Hey guys, welcome back! This time I’m back with a bit of a more behind-the-scenes science-oriented blog post, but bear with me through it cause it’s actually super exciting. This will help you gain an understanding of what it is that we are recording with our receivers. Even if you just skim this article (which is totally okay! Any attempt to expose yourself to science is great in my book!) it will hopefully help at least a little with your understanding of what we are trying to do.
When you hear the terms “electric field” or “magnetic field”, it’s easy to mistakenly think of them as completely separate things. In reality, they are essentially two sides of the same coin: electromagnetic fields. When you have a magnetic field that varies with time, it generates an electric field that can induce electric currents in conductors (this is how an alternator in a car works to keep the car battery charged while we drive, and also how generators at powerplants work to produce electricity). The same holds in reverse: a changing electric field generates a magnetic field. This means that one creates the other and vice versa in an unending cycle that we call “electromagnetic waves” (also known as “light”). Similarly, when an electric current is flowing in a conductor, it also generates a magnetic field. For example, if you were to place a compass near an electric wire, the needle would be deflected by the motion of electricity through the cable. This relationship between electricity and magnetism is how we end up with the phrase electromagnetism. Basic electromagnetic principles are fundamental to a huge amount of our modern society including power generation, WiFi signals, cell phone transmission, etc.
On Earth, electromagnetic principles also drive important geophysical phenomena. For example, deep inside the Earth, there is a liquid outer core where molten iron (a highly conductive fluid) experiences convection. The convecting molten iron is essentially a moving electric current which then generates Earth’s magnetic field. This is often represented as a huge bar magnet through the center of Earth with a geomagnetic field that surrounds our planet.
This geomagnetic field allows us to navigate with a compass and also protects the atmosphere from essentially being blown away by the gusts of charged particles that the sun is constantly emitting, which is called the “solar wind.” It also protects us from being bombarded by harmful electromagnetic radiation, which is very, very, very bad for living organisms, including us humans.
As it just so happens, at this very moment (February 27th) we are in the midst of a large “geomagnetic storm”. This means our geomagnetic field is currently being pushed by a stronger-than-usual solar wind force, thanks to a solar flare from a sunspot. A beautiful side effect of this will be an increase in auroras, because of the strength of this storm they will be visible farther south than usual, including in the northern portions of the continental United States.
Why is this important for our method?
So, given all the above, let’s put this all together to explain what we are measuring on the Nicoya Peninsula using the magnetotelluric method. When the solar wind (i.e. a stream of charged particles) pushes against our geomagnetic field, it changes the geomagnetic field by a tiny amount, and—as I mentioned earlier—a changing magnetic field creates an electric field! Electric fields create magnetic fields create electric fields everywhere!
So what does that mean for us and the instruments we have collecting MT data right now?
Well as it turns out, the time-varying geomagnetic field is the very source of energy that powers the magnetotelluric (MT) method! This energy source can induce electric currents, in the same way that we can charge our smartphones without connecting them to a physical cable. However, since the ground is relatively conductive while the atmosphere is highly resistive, no current is induced in the air, only in the ground beneath our feet. No matter where you are, there are always extremely weak currents flowing beneath your feet!
Our MT instruments record the naturally occurring electric and magnetic fields over time. By measuring these fields, we can create an image of the electrical conductivity structure below the surface and how it varies spatially and with depth because electrical currents preferentially flow through conductors. The larger the perturbations to the geomagnetic field, the more signal we have and the easier it is to constrain the conductivity structure. Therefore, geomagnetic storms are a good thing for MT, since it provides us with a massive signal boost that improves the quality of our data. This helps us to get more information out of the data to peer deeper inside Earth.