Special issue on waterborne geophysics in Near Surface Geophysics
Guest editors Karl E. Butler and Luigi Sambuelli together with editor-in-chief Ugur Yaramanci report on the special issue on waterborne geophysics being published in the August 2009 issue of Near Surface Geophysics.
Water has been defined a ‘strategic resource’ and many analysts foresee that it will be the subject of conflicts in the future. Its increasing value, concerns about quality and quantity, industrial/agricultural versus public allocations, and the likely impacts of climate change have recently given rise to a geo-political way of looking at water resources and to coin the term ‘hydro-policy’.
Worldwide, 214 river or lake basins, populated by 40% of the world’s human population and covering more than 50% of the Earth’s land area, are shared by two or more countries. India and Bangladesh share the Ganges; Mexico and the USA the Colorado River; the USA and Canada share the Columbia, Red and St. Lawrence Rivers; Egypt, Ethiopia, Sudan, and Uganda the Nile; Switzerland, France, Germany, and the Netherlands share the Rhine while the Euphrates is shared by Turkey, Syria, and Iraq; China, Myanmar, Thailand, Laos, Cambodia, and Vietnam share the Mekong and many other examples can be found throughout the world. The same holds for some of the largest lakes on Earth including the Great Lakes of North America, the Tanganyika Lake in the East African Rift, and the Aral Sea in Central Asia. Bodies of fresh water, like the coasts of seas and oceans are attractors for human settlement, a fact that is immediately apparent from satellite photographs taken at night. As a result, knowledge of shallow water environments is a topical subject of dramatically growing importance.
Climate change, through the greenhouse effect, is giving more energy to the water cycle engine so that the definition of an extreme event will probably have to be revised. Heavy rains and consequent floods are likely to be more frequent than in the past and will be exacerbated by sea level rise. Improved knowledge of the connections between surface waters and groundwater will allow better management of water resources and can also contribute to emergency management and disaster planning.
Knowledge of sea, lake, and riverbed sediments are also important for a variety of other societal needs. Civil engineering projects, such as the construction of pipelines, bridges, and ports, and the dredging of harbours have obvious requirements for information on subsurface geotechnical properties and hazards. Information on sediment transport is important for assessing the environmental and hydrographic impact of floods as well as bridges, dams, and other structures designed for coastal protection or hydroelectric/wind/tidal power generation. The oil and gas industry requires information on shallow subsurface conditions relevant to drilling hazards and pipelines, while the mining industry recognizes vast seafloor mineral resources ranging from sand and gravel to massive sulphides and placer diamonds.
Returning to environmental applications, geophysical methods are increasingly being used to help assess how groundwater-surface water interactions are influencing water quality and supply. As one example, information on the hydraulic connections between rivers and adjacent well fields can be inferred from geophysical imaging of the riverbed. Such information can be used to help position wells in regions where they are less likely to yield ‘groundwater under the direct influence of surface water’ (i.e., a GWUDI designation). In other areas, electrical resistivity and EM induction techniques are contributing to efforts to mitigate the effects of saline groundwater that is threatening rivers and lakes. They are also being used to identify submarine discharge of fresh groundwater in coastal zones, which can influence marine ecosystems either positively or negatively (e.g., through fertilizer-driven eutrophycation of estuaries adjacent to agricultural areas).
In this special issue of Near Surface Geophysics we present a subset of the papers that were presented at a workshop entitled ‘High-resolution Geophysics for Shallow Water’ held in London, UK on 10 June 2007 in association with the 69th EAGE Conference and Exhibition. We do not pretend to give an exhaustive analysis of the geophysical methods that can be applied in shallow water surveys (seismic methods, although presented during the workshop, have not been collected here). Rather, we aim to show some interesting experiences around the world in this field.
Our intent, encouraged by Near Surface Geophysics, which kindly accepted to host a ‘bouquet’ of the papers presented in the workshop, is to make a broader geophysical community aware of the potential of well-known methods in less well-known environments and fields of application. The full set of papers presented at the workshop are accessible through the EAGE’s online publications database EarthDoc by selecting the Event titled ‘69th EAGE Conference and Exhibition – Workshop Package’ and entering ‘Wo2’ for the name of the Session.
Readers will find an illustration of the trends in waterborne electrical and EM induction methods for high-resolution sub-bottom imaging. A broad panorama of methods and environments, ranging from the well-field scale to long river stretches and sea-bottom surveys, is shown with a rich list of references. They can also read about multi-offset GPR methods for investigating hyporheic exchange processes (i.e., stream–streambed water cycling) in temperate and cold environments. In this paper detailed and accurate GPR data acquisition and processing allow the estimation of streambed porosity variations important for hydraulic and thermal modelling.
On a much larger scale, an airborne EM survey along about 700 km of streams in the upper Colorado River region is presented. Two days of airborne surveying, integrated with ground EM data and geochemical sampling, allowed a better comprehension of the origin of anomalous salinity values in some stretches of the river.
A paper on resistivity imaging presents a case study from Australia, where towed resistivity and EM induction methods are becoming common tools in assessing connections between rivers and aquifers, including natural saline groundwater that threatens sources of irrigation and drinking water. In this case, continuous resistivity imaging is used to identify the stretches of a perched river that contribute recharge to an underlying semi-confined aquifer.
Readers will also find a more theoretical work dealing with uncertainty propagation using some common mixing rules. This paper investigates the uncertainty that can be expected when, for example, from the bulk conductivity obtained from an electrical tomography, one wants to estimate the porosity of a saturated geological material.
Geophysics on shallow water is a growing field, destined to become more varied in response to evolving applications and more common as experimental approaches give rise to purpose-built commercial instruments. The accompanying figure clearly shows the increase of interest during the last 35 years, in just one of the non-seismic techniques suitable to study shallow water environments. It is our opinion that this field of application will require further improvements in acquisition, processing, and interpretation tools to be able to tackle the challenges presented by growing populations seeking water, energy and mineral resources exploited in an always more sustainable way.
(This article is published in First Break August 2009)
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