Electromagnetic (EM) Surveys – Data Contour Maps or Not?
Traditionally, results of electromagnetic surveys utilizing EM instruments such as Geonics EM31 or EM61 are usually presented in contour maps. A picture is worth a thousand words -- these data contour maps, especially when drawn in colors, can be very powerful presentation materials showing details of investigation results.
However, when the budget of a project is limited, it’s possible to lower the survey cost, but still achieve the same investigation goal without contour maps as the final output. This can be done with the reconnaissance survey mode without setting up GPS or a manual grid. By doing so, both field time and office time are saved, hence the cost is lower.
So, should you set up an EM survey with or without data contour maps as the final output? It depends on a lot of factors. Here are a few things to consider:
Cost v.s. value. Data contour maps could be very good records and they certainly can preserve and present great details of the investigation results. They can be eye-catching and sometimes easier to understand than many words. However, they usually come with a cost. It’s hard to predict their exact values but the responses to them from your clients or the end users might be helpful.
The GPS satellite coverage of the survey area. If the survey area is an open field without nearby tall buildings or tree canopies, etc, the field times for the EM survey are similar either way. Note that even only a small portion of survey area couldn’t be covered by GPS, the cost could be significantly increased due to manual grid setup and the issue of merging non-GPS EM data with GPS-positioned EM data.
The target of the survey. If you are looking for underground metallic objects, which are easily identifiable as you go along with the survey (i.e., in the reconnaissance survey mode), they can be marked on-site and data contour maps might not be necessary. In the other hand, if you are looking for subtle variations of soil electrical conductivity, such as locating the septic field of a septic sewer system, data contour maps are usually necessary since the targets are hard to “see” without the “big picture”.
The size of the survey area. It’s usually a good idea to collect the data with positioning, i.e., making data contour maps eventually for a large survey area. Some targets, potential targets or surprises can be readily recognized only through the “big picture” - data contour maps. A site can be defined as a large site with the size of tens of acres or even only a few acres. However, if the area is congested with interested targets, it might still be a good idea to map them using the data, even when the area is fairly small. Sometimes it’s actually easier or more accurate to mark the anomalies on-site after the contour map is generated.
Before starting an EM survey, the client requesting it and the contractor conducting it should reach a general understanding whether the data contour maps are needed or required. Usually it’s too late to request data contour maps if the survey is done in reconnaissance mode without positioning information. Please feel free to contact us to discuss the options of conducting EM surveys at your sites.