Magnetic Methods Used In Prospecting

[Carl]

>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>Can one build a better detector than what can be purchased commercially? For underwater use?


>I would love to build a detector but don't know where to start.


A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


- Carl

[Carl]

Re: To Build or Buy?

>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>Can one build a better detector than what can be purchased commercially? For underwater use?


>I would love to build a detector but don't know where to start.


A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


- Carl

[Carl]

Re: To Build or Buy?

>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>Can one build a better detector than what can be purchased commercially? For underwater use?


>I would love to build a detector but don't know where to start.


A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


- Carl

[Carl]

Re: To Build or Buy?

>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>Can one build a better detector than what can be purchased commercially? For underwater use?


>I would love to build a detector but don't know where to start.


A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


- Carl

[Bill]

Re: To Build or Buy?

>>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>>Can one build a better detector than what can be purchased commercially? For underwater use?


>>I would love to build a detector but don't know where to start.


>A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


>- Carl


Carl, we have been building and using underwater


camera housings with strobes and such for over 35


years. Water proofing is not a problem today. I


agree that PI is the way to go, also getting back


to the "click" type of indicator, rather than the


continuous tone is much better for UW work.

[Bill]

Re: To Build or Buy?

>>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>>Can one build a better detector than what can be purchased commercially? For underwater use?


>>I would love to build a detector but don't know where to start.


>A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


>- Carl


Carl, we have been building and using underwater


camera housings with strobes and such for over 35


years. Water proofing is not a problem today. I


agree that PI is the way to go, also getting back


to the "click" type of indicator, rather than the


continuous tone is much better for UW work.

[Bill]

Re: To Build or Buy?

>>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>>Can one build a better detector than what can be purchased commercially? For underwater use?


>>I would love to build a detector but don't know where to start.


>A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


>- Carl


Carl, we have been building and using underwater


camera housings with strobes and such for over 35


years. Water proofing is not a problem today. I


agree that PI is the way to go, also getting back


to the "click" type of indicator, rather than the


continuous tone is much better for UW work.

[Bill]

Re: To Build or Buy?

>>Hi. I am new to this forum and do not have a terrible amount of electronics experience under my belt.


>>Can one build a better detector than what can be purchased commercially? For underwater use?


>>I would love to build a detector but don't know where to start.


>A pulse induction unit, maybe, but not a target I.D. VLF type detector. PI is what you want for underwater, but making everything waterproof is another challenge.


>- Carl


Carl, we have been building and using underwater


camera housings with strobes and such for over 35


years. Water proofing is not a problem today. I


agree that PI is the way to go, also getting back


to the "click" type of indicator, rather than the


continuous tone is much better for UW work.

[Carl]

Re: To Build or Buy?

>Carl, we have been building and using underwater camera housings with strobes and such for over 35 years. Water proofing is not a problem today.


Maybe you can offer some info on how to make or where to get good waterproof housings. I have no knowledge in this area. Also a source of waterproof sound transducers.


>I agree that PI is the way to go, also getting back to the "click" type of indicator, rather than the continuous tone is much better for UW work.


Yes, I agree.


- Carl

[Carl]

Re: To Build or Buy?

>Carl, we have been building and using underwater camera housings with strobes and such for over 35 years. Water proofing is not a problem today.


Maybe you can offer some info on how to make or where to get good waterproof housings. I have no knowledge in this area. Also a source of waterproof sound transducers.


>I agree that PI is the way to go, also getting back to the "click" type of indicator, rather than the continuous tone is much better for UW work.


Yes, I agree.


- Carl

[Carl]

Re: To Build or Buy?

>Carl, we have been building and using underwater camera housings with strobes and such for over 35 years. Water proofing is not a problem today.


Maybe you can offer some info on how to make or where to get good waterproof housings. I have no knowledge in this area. Also a source of waterproof sound transducers.


>I agree that PI is the way to go, also getting back to the "click" type of indicator, rather than the continuous tone is much better for UW work.


Yes, I agree.


- Carl

[Carl]

Re: To Build or Buy?

>Carl, we have been building and using underwater camera housings with strobes and such for over 35 years. Water proofing is not a problem today.


Maybe you can offer some info on how to make or where to get good waterproof housings. I have no knowledge in this area. Also a source of waterproof sound transducers.


>I agree that PI is the way to go, also getting back to the "click" type of indicator, rather than the continuous tone is much better for UW work.


Yes, I agree.


- Carl

[Dale Seppa]

Magnetic Methods Used In Prospecting

MAGNETIC METHODS from SUGGESTIONS FOR PROSPECTING, U.S. Department


of the


Interior/Geological Survey, USGPO, 1978.


