Undergrad Behaviour of Magnetic Fields Underwater

[sjr]

TL;DR

Expected decay rate and speed of magnetic field changes underwater.

I've been trying to pick up the fundamentals of electromagnetic theory and I reckon I'm doing alright, but I've run into a some questions about how magnetic fields behave in an anisotropic fluid medium like the ocean. There is an awful lot of conflicting information online. Ultimately, I'd like to be able to detect magnetic field anomalies at a range of up to 100m under the water as accurately as possible.

A couple of questions:

1. The rate at which the strength of a magnetic field decays with distance remains somewhat unclear to me. I understand there are near field and far field implications, and in general the field strength will decay at a cubic rate. Can someone offer me a clear explanation of the expected rate of magnetic field decay with distance in a medium like the ocean, and how it's affected by conductivity, permittivity and permeability, and so on? This part I haven't figured out yet.

2. The 'propagation' of magnetic field changes in the ocean. Consider a basic problem. A point source $O$ lies in the ocean and directly ahead of it there is a sensor $A$ a range $r$. We then activate a magnetic field at $O$. Assuming it is capable of doing so, how long will it take the sensor $A$ to detect a resultant change or 'front' in its vicinity? I'm consistently reading very different answers to this question.

If you could offer me any guidance on this I'd really appreciate it :)

 

[berkeman]

Welcome to PF.

I've been trying to pick up the fundamentals of electromagnetic theory and I reckon I'm doing alright, but I've run into a some questions about how magnetic fields behave in an anisotropic fluid medium like the ocean.

Why do you say that you think that conductive seawater is anisotropic with respect to magnetic fields? And are you expecting a strong interaction, or a weak interaction?

https://svs.gsfc.nasa.gov/12450

 

[sjr]

I should have worded that line differently, I wasn't specifically referring to magnetism.

I'm interested in how physical characteristics of seawater may cause magnetic fields to behave slightly differently in the ocean than they would in free space. I'm not making any assumptions at all right now, I'm just trying to make sense of the all the conflicting stuff I'm reading.