The Way of the Geophysicist.

Publish or perish!

This is often the case in academia. The knowledge base is ever growing and keeping up can be quite a hassle.
Checking all the different websites for valuable information or the newest papers can be very time consuming. Fortunately, the early internet has brought forward RSS. It's an acronym for Real Simple Syndication, so basically you can get all the information from different websites really simple.

I've been meaning to write this article for quite a while and I wanted to recommend the Google Service - Google Reader. However, today the big G announced that they would discontinue the service¹. Everyone can pack their so called feeds of websites and go somewhere else. So why still write about it? All hope is gone! Well not really and even if you have never been into RSS feeds before, keep on reading this will change your view of the internet!

My first reaction after seeing the anouncement, I tried to install a RSS Reader on my own webspace. That failed miserably so I started looking around for alternatives and they delivered. There is a very extensive list on Wikipedia, but I almost died from information overload while browsing that thing. So let me sum up what I got out of it:
You can chose between the usual desktop program or cloud-based services like Google Reader.
For me this choice was pretty easy, I work on three different computers and have a smartphone, I need something that syncs all across those devices without problems. A desktop solution wasn't an option for me. When looking for cloud-based RSS services, I found that two stuck out.

There is Newsblur, which looks pretty much like Google Reader and will be fine.


However there was another option called Feedly.
It's more of a magazine style feed reader and I like the style very much. Especially for News feeds this can be a nice addition. They reacted very quick to Google Reader closing down and provided a guide for new ex-Google users that will show you through the import of your feeds. It's a two-click process then you're done! Just allow them to connect to Reader and that's it all your feeds are there.
For the true minimalists that liked the Reader design a lot, they can just use the "Tile View" that will give you the same plain Google Reader interface².


I used this little switch to Feedly to clean up my feeds and I would very much like to share some of them with you.
Some other bloggers published their lists of publications they follow^{3 4 5 6}. I will present some highlights from the list, however I have taken the freedom to compile files that you can simply import in your favorite reader. They're called OPML-Files and if you're not sure how to import them into your reader you can google it. I have uploaded five different files that include:
Continue reading "Maximizing your Geo-knowledge in minimum time!"

The basic concept of seismic is pretty easy.
We send sound into the ground and listen what comes back to the surface. There are a lot of sources we can use to make this sound. A very basic but effective approach is to use a sledgehammer and bang it on a steel cap on the ground. This works well to get some acoustic waves into the top layers of the subsurface, but there are even some ways to get images from the boundary between Earth's crust and the mantle. We need more sophisticated ways that get this amount of energy into the ground.


The easiest way to do this is to bust out the big guns and hammer more energy into the ground. We could call this the mythbusters approach.

But once we start to use a charge of explosives that is big enough something like this might happen.




We also have to consider that using explosives isn't exactly the green way to go. The environment will get damaged and it will obviously be harder to justify the use of explosives. However, if we can't use brute force there is another way to go. Instead of sending a big amount of energy into the ground at once we can send less energy over a sustained time.
This is a method called Vibroseis. A hydraulic system mounted on a truck sends vibrations into the ground. This vibration will usually last twelve seconds and change in frequency. I have attached a video of one of these trucks below. It's not too spectacular but don't underestimate that truck it weighs around 30 tons.

How does this give us any information about the subsurface?

All the time I have been talking about sound in the subsurface, but when we look at sound it is basically an acoustic wave. This wave travels through the ground just like our favorite piece of music travels from the speaker to our eardrum. The air that transports the sound of music is fairly homogenous, however the ground beneath us had 4.54 ± 0.05 billion years to form and a doesn't mix quite as well as air. There is quite a lot of history hidden under our feet. Over time the depositional patterns change and additionally we have processes like tectonics that cause unconformities in the subsurface. When an acoustic wave encounters a change in the subsurface part of it will be reflected and another part will be transmitted. The part that was transmitted will be reflected at another change in the subsurface and so on. On the surface we record the reflected waves with something called geophone and through thorough analysis, we may obtain an image of the subsurface. Just take a look at the picture of the grand canyon. You can easily identify different layers of rock just like seismics would in the subsurface.

Marine seismics

This works very well on land, but 71% of the Earth are covered by water and those trucks don't swim too well.
Fortunately water can transport acoustic waves as well, so we just need to find a suitable source to send sound into the water that travels down into the subsurface. This works quite well with something we call airgun. It injects a bubble of air into the water that collapses. This collapse will create the acoustic wave we need.
Listening for that wave is the next thing we have to solve. Ships create a lot of vibration, so we have to record the acoustic waves somewhere else. The listening devices are stored in something we call streamer that gets dragged behind the ship.

Seismics is pretty easy, when we boil it down to its core.
You send sound into the ground with some sort of source and then have devices that will record the reflected waves back at the surface.

This week a boat in Hamburg was carrying dangerous cargo.
It was an unexploded grenade that had been found on a building site. Now it might seem weird that this kind of cargo is being transported on a boat on the largest lake in Hamburg, but a time fuse made it impossible to defuse the bomb. The safest way to handle this, was to carry the grenade out into deeper water and explode it under controlled circumstances.



This is quite a problem in Hamburg and Germany in general, when there is a building site, it's quite common to find blind shells from WWII. It can be quite risky to just start building somewhere without knowing the subsurface.
So geophysics knows a solution to this problem¹⁰. Ironically, this solution was developed for the purposes of war.
When submarines were in vogue for sea battles, people obviously wanted some way to find those sneaky machines. Since they're built of metal there was an easy way to detect them using some form of magnetic anomaly detector. These could be used from an airplane to search the seas for a threat.

Grenades, bombs and submarines have one very important thing in common. They're made from metal.
When searching for blind shells we could just use these planes that were used before, right?
Unfortunately we cannot. Submarines are a little bigger than blind shells. Another "problem" is that geophysicists like to be paid for their work, so a lot of building sites use an "on-the-fly prospection", which basically means they start digging and have a look for unwanted surprises, instead of doing a proper inspection beforehand. This can be quite expensive when the entire building site has to be evacuated but many builders are willing to take the risk. However, some people take the precaution and this is how a magnetic measurements work.

We all know the comparison of the Earth's magnetic field to one of these old magnets with two poles. However, the global magnetic field underlies some variations over time. Every five years a global model is issued to account for the newest measurements. This is called the International Geomagnetic Reference Field (IGRF)¹¹

When a magnetic measurement differs from this IGRF we call this an anomaly, which could be an indicator for metal in the subsurface. But how do we even take this measurement? Continue reading "Dangerous cargo and magnetic prospection"