seems he want to do a pi


as proscan #2---ok fine--but still have not


heared any say how well proscan works---or


my memory faulty...seems that someone said


a little disapointed--not sure---a lot of guys


respond on many issues and me keep track all----


you keep track on one--me......without go back and


re-read all here and erics too.....maybe another


board is to be specifically at proscan #2/vlf/coils/


finds/etc. to try to localize subject matter a little


closer...anyway -115db----me go do vlf now


check back later


-rick-

[rick]

Re: -120db

hi carl-


thanks responce--almost did same (email) to


randy on pi unit--perhaps people want a more


immediate responce to ?/statements/etc than


a forum provides...seems he want to do a pi


as proscan #2---ok fine--but still have not


heared any say how well proscan works---or


my memory faulty...seems that someone said


a little disapointed--not sure---a lot of guys


respond on many issues and me keep track all----


you keep track on one--me......without go back and


re-read all here and erics too.....maybe another


board is to be specifically at proscan #2/vlf/coils/


finds/etc. to try to localize subject matter a little


closer...anyway -115db----me go do vlf now


check back later


-rick-

[Dale Seppa]

Diurnal & other variations - Compensation required???

As I read them, the sum total of the excerpts following are as


follows:


1. If you use a "magnetometer" (single sensor) you must be


VERY aware of diurnal variation and you must:


a. Do your survey between the hours of 10 in the morning


to 2 in the afternoon, according to Wolff.


b. Do your survey early in the morning, late in the after-


noon, or better yet during the middle of the night


accord- ing to Heylmun.


2. If you use a "gradiometer" (multiple separated sensors)


you


+:


a. Need to pay virtually no attention to diurnal variation


acccording to Koehler and Dalton.


b. are dealing with "reduced" interference and presumable


still have to make some allowance for diurnal variation


according to http://www.digistar...


Item "1." above may be moot depending on whether we select "a." or


"b." in item "2." above but at the very least I am wondering


how two people who are both experts by my standards can be in


such marked disagreement on such an important point. I have


not yet started to study overall magnetic variations so cannot


come to my own conclusions on this so would appreciate any in


put from anyone.


Item "2." above, where I personally have a tendency to agree


with Koehler and Dalton, is really very important. Many of


the practical considerations for a given survey may vary con-


siderably depending on one's conclusions.


"In the gradient method, the total field is measured simulta-


neously at two elevations by using two sensors on one staff


separated by a fixed distance. The difference in magnetic in-


tensity between the two sensors divided by the distance between them


is the vertical gradient. This technique reduces interference from


solar magnetic storms and regional magnetic


changes. This technique is particularly useful for locating


small, shallow objects and also is useful for estimating burial


depth of objects." (http://www.digistar.mb.ca/minesci/finding/


magnetic.htm)


"Magnetic storms are periods during which the earth's field var-


ies widely and rapidly. The earth's field on a "quiet" day un-


dergoes a small smooth variation known as the diurnal


variation. It is on such a quiet day that the magnetometer


surveys must be made during the time least likely to be dis-


turbed, the period of two or three hours before and after noon.


The earth's field is influenced by particles emanating from the


sun, and since the same spot on the sun points to the earth


about every 27 days, there is a tendency for quiet or disturbed


conditions to repeat every 27 days." (Wolff)


"In a gradiometer, there are two sensor heads. Local time


variations due to upper atmospheric currents will cause the


same change in both sensors; both will increase or decrease


essentially the same amount because the cause of the change is very


far away. The difference between the sensors will be due


to magnetic material in the vicinity of the two sensors alone."


(From - PROTON PRECESSION MAGNETOMETERS BY Jim A. Koehler.)


"Surveys are best done in the early morning hours and in the


late afternoon. They would probably be even better if done


during the middle of the night, as the sun is all the way be-


hind earth and would have the least effect upon the readings at


that time." (From: USING A PROTON MAGNETOMETER by Edgar B.


Heylmun, Ph.D.)


"Whether 2 or 4 sensor are used, magnetic storms, micropul- sations


and diurnal drift effects are cancelled at the gradiometer's] input


transducer." (From M. L. Dalton promo-


tional material.)


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