Yesterday, I took a group of enthusiastic third year geologists and environmental scientists to the British Geological Survey in Keyworth for a tour of the facilities and discussion/demonstration of their geophysical equipment. The BGS staff did a fantastic job of entertaining and educating us all – thanks BGS! – and this post primarily provides back-up notes and further information about the geophysical surveying techniques we saw – with a cryosphere twist!
Why Use Geophysics?
If we want to find out what is underground, we must either dig down and look, or use geophysics to gather data that we can interpret. Geophysical surveys measure the properties of the earth’s subsurface, allowing us to visualise structures we can’t access or see directly. This is usually achieved by taking measurements at or above the Earth’s surface, eliminating the need to drill or dig. Theses methods are therefore referred to as “non-intrusive”. Non-intrusive surveying is usually far cheaper, far quicker and much less disruptive than intrusive methods such as borehole drilling and digging pits. Non-intrusive geophysical surveys may also be able to resolve structures and stratigraphies that are inaccessible by intrusive means, and cover a wider area in much greater resolution. The disadvantages of geophysical surveys include the need for very sensitive, specialist equipment and expertise in data analysis and interpretation, and the lack of physical samples for physico-chemical analysis.
What are the Main Geophysical Methods?
Ground penetrating radar uses the reflection of electromagnetic radiation to build an image of a material’s subsurface. A high-frequency radio wave is emitted from the device which travels downwards until it meets a boundary between materials of different density. Here, the wave is reflected and detected by a sensor at the surface. By using differently calibrated antenna, the device can be used to image to a wide range of depths and has been used to identify buried objects, rock and ice stratigraphies. The equipment usually comes mounted onto a lawnmower-like frame that can be wheeled across surfaces at a brisk walking pace and a lot of data can be gathered in a relatively small time, but this depends on the nature of the terrain – heavily crevassed glacier ice might be more challenging!
Everything has a gravitational attraction to everything else. The closer things are and the denser they are, the stronger the gravitational force exerted. This concept is exploited by gravity surveys to identify subsurface density variations. Gravity surveys measure minute shifts in Earth’s gravity due to the density of the subsurface. It does so using a weight on the end of a horizontal bar. This bar is suspended by a spring. The extension in the spring varies in different locations despite the weight remaining constant, as a result of changes in the gravitational force acting on it. This is measured by the number of turns of a sensitively calibrated wheel on the device required to return the weight to a standard starting position. This device is extremely sensitive and provides only relative measurements. It requires frequent recalibration against standards. These standards are found at known locations on Earth and require visitation with the device. There also several corrections that need to be made, including for the “free air” effect. This is produced by the decrease in gravitational pull as the distance from the centre of the earth increases. Next, the “Bouger correction” must be applied to account for the density of material raising the gravimeter above sea level. Both corrections act to reduce gravimeter readings to sea level, or in other words to account for the changes in gravity resulting from topography. Furthermore, there are variations in the centrifugal force generated by Earth’s rotation depending upon latitude. Therefore, a latitudinal correction must also be applied. The earth’s tides and instrument drift also need to be considered. Temperature would also have an effect, but most gravimeters come in temperature controlled housings. The remainder of the variance in gravity measurements is assumed to be due to subsurface density variations.
Further information can be found here: http://www.ga.gov.au/image_cache/GA2236.pdf
Seismic reflection is a commonly used method of determining subsurface density variations. It is conceptually simple – a seismic “shot” is created which sends elastic waves through the surface layers and also down through the subsurface mass until it reaches an interface between materials of different densities. Here, some of the wave energy is reflected back up towards the surface where it is recorded using a “geophone”. If the distance between the shot point and the geophones is known, the depth to the density boundary can be calculated from the time between shot and detection. However, this requires knowledge of the velocity of the seismic waves in the local medium, its variation with depth and the expected wave refraction. The result of seismic surveying is a “seismogram” – a record of seismic activity represented as a horizontal line with deviations in the vertical dimension representing seismicity (greater deviations = stronger seismic waves).
The type of shot used in seismic surveys depends upon the density of the medium being tested and the depth/resolution desired by the survey. Shots can range from tapping the surface with a hammer, heavy strikes with sledge-hammers onto high density rubber discs, mechanical impacts and dynamite blasts. The distance between the shot point and the geophones can also be manipulated depending upon the data requirements of the survey.
Seismic data is conceptually simple but can be tricky to interpret. Reflected waves have to be separated from direct compressional waves that travel horizontally along the surface, and also from background noise. This could be footfall, traffic noise, seismic noise from industry, mining, wildlife, geological activity and that produced by the seismic operators themselves.
See the video here for a deeper explanation:
Geophysics in the Cryosphere
Geophysical methods have long been extremely useful to glaciologists because Earth’s ice cover is thick, slow moving, and the subsurface is inaccessible. Understanding the structures inside and underneath ice masses can provide great insights into how they behave now, how they have behaved in the past and how they will behave in the future. It is through geophysical methods that we know how thick the Antarctic and Greenland ice sheets are, what the topography is like underneath them, where water is routed, the role of internal deformation, the location of subglacial lakes and the extent of isostatic rebound throughout the cryosphere. A particularly interesting book that details extensive geophysical surveying on the Greenland ice sheet in the mid 1900’s is “Venture to the Arctic”, edited by R.A. Hamilton. More recently, geophysical survey techniques have been adapted to image the calving fronts of ice shelves and marine-terminating glaciers in Greenland – see the BBC’s Operation Iceberg! The links and videos below provide just a few examples of geophysical applications in the cryosphere… and of course check NSIDC and NASA Earth Observatory for more information and stunning images!
Video: Subglacial ice sculptures
Video: Vintage Seismic Reflection
Enderlin et al (2014) Improved mass budget for Greenland ice sheet
Ferraro and Swift (1995) Measuring geophysical parameters of the Greenland ice sheet
Maurer and Hauck (2007): Geophysical imaging of alpine rock glaciers
Navarro (2014): Ice volume estimates from GPR
Palmer et al (2013): Greenland lakes detected by radar
Shean and Marchant (2010): Seismic and GPR surveys of Mullins Glacier