Geophysics for Environmental Consultants

by management@detectation.com

Shallow electromagnetic (EM) mapping

During the Phase 1 Environmental site Assessment (ESA), the visual inspection and the document review of the target property and surrounding areas are performed. The purpose of Phase 1 of ESA is to identify potential environmental concerns and determine whether further actions (such as the Phase II ESA) are needed. The visual inspection and the document review have limitations. You can not see what's hidden underground and the property record (as much as you can find) can not show a complete history. A shallow EM mapping would be a very good way to make the Phase I ESA more thorough and still cost effective. The underground storage tanks, drums, pits, legal and illegal dumps, secrete burials, contaminant leaks and spills, could cause the change of the ground soil conditions including the electrical conductivity and presences of metals. The EM mapping provides a great tool to get a general base map of shallow underground conditions, find the traces/remains of past activities which could lead to important discoveries for environmental concerns. Recent development of geophysical hardware and software make the scanning much faster with minimal manpower and resources, and more practical for smaller sites with limited budgets. Depending on the site specifics, 2 to 5 acres or even more can be scanned in one day by one geophysicist. And the initial results/maps can be obtained in the field at the end of the day.

Magnetic/Gradiometer mapping

Magnetic surveys have been there for long time and have been used extensively in geological mappings and mineral explorations. Magnetometers, the instruments for magnetic surveys, also frequently appear in the news with their archaeology applications. Magnetics in environmental applications is not so hot mainly due to the "culture noises": steel building, overhead and underground utilities, fences, reinforced concrete and scrap surface metals, etc. For environmental applications, we typically search for smaller and shallower targets, instead of large and deep targets in its traditional applications. EM mapping will suffer much less noise problems. Still, magnetic surveys for environmental applications can be used to map deeper targets than a general shallow EM mapping does, although its resolution is worse. If the burials are suspected as more than 10 feet, a magnetic survey could be a great help for past human activities often involving ferrous metals like steel. The recent advanced GPS technology has been integrated into the data collecting system, making the data collection and processing more cost effective.

Ground penetrating radar

Ground penetration radar is being used extensively in the environmental field. It's becoming very common for underground utility locating for proposed soil boring/monitoring well locations. A lot of utility locating companies are equipped with GPR. However, due to complexity of the GPR signals and shortage of experienced geophysicists, the GPR applications on environmental targets like underground storage tanks, septic tanks, septic field, dry wells, cesspools and underground voids, etc. are not well known and trusted. The GPR penetration depths are limited at a lot of sites, but in a much less extend than a lot of people believed. The lack of experiences of the GPR operators and the lagging GPR data processing software are the two primary forces impeding GPR applications in environmental fields. These two situations are improving every year. In the near future, we'll see extensive usage of GPR applications on environmental sites. Those environmental consultants understanding GPR services better will have better positions in the competitive environmental consulting industry.

Direct current electrical resistivity and seismic surveys

Resistivity profiling (2D) and electrical resistivity tomography (3D) could be a great help for large scale contaminant spills. When the ground water is replaced by contaminants with different electrical conductivities, resistivity mapping in 2D or 3D will be a great guidance of the contamination scope, in both horizontal and depth directions. Instead of spending months doing hundreds of soil borings blindly, performing a quick resistivity survey for one or a couple of days could save a lot of sweating effort.
Recently development of the seismic surveys, especially in surface waves and seismic refraction tomography, has greatly changed the prospect of seismic applications in environmental and engineering fields. The methods utilizing the surface waves have much better resistance to the traffic noises - a main discouragement for past seismic survey attempts in the cities.
Both resistivity and seismic surveys can be utilized to find ground water pathways in the soil and bedrocks. Both surveys can be used for mapping bedrock fractures and cavities depending on the site specifics. As the computer software is improving, the geophysicists are gaining more experiences in dealing with those data noises which could be killers for effective surveys in the past.



Written by chrisvecan from Environmental and Engineering Geophysics Forum