If you have spent most of your life studying audiology and sound properties, it is quite possible that you will find this post of interest.  In our studies of sound and its uses, most of us do not think about how it can be used to map various characteristics under the surface of the earth or below the sea.  That is our topic this week at Hearing International

Ludger Mintrop (1880-1956) was a German mine surveyor and geophysicist who is considered the inventor of the seismic method for exploration of hydrocarbons and minerals through the seismic refraction method (Patented in 1916).  Born in the Northwest German city of Essen in the 1880, he used his method in WWI to find allied artillery and later to find salt mines and other underground structures and won many accolades and awards for his seminal work in seismic prospecting.  While your fish finder  in that new bass boat uses a variation of sonar technology, Mintrop’s seismology uses some interesting applications of sound propagation to find things and has now become the primary tool of exploration companies in the continental United States, both onshore and offshore. Seismic surveys can help locate ground water, land fill sites and are famous for characterizing how the land will shake in an earthquake but are primarily used for oil and gas exploration as well as research in some parts of the world. Seismic surveys are conducted by creating a shock wave, or a seismic wave, on the surface of the ground along a predetermined line, using an energy source.  The seismic wave that travels into the earth is reflected by subsurface formations and returns to the surface where it is recorded by receivers called geophones a device similar to microphones. Basically, it is a device that converts ground movement (velocity) into voltage, which may be recorded at a recording station. The deviation of this measured voltage from the base line is called the seismic response and is analyzed.  Typically, these seismic waveforms are created either by small explosive charges set off in shallow holes (“shot holes“) or by large vehicles equipped with heavy plates (“Veibroseis” trucks also called thumpers) that vibrate on the ground. By analyzing the time it takes for the seismic waves to reflect off of subsurface formations and return to the surface, a geophysicist can map subsurface formations and anomalies and predict where oil or gas may be trapped in sufficient quantities for exploration activities or even map the bottom of the sea.

Seeing Through the Antarctic Ice Cap

Seismic surveys can be used to map the ocean floor underneath the ice cap in Antarctica.  Applications of these techniques are now being used to see under the ice to map the floor of the ocean with the use of a thumper truck (sometimes called a weight-drop truck).  As an alternative to the use of dynamite as a stimulus generator, the thumper technique was introduced in 1953.   Thumping is usually less damaging to the environment than firing explosives in shot-holes, and its especially useful in politically unstable areas where fracking has significant controversy.  A thumper truck is a vehicle-mounted ground impact system which can be used to provide a seismic source. A heavy weight is raised by a hoist at the back of the truck and dropped, generally about three meters, to impact (or “thump”) the ground. To augment the signal, the weight may be dropped more than once at the same spot, the signal may also be increased by thumping at several nearby places in an array whose dimensions may be chosen to enhance the seismic signal by spatial filtering.  These days, the more advanced Thumpers use a technology called “Accelerated Weight Drop” (AWD), where a high pressure gas is used to accelerate a heavy weight Hammer to hit a base plate coupled to the ground ufrom a distance of 2 to 3 m. Several thumps are stacked to enhance signal to noise ratio. AWD allows both more energy and more control of the source than gravitational weight-drop, providing better depth penetration, control of signal frequency content.

How Does it Work?

Antarctic researchers are interested in the looking at the geological history of the area by finding sediment on the sea floor so it can be sampled to monitor the changes in the future.  They are interested in how global warming is making changes in the polar ice cap that could affect the amount of water on our planet and cause major changes in coastlines around the world in the future. Looking at these sediments and other changes to the ocean floor can possibly predict, modify, slow down or otherwise prepare what may be to come from these changes.  How do you see what’s below the ice in Antarctica?  -Seismic measurements- Procedures and protocols are always, by necessity, a bit different in the land of the ice, snow and bitter cold that is Antarctica.  The application of seismic technique in the mapping of the Antarctic ocean floor requires similar but special modified seismic equipment.  In this environment, the Thumper truck is a snow cat that pulls a trailer containing the geophones.  The geophones are laid out is a special patter and the Thumper that generates the signal to the ocean floor.   The way it works is that as the thumper hits the ground, in this case the ice, and simultaneously the geophones are triggered to listen for the refractions, calculations are then conducted by computers.  In the warmth of the snow cat, researchers using special software will analyze the response and can see under the Antarctic ice for the structure of the ocean floor.  These seismic measurements allow them to map the research areas ensuring that there are no big rocks below the ice that can hamper accessing their research samples. 

Click on the Thumper picture (Left) for an interesting video and you will see why we say …………………

             Thumper is NOT a Rabbit!

 

References:

Malehmir, A., Urosevic, M., Bellefleur, G., Juhlin, C., & Milkereit (2012).  ”Seismic methods in mineral exploration and mine planning — Introduction.    

     Geophysicis, 77(5).  Retrieved April 12, 2017.

Exploration Instruments (2017).PEG-40 Accelerated Weight Drop Seismic Source.  Retrieved April 12, 2017.

Nolen-Hoeksema, R. (2014).  A beginners guide to seismic surveying.  Oilfield Review, 26(1).  Retrieved April 12, 2017.

Seg Wiki (2017). Ludger Mintrop. Retrieved April 12, 2017.

US Geological Survey (2014).  A Brief History of the use of sound in ocean exploration:  But first a few facts and definitions.  Retrieved April 10, 2017.

Images:

Disneyclips.com (2017).  Thumper.  Retrieved April 10, 2017.

Enjolras, J. (2013). Total pioneers “cable-less 3D seismic surveys in Uganda.  Retrieved April 12, 2017.

National Geographic (2017).  The Ice Thumpers. Continent 7:  Antarctica.  Youtube.com Retrieved April 11, 2017.

Niobrara News (2017) Thumper Truck.  Retrieved April 12, 2017. 

6th Parallel Map & Keywords (2017). Antarctica Map.  Retrieved April 12, 2017.

Videos:

National Geographic (2017).  The Ice Thumpers. Continent 7:  Antarctica.  Youtube.com Retrieved April 11, 2017.

 

 

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