Magnetic prospecting is based on the natural magnetic properties of


some minerals


such as magnetite. When held near a magnetite-rich rock, the needle


in a compass


behaves erratically because the Earth's magnetic field is distorted


by the local


magnetic field. Minerals such as ilmenite (iron-titanium oxide),


hematite (iron


oxide), and pyrrhotite (iron sulfide), are weakly to moderately


magnetic, a prop-


erty that can be recorded by sensitive magnetic instruments.


The common unit of measure for the strength of a magnetic field is


the gamma


[1gamma = 1 nT - das]. Where not disturbed by highly magnetic


rocks, the strength


of the Earth's magnetic field in the conterminous United States in


1975 ranged


from a low of about 48,000 gammas in Texas and Florida to a high of


60,00 gammas


in Minnesota.


Instruments called magnetometers are used for direct detection of


magnetic


anomalies (that is, the distortion of magnetic minerals in crustal


rocks superim-


posed on the Earth's magnetic field). The magnetic readings over


weakly magnetic


host rocks may depart from local average, or "background," values by


10 to 500


gammas, [Heylmun says: "The typical anomaly ranges from 10 to 1,000


gammas and the


stronger anomalies warrant attention." - das] but over magnetic iron


formations


the readings may depart from background by 100 to 100,000 gammas.


The magnetom-


eter can also be used to trace concealed rock formations that have


magnetic prop-


erties differing from those of adjacent formation. For example a


prospector may


know that copper is associated with an igneous rock such as quartz


monzonite, if,


as is often true, the quartz monzonite differs noticeable in


magnetic response


from the surrounding rocks, the magnetometer can be used to detect


it beneath


soil, talus, or other cover. Similarly the "black sand" of placer


deposits com-


monly contains grains of magnetite or ilmenite that affect the


magnetometer. This


instrument, therefore, can be used indirectly in the search for gold


or other


heavy minerals present in the black sand.


Some years ago the dip needle, a simple instrument using a pivoted


magnetized


needle, was often used to measure the magnetic field when great


sensitivity was


not required. Dip needles are no long manufactured, [??-das] as


various types of


more sensitive magnetometers have replaced them. These modern


magnetometers can


measure directly the total intensity of the Earth's magnetic field


at any one


place or can measure the intensity in either the vertical or the


horizontal direc-


tion of the field.


In a torsion magnetometer, a small magnet is attached to a fiber in


such a ways


that the magnet twists the fiber in proportion to the intensity of


the magnetic


field as measured in the vertical direction. The fluxgate


magnetometer is an


electronic device that uses a magnetic core that becomes "saturated"


in the


Earth's field to measure the strength of the field in the vertical


direction.


The basis sensing element for a proton magnetometer consists of a


container filled


with a proton-rich liquid such as water [Everybody now says


DISTILLED (with no


dissolved oxygen per Jim Koehler) water - das] or kerosene


surrounded by a coil of


wire. The protons within the liquid are subatomic particles that


spin about rota-


tional axes somewhat like the Earth spins on its axis, completing


one turn per


day. The frequency with which the spin axes of the protons wobble,


or "precess,"


after being excited by a current passed through the coil, is


directly related to


the strength of the Earth's field. This frequency is measured and


converted into


readings in units of gammas. The proton magnetometer measures the


total intensity


of the Earth's field rather than the intensity in the vertical or


other direction.


Many commercial magnetometers range in price from US$1000 to


US$5,000. [1978


prices - das] The less expensive ones are accurate to only about 20


gammas or


less. The most expensive are accurate to about 1/4 gamma.


Magnetic surveys may be conducted either along a series of lines or


in a grid pat-


tern. The size of the area being prospected and the type of deposit


being sought


determines the spacing of station. Stations spaced 10 to 20 feet (3


to 6 meters


approx.) apart [Jim Koehler mentions 1 meter spacing, I believe and


for placer de-


posits, I THINK I may agree with that - das] may be required to


locate small mag-


netic anomalies associated with weak or moderately magnetic rocks,


but stations


spaced 100 feet (30 meters approx.) or more apart may suffice if the


presence of


highly magnetic rocks is suspected in a large area. Powerlines,


rails, automo-


biles, and other large metallic objects should be avoided in any


type of magnetom-


eter survey because the create strong local magnetic fields that


mask the


anomalies inherent in the rocks.


Today most magnetic surveys are airborne or marine and use


total-field detecting


systems rather than measuring either the vertical or horizontal


component of the


Earth's magnetic field. These surveys provide comprehensive


reliable data about


regional magnetic trends. Ground magnetometer surveys are still


used however, to


locate anomalies from small subsurface structures.


End of "Magnetic Methods".


"Where geologic evidence indicates the possible presence of an


orebody, magnetom-


eter measurements may help discern anomalies likely to represent


valuable mineral


deposits."