Two words - ACTIVE SOURCE! I have (temporarily) fled the realm of passive source seismology for the exciting land of active source seismology. What's active source seismology you ask? Good question - let's get into it.
Active source seismology employs the same same tools and techniques as passive source (think Earthquake) seismology but using man-made sound sources (i.e. airguns or explosions) rather than earthquakes. The beauty of active source seismology is that it allows us to investigate the Earth using (almost) exactly the parameters we wish. In traditional passive source seismology (admittedly ambient noise experiments are excused from this) one must wait for earthquakes to occur so that you can use the generated seismic waves to map earth structure. However, in active source seismology we generate the sources ourselves at the sizes and locations that we desire.
One common method of active source seismology occurs in the oceans where we use air gun arrays to produce high-pressure bubbles. These bubbles expand until the pressure of the surrounding water forces the bubbles to collapse powerfully hence causing a "bang!" and generating seismic waves that can travel hundreds of kilometers away. The airguns fire at set intervals depending on how far away one is recording the arrivals and also how fast you can re-pressurize the airguns - common intervals are between 12 and 150 m.
The arrivals from the airguns can be recorded on both onshore and offshore (i.e. ocean bottom) instruments as well as a towed seismic streamer. A seismic streamer is a cable made up of hydrophones that records the nearest offset arrivals from the airguns - though don't be fooled, streamers can be 10's of kilometers long!
Experiments that analyze records from seismic streamers are mainly concerned with seismic reflections - waves that reflect off of sharp boundaries in earth structure. Contrast that with experiments that analyze records from onshore/offshore instruments which are mainly concerned with seismic refractions - waves that bend back to Earth's surface as they travel through faster and faster velocity material. The principle that defines how waves interact at velocity boundaries (i.e. reflect vs. refract) is termed Snell's Law and is actually quite simple to understand (plus cool!) so definitely check it out if you're interested. In the figure above the waves labeled Pn and Pg are seismic refractions through the mantle and crust, respectively. the PmP wave is a wave that travels through the crust and reflects off the boundary between the crust and mantle.
Phew! That was a long introduction into what I've been up to and we're not even there yet! Hold on for just a little more - I promise the best is yet to come.
I'm primarily interested in the seismic refraction side of things because these seismic refractions can give exceptional information on crustal and even mantle structure over a large region - remember offset is everything! The further away your receivers that record the sources the deeper the depth penetration. Thus to obtain information about the lower crust and mantle we typically have to depend on onshore/offshore instruments placed at a far distance from the airgun source.
To analyze seismic refractions from the SEGMeNT active source experiment I've created receiver gathers for all of our lake bottom seismometers (LBS). This is where we take the continuous time series for each instrument that recorded all the airgun shots and cut it into segments at each time that we shot the airguns. By doing this we can track refractions through the layers of the Earth as they arrive at the instrument from different offsets.
Above is an example of a receiver gather. As always the axes are important!! The x-axis is the distance between the instrument and airgun, termed offset. The y-axis is reduced travel time which is the original time axis reduced by the offset of the trace divided by a set velocity, in this example 6 km/s. These axes, distance and time, thus produce plots where the slope of lines indicate velocity - whoa, we can already say something about earth structure!! The arrivals at the closest offsets and the shallowest slopes (shown by the blue arrow in the figure) are refractions though the sediments which travel at slow velocities (remember that velocity generally increases with depth in the Earth). The arrivals at further offsets and with an approximately straight slope (indicated by the green arrow) are refractions through the crust. Because the slope of the crustal arrivals are approximately horizontal we can assume that the mean velocity that they travel through is close to the reduction velocity we applied (i.e. 6 km/s).
By picking the first break (the first positive or negative pulse associated with a seismic arrival) of these arrivals on each and every trace within a given receiver gather we can begin to create an image of the Earth structure that these waves traveled through. In that way active source seismology is just another breed of passive source methods - see it's all coming together!
There's a lot more than we could get into about active source seismology but for now let this whet your appetite for broader world of seismology.
Until then go check out YouTube videos of airguns in action - they're pretty awesome!