Seismic is essentially anything to do with the movement (vibration) of the earth. Tectonic plates float on the surface of the earth and are forever moving and crashing into each other. At the leading edge one plate is pushed under the other leading to mountain building on one side and marine trenches (subduction zones) on the other. These faults are called thrust faults. It is along these, particularly on the uplifted side, that the majority of the earth’s large earthquakes occur – eg the Pacific “ring of fire”.  Volcanoes are also likely to be created at these boundaries. The Nepal earthquakes are of this type.

On both sides of the tectonic plate lateral movement occurs where the plates are sliding past each other. In these locations the faults that are created are called transform faults (or strike slip) and are likely to build up stress over time and then release it suddenly ( eg San Andreas and Christchurch). The shaking associated with these can cause liquefaction where normally solid ground essentially becomes liquid for a time.

At the back of the plates extension zones are created, where fresh magma from deep in the earth rises to the surface and fills the gap. Iceland is located right over one of these zones and is growing in size due to the new magma.  The extension zone extends right down the middle of the Atlantic Ocean and is called the mid Atlantic Ridge.

Earthquakes can also occur within tectonic plates as these take up stresses from the boundaries. The recent 6.1 quake in central Australia was one of these.

As you can see from the map (Courtesy of Geoscience Australia) there has been movement on the Australian plate over the last week and part of this stress is taken up in Central Australia. Volcanoes can also occur within tectonic plates. The current theory has it that the tectonic plates move across hotspots in the mantle and when a weakness is found the magma reaches the surface. This gives the linear formation of volcanoes over time ( eg east coast of Australia)

Earthquake Size and Measurement

Earthquake size is measured on a base 10 logarithmic scale so that a 7 is 10 times the amplitude of a 6. The original scale was the Richter Scale and this is still the basis for modern scales. The currently used scale is the MMS (Moment Magnitude Scale) as it better identifies large earthquakes. The amount of damage that an earthquake does is determined by its location ( ie in a built up area), its depth, the type of movement (particularly at the surface) as well as its magnitude. Occasional tsunamis caused by earthquakes can do massive damage.

Seismic energy is recorded on accelerometers, just as you have in your modern phones (download the seismometer app for a view). Where these vibrations move through the air they are called sound and travel at about 330 m/sec. In water they travel at about 1500 m/sec (depending on temperature and pressure). Submarines can hide in thermoclines (rapid changes in temperature).  In the solid earth they travel at about 2-5000 m/sec for the Primary wave (compressional wave) with a Secondary Wave (Shear Wave) travelling about 60% of the P wave velocity.  Studying the arrival of these two waves at various recording stations allows the location of earthquakes to be calculated.

We hear about the big earthquakes but in reality earthquakes are very much a continuous event. We have thousands of earthquakes recorded down to about 1 on the MMS scale each day in most areas of the world. As your (iphone) seismometer app shows, it is easy to record a light tap on a desktop. Daily natural events create these earthquakes including plate tectonics, landslips, water movement, ice movement, tidal movement etc.  These are supplemented by the many human activities including dam building, mining, vehicle movement, pumping in and out of bores (including fraccing), explosions and building.

Seismic Surveys

As seismic energy moves through the earth, it is reflected at an interface where the velocity of the rocks changes (and / or the density). For example velocity through near surface sediments may be about 2000 m / sec but may be up to 4000 m /sec through an underlying limestone.  This will create a massive reflection event on recorded seismic data.

We take advantage of this to map what is happening in sedimentary basins and this is the major technique used for oil and gas exploration. In a seismic survey a set of seismometers (geophones, accelerometers or jugs) are placed evenly along a line and a series of small explosions set off. The seismic activity is recorded and the massive amounts of data processed and interpreted to give a picture of what layers exist below.

These days the explosions have been replaced by truck mounted vibrating weights (vibrators) and the 2D data (ie a virtual slice through the earth) has been replace by 3D seismic by having the seismometers areally distributed. The only method that we have to accurately map deep sub surface faults is by recording seismic. We often do not have an accurate picture of faults below cities as no seismic surveys have been conducted. Seismic can easily be recorded along roads in built up areas with no damage but this is rarely done.  We probably should do more surveys through cities to assess the earthquake risk. Currently we map surface faults together with the occasional drill hole information.

Seismic data has improved greatly over time, particularly in its quality. Seismic data processing is a massive user of computing power and has used many hours of supercomputer time. Today we are more likely to have many thousands of connected CPUs to do the work.  Current seismic can be used to identify fluids as opposed to solids due to the change in velocity and due to S waves not travelling through fluids.

While we can make accurate maps of the subsurface these are generally in Time (the measured time for a wave to travel to a surface and be reflected back). Converting time to depth requires an accurate picture of velocity through the sediments. As this is highly variable it remains a major issue with seismic mapping accuracy.

Depth maps are used to identify areas where hydrocarbons are likely to be cooked up (deeply buried shales and coals) and where the gas and oil may have moved to and become trapped. These targets may be incorrectly mapped in depth, may leak hydrocarbons into other structures and mapped fluids may be water (with some dissolved gas). Until a well is drilled, the presence of commercial hydrocarbons is just conjecture.  These conjectures are more often wrong!

Seismic can safely be recorded over land with little environmental effect. While seismic lines were once built like roads, these days they have little to no dozing and are generally completely rehabilitated within a short time. A seismic crew is often the first group of vehicles into an area and are often the first road builders. The French Track and QAA across the Simpson Dessert in Australia are old seismic lines. QAA was the original seismic line recorded by Delhi Petroleum in Queensland (line AA). The French Track is named after the seismic line recorded by Total, the French oil company.

Offshore, marine seismic uses hydrophones towed behind seismic boats with airguns as the energy source. Airguns use the explosive release of compressed air. The effect on marine life is minimal, no more than general shipping. Surveys are not recorded when whales are nearby (although there have been no indications of distress from whales or other marine life).  Marine surveys can be done over large areas, with any disturbance being short term.

While seismic surveys record deep layers in the earth, they are generally not focussed on the near surface. Detailed surveys can be recorded for this purpose but this is expensive. The ocean floor can be mapped by Side Scan Radar or Echo Sounders aboard ships. Land surface can be mapped from satellite imagery. Electrical surveys can map shallow sediments in areas with resistivity changes. Gravity surveys can provide low resolution deep within the earth (measure changes in density).  Magnetic surveys can be ground based or airborne and measure magnetic susceptibility changes within the earth (and are used to identify areas of mineralisation in hard rock areas).

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