15Mar / 2015
Ground-gas monitoring programmes can hold up brownfield development. Spot monitoring periods can be prescribed for up to 12 months. Continuous monitoring can provide a solution to this.
Continuous monitoring involves the use of unmanned data collection instrumentation. There are many benefits to collecting continuous data, including improved risk prediction, but continuous monitoring also saves you time. Here’s how:
Shortened monitoring periods
While the traditional periodic monitoring method has its place, the low frequency of monitoring means that on sites where ground-gas generation potential is very high, up to 12 months of weekly monitoring visits can be required.
When time is of the essence, consultancies, local authorities, developers and landowners cannot afford to wait for 12 months before they can develop a potentially gassing site.
Continuous monitoring typically increases the frequency of monitoring 300 fold. This means that typical monitoring periods run for just 3-4 weeks and the quantity of data collected is still significantly higher than using traditional methods. Regulators, such as the NHBC, Environment Agency and local authorities are increasingly accepting of shortened monitoring periods using this method, understanding the strength of more data as well as the ability to better interpret risk.
Fewer site visits
Where periodic monitoring requires a site visit to collect every data point, continuous monitoring makes use of unmanned data collection instrumentation, meaning data is being collected while you can focus on other tasks. To collect around 800 data points for each measured parameter, typically only 2 site visits are required. During these visits, any monitoring wells where continuous monitoring is not being utilised may be monitored with periodic monitoring equipment while a Purge & Recovery Test (GGS PRT®) is being carried out – ‘Interpreting PRT results’ blog to come, watch this space.
Saves reviewing time
Collection of continuous data, as opposed to discrete data means that trend-lines connecting all the data points can be drawn and the data presented as a graph. Graphical representation means that the variability in ground-gas concentrations, as well as measured environmental parameters can be quickly identified together with the key relationships that characterise the ground-gas migration drivers. Such graphs avoid the need to sift through tables of discrete data points.
Figure 1 below shows an example of continuous monitoring data graphs
Figure 1: Continuous ground-gas monitoring graphs. Note – inverse relationship evident between pressure and methane concentration
In addition to plotting time series graphs (above) continuous datasets allow other data analysis tools to be used to assist risk assessment. One such technique is the Concentration Duration Curve; this can be quickly developed to indicate the proportion of the monitoring period during which a significant concentration is present. Figure 2 below shows an example of a concentration duration graph. A reviewer can quickly see that methane concentrations are over the Lower Explosive Limit (LEL – 5% v/v) for more than 80% of the monitoring period.
Figure 2: Concentration Duration Curve, note methane is over LEL for more than 80% of the monitoring period.
If time is of the essence for you and your clients, continuous monitoring can dramatically shorten monitoring periods, reduce the number of site visits and speed up reviewing and interpreting ground-gas data.
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