magnetometer

[charlie23]

Hello, I want to ask some questions about magnetometer. I show you as a sample thsi graphic:




I want to know about the "nT". As you can see on the graphic, there is an anomally, and the spikes go up and then down. As you can see, at some points the spikes are into a green color, then there is a huge spike in a red color at the end, and then a spike that it goes down, in a blue color.

Why is that, why does it go up and then down? what is the diference between the positive and the negative "nT"?

Also, can you know the size, the weight and how much buried the anomally is by checking this 3D graphic?


Thank you guys

 

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[stevemc]

A magnetometer can only pick up iron. So you could assume the bigger spikes are larger pieces of iron, such as cannons, or anchors, in a pile, and the smaller spikes are singles of those or spikes or straps. The whole area is worth looking at when you find spikes like that.

 

[bronzecannons]

The nT stands for 'gammas'. The higher the gammas, the larger (or closer) the mag unit is to the 'target'. Perhaps, depending on the gamma strength and area, some targets may even produce 'negative' gammas. Here I've attached a reading from one of our SeaSpy mag surveys. You can see that the negative readings spiked so high that they even went off the chart and came back again.

Because of the very 'short/quick in time/distance' gamma readings there, I would say that those two main targets are 100% positive but either small in size (perhaps one small iron target and the 60 g is a small anchor?) or your mag unit is still not close enough to the actual target(s) - meaning, you could perhaps let more cable out or slow down the boat speed (or both) so it can be closer to the bottom to see if the gamma readings increase or not.

TW

 

[Salvor6]

Tony is right. nT means "nano Tesla" which is a measure of gammas of magnetic intensity. I have seen that software that records the intensity of the magnetic field. It tells you how much iron is present in your hit. It also tells you how many pounds the hit weighs but not how deep it is. It goes up and down as you pass over the object.

 

[inletsurf]

Magnetic fields are vector based, having a magnitude and direction. Key word being direction.

 

[charlie23]

Thank you for all the answers.

I just want to know another thing. Magnetometer do not pick up gold or silver, only iron objects.

Is the magnetometer, nowadays, the only way to actually locate a buried wreck, I know that the side scan sonar is one of them, but are there more? or is the magnetometer the better option?

Mostly treasure hunters want to go for the gold and the silver, but the magnetometer doesnt detect them, so why do they still buy them, do treasure hunters locate an anchor or a cannon and go from there, blowing wholes until they find some treasure?? Isn´t there a way to find the actual treasure more quiclkly?

 

[Dinkydick]

Charlie23

First let me tell you that operating a mag does not make you a professional at
analysing the results obtained.
I would suggest that you obtain a copy of "Applications Manual for Portable Magnetometers" pages 14 through 58 or a book written by Mr. Anthony Clark
entitled " Seeing Beneath the Soil". The article I believe is still available at the
Geometrics website free. I know that it is being sold on EBay. These two references will give you a better ability to actually see what the magnetometer
is telling you. Then you can determine what your next project will encompass
regarding your find. Just remember that it's all "dipoles and pipes" terms that
I don't think you are aware of.
I am not trying to be hyper-critical of your efforts but just would like to help
you out.
I design and build magnetometers non of which are for sale.

 

[Au_Dreamers]

You pretty much have it correct.
The ferrous items on ships are much larger than the precious metals so you can detect them from a greater distance. So in a “search” aspect you don’t have to be so close.

Your area of detection is so much larger even if you were using a large coiled metal detector. If the objects are buried one could miss them with a sonar unit.

One must also take into consideration of why/how the ship sank. You could have a large scatter field or a compact area or something in between depending on the conditions that caused the ship to wreck.

 

[bronzecannons]

Yup, like Au_Dreamers confirmed with you, that's correct.

One other piece of equipment that could be used to locate a buried wreck would be a sub-bottom profiler. This would only work in the case that the wreck is not dispersed because you would have a good chance to see the buried cargo and/or ballast stones. Keep in mind also that the sub-bottom profiler doesn't work like a side-scan sonar whereby you're using high frequencies (125 up to 500 or more Khz) can adjust the range up to a few hundred yards/meters to each side of the boat. The sub-bottom profiler is using a very low frequency (5 Hertz or even lower) to penetrate the seabed and has a very narrow swath so is pretty much looking straight down below the boat.

So, once you have located the mag target area and if there's no trace of any wreck there on the seabed, perhaps utilizing a sub-bottom profiler would show you what's buried there. Keep your line spacing very tight (no more than 5 yard/meters) and go over the wreck in both east/west and north/south directions for the best results.

TW

 

[Desy5]

Hello! I'm not an expert in all this yet, but I'm interested in such questions. I esteem your opinions, I think it will come in handy.

 

[Photo24]

I have been following this for years and have a homemade mag. I was wondering if their is a line of lower priced subbottom profilers or any other approach to the problem. Also wonder can a sub profiler be use on the beach near the water? Is there a combo GPR/subprofiler?

 

[seekerGH]

For a novice search, the sidescans are the best, and least expensive option. Mags will only find large iron on the novice level you are looking at. You will likely find an anchor with a SS before you find it with a mag...

The sub-bottom profiler is using a very low frequency (5 Hertz or even lower) to penetrate the seabed and has a very narrow swath so is pretty much looking straight down below the boat.

Sub-bottom profilers are multi-frequency (and are not chirp, there is no such thing as chirp) they range from around 500Hz to 24 kHz, and most are dual band. ie 4/24 or 2/16 depending on bottom conditions. Some high end models broadcast 3 freq, but isolating the freq/retun doppler is difficult and expensive.
Lower frequencies tend to penetrate further, but with lower res, while higher freq show higher res and less penetration.
ie in coarse sand,
500hz/12kHz will go 20m/200, (but with 20cm resolution)
2kHz/16kHz will go 6m/80m, (10cm res)
and 4kHz/24khz will go 2m/40m...(8cm res)
its all about the res and what you are looking for.