A nuclear magnetic resonance (NMR) borehole logging technique has been developed for near surface mining, groundwater and environmental contamination projects. The NMR technique provides a direct and unambiguous measurement of groundwater. With a direct view into the pore space, NMR data enables precise quantification of moisture content and porosity of the rock surrounding a borehole. Additional parameters measured include an estimation of other critical hydrogeological parameters such as permeability, specific yield and pore size distribution.

<h4>The logging technique</h4>

The NMR logging technique is a magnetic technique and does not require a radioactive source. This allows the NMR logging technique to replace the conventional neutron logging for moisture content and porosity. The NMR logging technique has the ability to distinguish between bound water in clays and free water in the pores allowing the identification of clay/shale layers in the sub surface and support interpretation for conventional logs such as resistivity and gamma.

<h4>The Vista Clara Javelin system</h4>

The Vista Clara Javelin NMR logging system is designed specifically for near surface applications. A range of different probes are available to suit the casing and drilled diameter. The objective of the different probe size is to take measurements beyond the disturbed zone created during the drilling of the borehole. The system includes adaptive noise filters to get over potential near surface environmental noise, such as overhead power lines, which could affect the NMR data.

<h4>Applications</h4>

Some examples of the potential applications of the NMR are technology:

• Measuring the moisture content of the “vadose zone” above the water table. More environmental contamination projects are beginning to show the importance of the moisture content controlling the migration pathways of certain contaminants.
• As the NMR technology is able to distinguish between water bound into the structure of clays (hence unavailable for extraction) and water present in the pore space (hence potentially available for extraction), it will have great significance in the placement of screens in monitoring and/or production bores.
• Many environmental contaminants at the present day are hydrocarbon based. The NMR technology detects hydrogen in water or hydrocarbons.

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Most are associated with previous heavy manufacturing industries which have since moved on.

However, the pollution plume is now affecting surrounding residential areas.

Identifying and mapping the geology of Clovelly Park

Our gamma probe is a powerful method for rapid identification of the local geology.

Once the data from several boreholes has been recorded, we can start tapping this powerful method for mapping the geology across the suburb.

We favour this approach because it is not unusual for near surface sediments to be a mix of recently deposited thin sands and clays.

The sand layers are likely to be the “transport” zones through which the pollutant has migrated throughout the suburb. Identifying these zones will aid any remediation plan.

Sampling the local groundwater

One of the geophysical logging methods involves a specialist fluid sampler which takes a sample of the fluid in the investigation borehole at any depth.

In the case of equipment we have at Borehole Wireline that would be a one litre sample taken during one logging run.

The benefits of this approach are:

• Precise depth control for the sampling
• Better sample handling at the surface

Identifying the pollutant plume

Common pollutants from heavy industries of the 20th century are generally hydrocarbon based.

As long as the pollutant impregnates a portion of the groundwater or rock, it can be measured effectively by borehole resistivity and/or induction/conductivity logging where the typical signature to look for is unexpectedly high resistivity.

Our team always favours use of the induction/conductivity probe because measurements can be obtained in air filled and/or fluid filled conditions and within PVC casing.

Repeating induction/conductivity logging over time can show progression of a pollutant plume through the ground as pollutant percentage will alter the resistivity of the groundwater/rock.

Alternatives to borehole geophysical logging

While it can be tempting to apply fluid sampling techniques using a small pump, this technique has little or no depth control. So the question would remain, where are your samples coming from?

Another approach is auger drilling, a common drilling method for environmental site monitoring. The technique can produce a continuous sample of the rock that’s been drilled through, but the sample is typically messy and gives only a “bulk” indication.

Compare this to gamma logging which clearly shows intricate interbedding of thin sands and clays hence showing the likely “transport” zones and their respective depth in the ground and thickness.

Finally, as with any logging program, geophysical logging will provide fast, reliable and repeatable results complimenting any site geological information. In fact, we’d argue that borehole geophysical logging would significantly increase the efficiency of any program employed to solve mysteries like the one facing Clovelly Park.

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