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Monday
Aug292011

Geophysical stamps 4: Seismology

This is the last in a series of posts about some stamps I bought on eBay in May. I don't collect stamps, but these were irresistible to me. They are 1980-vintage East German stamps, not with cartoons or schematic illustrations but precisely draughted drawings of geophysical methods. I have already written about the the gravimeterthe sonic tool, and the geophone stamps; today it's time to finish off and cover the 50 pfennig stamp, depicting global seismology.

← The 50 pfennig stamp in the series of four shows not an instrument, but the method of deep-earth seismology. Earthquakes' seismic traces, left-most, are the basic pieces of data. Seismologists analyse the paths of these signals through the earth's crust (Erdkruste), mantle (Mantel) and core (Erdkern), right-most. The result is a model of the earth's deep interior, centre. Erdkrustenforschung translates as earth crust surveying. The actual size of the stamp is 43 × 26 mm.

To petroleum geophysicists and interpreters, global seismology may seem like a poor sibling of reflection seismology. But the science began with earthquake monitoring, which is centuries old. Earthquakes are the natural source of seismic energy for global seismological measurements; Zoeppritz wrote his equations about earthquake waves. (I don't know, but I can imagine seismologists feeling a guilty pang of anticipation when hearing of even quite deadly earthquakes.)

The M9.2 Sumatra-Andaman earthquake of 2004 lasted for an incredible 500 seconds—compared to a few seconds or tens of seconds for most earthquakes felt by humans. Giant events like this are rare (once a decade), and especially valuable because of the broad band of frequencies and very high amplitudes they generate. This allows high-fidelity detection by precision digital instruments like the Streckeisen STS-1 seismometer, positioned all over the world in networks like the United States' Global Seismographic Network, and the meta-network coordinated by the Federation of Digital Seismograph Networks, or FDSN. And these wavefields need global receiver arrays. 

The basic structure of the earth was famously elucidated decades ago by these patterns of wave reflection and refraction through the earth's fundamentally concentric spheres of the solid inner core, liquid outer core, viscous mantle, and solid crust. For example, the apparent inability of the outer core to support S-waves is the primary evidence for its interpretation as a liquid. Today, global seismologists are more concerned with the finer points of this structure, and eking more resolution out of the intrinsically cryptic filter that is the earth. Sound familiar? What we do in exploration seismology is just a high-resolution, narrow-band, controlled-source extension of these methods. 

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