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ABSTRACT
This paper traces the origins of common geophysical techniques and their developments for use in the UK. It is recognised that most techniques for ground surveying have their origins in the US oil and mineral industries, but were developed in the UK for use in archaeological surveying. Systems and techniques invented for archaeological purposes now have varied use in the UK, from environmental to engineering applications.
Keywords: geophysics, ground investigation, archaeology, surveying, Britain
1. INTRODUCTION
Geophysical techniques for ground surveying grew around the desire for cheaper methods of discovering subsurface characteristics, as an alternative to borehole drilling and excavation. As with many technologies, there was a boom in development during and after the World Wars. Although borne primarily from the search for oil and minerals in the US, geophysical techniques now have many applications in ground surveying, from detecting contamination and leaks in environmental studies, to finding archaeological remains and relict engineering sites.
2. ORIGINS OF TECHNIQUES
Seismology
The subject of seismology has a long history in earthquake measurements and investigating the internal structure of the Earth, but the shallow (crustal) investigation using human-generated seismicity was largely developed by the oil industry in the United States in the 1920s. The idea that time-distance graphs of artificial earthquake waves could be used to infer the variation of velocity with depth was proposed in the late 1800s, but it wasn’t until 1923 that the first commercial application of the refraction method was used, and the first oil deposit found in 1924. Seismic reflection was proved very useful just a little later, at the beginning of the 30s, and soon overtook refraction as a popular oil prospecting method.
Gravity
The fact that gravity varies slightly at different locations was discovered in 1672 by Jean Richer (Jakosky, 1949). ‘Gravimeters’ went through many changes over the next couple of centuries, but the first practical field instrument for geophysical exploration was designed and tested by Eötvös at the turn of the 20th Century. It was again in oil prospecting that surveying took on a commercial purpose, as several people began to correlate anomalies with anticlines and domes. The first oil prospecting using gravity was carried out by the Marland Oil Company in 1925 (Eckhardt, 1940).
Magnetometry
Magnetic methods of ground investigation also have a long history, beginning with the discovery of the inclination of the compass by Hartmann in 1544. Using the compass to look for magnetic anomalies began around 1640 when it was used to look for iron ores in Sweden. Prospecting began in earnest in the late 19th to early 20th Century in the US, over iron and gold placer deposits, and took off with the invention of the Schmidt field magnetometer in 1915.
Electrical methods
Electrical conductivities of rocks were tabulated in 1720 by Gray and Wheeler. The application of the phenomenon of spontaneous polarization of rocks to ore body location was theorised in the 1800s, and the first commercial use was in 1913 by C. Schlumberger. Methods using the principle that subsurface bodies with high conductivity affect conductivity measured at the surface were tested on copper and iron areas in the US at the end of the 19th Century. It became clear that the subsurface structure influenced current distribution at the earth’s surface, and electrical methods began to see use in oil prospecting and civil engineering.
Electromagnetic (EM) techniques
EM techniques were largely developed by the ore industry specifically to look for sulphide prospecting in the base metal industry. The ground EM technique was developed in Scandinavia, the USA and Canada in the first half of the twentieth century (Sheard et al., 2005). Airborne EM (AEM) methods were developed in Canada in 1948, and in 1954 the first deposit discovery attributed to an airborne EM survey, in Canada, was announced.
3. DEVELOPMENT AND USE IN THE UK
In the UK, geophysical methods have not been used to find the majority of mineral and oil wealth. Most major deposits and associated mining activities have significant pasts. Discoveries in the North Sea oil field were mostly made by correlating offshore geology with the geology of Northern Britain and Europe that were already known to yield natural gas. However, around 700,000 miles of seismic surveys were shot in the whole of the North Sea between 1962 and 1973, of which about 380,000 miles were in the United Kingdom sector, and about 40 per cent were shot before 1968. Although poor in quality, they did reveal some important features (Fig 1)
The major use of emerging ground surveying techniques then was in the field of archaeology. The first archaeological geophysical survey in Britain was conducted by Atkinson in 1946 (Atkinson 1953). He used a resistance technique to discover a low-resistance anomaly associated with a ring ditch of a henge. The use of geophysical surveys grew quickly, in the consequences of a post-war Britain. These were that there was now a wealth of aerial photography that could be searched for possible sites and the rush for development and agriculture that threatened such sites (Clark, 1997).
