Single or dual axis? What's the difference?

Hi
If You thing of a reference system x, y and z, a single axis sensor will only sense in in the x direction, a dual axis sensor has to sensors and will sense to axes perpendicular to eachother i.e. x and y (often used for compassing oriented horizontaly). Triple axis sensor has three sensors and will sense all axes x, y and z (this gives you an opportunity to measure total field and adding tilt correction to your system).
Suggested search terms on the internet;
compass, tilt correction compass

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You are correct, two single axis sensors along the same axis are sufficient. If using two axis sensors you could achieve superior accuracy and bypass the troublesome work of manual precision alignment of the senors, via software. Two of the sensors would be for sensing the gradient and the two others as a reference for correcting the axial misalignment of the two sensors sensing the gradient. Normally a two axis sensor is used for compassing, but flipping two of those vertically could be a basis for a very well made gradiometer.


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Hello again.
I?ll try to answer you giving my best opinion. It would be nice to see if others have comments.
If you don?t intend to use 2 axis for alignment it would be a waste to have it. In the application notes for the speake sensors it?s commented how to compensate for a slight misalignment by setting up your search pattern in a consistent  way.
Picking the right sensor is a matter of what you are looking for, what are the engines made of? (steel, cast iron, aluminum  , ). The sensitivity of the sensor must be sufficient to pick up the gradient which the anomally (ie the engine) is giving in the local earth field and at the depth they are buried. (The gradient fall of at the rate cube root). So even if you set it up in a gradiometer setup, if the individual sensor can?t catch the (sensitivity) influence caused by your anomally (engine) at the expected depth and based on the materials it is built of, you have to go for a more sensitive sensor.


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 The engines are mostly aluminum but all have steel crankshafts that should give a good signal. Some on land some in water. Haven't got to thinking about the water mag yet. lol
 A few years back, a guy said that it doesn't matter how sensitive the sensor was because they are comparing the magnetic fields to each other (differential setup). Does this make since?
 What is your background? You sound like an engineer. I know just enough about electonics to be dangerous. I went to college to learn how to fix things, but when I graduated it was cheaper just to buy a new one. 
 I am starting a new post called $30 magnetometer. I hope you can comment on it too.

James

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About your question; A few years back, a guy said that it doesn't matter how sensitive the sensor was because they are comparing the magnetic fields to each other (differential setup). Does this make since?
It does not make sence! Or we could say it is to oversimplyfying it.
Imagine you are going to measure the differense in windspeed at location A and B. The sensitivity of the windspeed meters is so that they do not give an output for speeds under 1 knot, if the wind is blowing less than 1 knot you have nothing to subtract from eachother, ergo  no output (even though there is a draft in the air). 0 - 0 = 0

Hope I managed to clarify this for you.

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