Back in May I bought some stamps on eBay. I'm not really a stamp collector, but when I saw these in all their geophysical glory, I couldn't resist them. They are East German stamps from 1980, and they are unusual because they aren't schematic illustrations so much as precise, technical drawings. I have already written about the the gravimeter and the sonic tool stamps; today I thought I'd tell a bit about the most basic seismic sensor, the geophone.
← The 35 pfennig stamp in the series of four shows a surface geophone, with a schematic cross-section and cartoon of the seismic acquisition process, complete with ray-paths and a recording truck. Erdöl and Erdgas are oil and gas, Erkundung translates as surveying or exploration. The actual size of the stamp is 43 × 26 mm.
There are four basic types of seismic sensor (sometimes generically referred to as receivers in exploration geophysics):
Seismometers — precision instruments not used in exploration seismology because they are usually quite bulky and require careful set-up and calibration. [Most modern models] are accelerometers, much like relative gravimeters, measuring ground acceleration from the force on a proof mass. Seismometers can detect frequencies in a very broad band, on the order of 0.001 Hz to 500 Hz: that's 19 octaves!
Geophones — are small, cheap, and intended for rapid deployment in large numbers. The one illustrated on the stamp, like the modern cut-away example shown here, would be about 4 × 20 cm, with a total mass of about 400 g. The design has barely changed in decades. The mean-looking spike is to try to ensure good contact with the ground (coupling). A frame-mounted magnet is surrounded by a proof mass affixed to a copper coil. This analog instrument measures particle [velocity], not acceleration, as the differential motion induces a current in the coil. Because of the small proof mass, the lower practical frequency limit is usually only about 6 Hz, the upper about 250 Hz (5 octaves). Geophones are used on land, and on the sea-floor. If repeatability over time is important, as with a time-lapse survey, phones like this may be buried in the ground and cemented in place.
Hydrophones — as the name suggests, are for deployment in the water column. Naturally, there is a lot of non-seismic motion in water, so measuring displacement will not do. Instead, hydrophones contain two piezoelectric components, which generates a current when deformed by pressure, and use cunning physics to mute spurious, non-seismic pressure changes. Hydrophones are usually towed in streamers behind a boat. They have a similar response band to geophones.
MEMS accelerometers — exactly like the accelerometer chip in your laptop or cellphone, these tiny mechanical systems can be housed in a robust casing and used to record seismic waves. Response frequencies range from 4–1000 Hz (8 octaves; theoretically they will measure down to 0 Hz, or DC in geophysish, but not in my experience). These are sometimes referred to as digital receivers, but they are really micro-analog devices with built-in digital conversion.
I think the geophone is the single most important remote sensing device in geoscience. Is that justified hyperbole? A couple of recent stories from Scotland and Spain have highlighted the incredible clarity of seismic images, which can be awe-inspiring as well as scientifically and economically important.
Next time I'll look at the 50 pfennig stamp, which depicts deep seismic tomography.
Thank you to @hypocentre for the enlightening comments about these instruments. I changed some details in the text (in the square brackets) to reflect them.
In researching this question, I came across Michael Hons' MSc thesis. Michael was part of the CREWES project at the University of Calgary, and I've seen him talk a coupe of times—I should have looked at it before! At the beginning of his dissertation, he gives a nice comparison of geophone (i.e. coil) and MEMS physics. There was one surprise (for me): MEMS accelerometers actually measure displacement, but because operational frequencies are so far below the resonant frequencies of the tiny proof mass, displacement is conveniently proportional to, and only instigated by, acceleration. So under normal circumstances, they are de facto accelerometers.
Hons' thesis, Seismic sensing: Comparison of geophones and accelerometers using laboratory and field data, goes on to give a full comparison of geophones and MEMS and is well worth looking at if you deal with these instruments.