Advances in geophysics in other areas led to the adoption of new techniques in UK archaeology. The principle of proton free precession in the earth’s magnetic field (Packard and Varian 1954) led to the development of field magnetometers. An instrument developed at Oxford University formed the basis of the first magnetic archaeological survey (Fig 2). The innovation was again led by external factors – the
A1 road was being improved, the route taking it close to a known Roman site. The proton magnetometer successfully detected, among other features, a kiln at a site at Water Newton (Aitken, 1958). Kilns were the first features detected since they were permanently magnetised when heated above the Curie point. Iron artefacts may also be detected in this way, but the most common survey features are now ditches and pits, which accumulate soils with increased magnetic susceptibility. Human enhancement of susceptibility is primarily heat derived, through cooking, furnaces, pottery making etc.
The 1960s and 70s then saw a rapid growth in the development of archaeological prospection, especially with the growing sophistication of digital computers and the availability of transistorized electronics (Linford, 2006). In 1964 the fluxgate graviometer was invented, which drastically reduced the time needed to complete surveys by providing continuous reading in operation (Alldred, 1964).
Instrumentation continued to develop throughout the 1980s, as did interest in geophysical surveying. From archaeological and physics roots at Bradford University, Roger Walker established Geoscan in 1984, with the purpose of designing instrumentation. Today, the company still operates, designing instruments and software with its branch in the US, and providing equipment to the environmental, forensic, geological and civil engineering sectors. The value of mapping the subsurface was quickly realised, and it has become an integral part of planning policy in the UK, as of the early 1990s. Geophysical surveys are required to locate, map and classify remains before new development construction can begin. This has led to a surge in commercial geophysical surveys that were once the curiosity of archaeological bodies.
4. MODERN USES
Today in the UK, geophysical surveys are used for a range of applications, including environment, engineering and hydrology. Many of the same principles developed in the US oil rush and the UK archaeological exploration periods are still used, with improvements in processing, resolution and instrumentation.
In one engineering example from 1998, an electromagnetic survey revealed the remnants of oil depositories in Scotland supposedly cleared out in the 1980s (Hobbs and Vickery, 1998). Searching for buried parts of old developments have now become essential prior to new developments to avoid unforeseen issues in clearing and construction. UK advances in hydrogeology include electrical resistivity tests to determine the infiltration in the vadose zone (e.g. Hatzichristodulu et al. 1999).
However, one principle new technology has emerged over the past 30 years that has proved very effective in mapping shallow subsurface features such as bedrock depths, cavities and voids (including pipes and tunnels), ice thickness and stream channels, making it useful to a number of geophysical industries. Although pulsed techniques were developed in the 1930s, it wasn’t until the ‘space race’ of the late 60s and early 70s that ground penetrating radar (GPR) experienced a surge in popularity due its advantages over seismic investigations (namely that remote transducers of the radiated energy could be employed, rather than the contacting types used for seismic surveys (Daniels, 2004)). It has since become a widely used technique in the UK. Archaeology first used it to image a crypt in Worcester, and although in a very different discipline, was used in a similar context to locate grave sites at the home of Fred West in 1994. Ground penetrating radar was used already in 1987 to isolate leaks in the Frankley Reservoir, Birmingham, which had been leaking considerable quantities of water.
5. CONCLUSIONS
Although many fundamental techniques were developed for commercial use by the oil and mineral industries in the US, development in the UK was largely due to interest in finding and documenting archaeological remains. Emerging geophysical principles were applied to archaeological interests, with such success that techniques and instruments first developed for archaeological surveys are now used in a range of industries, from engineering to environment, hydrology and forensics.
