Formatted Title
Combined CPT-NMR Logging for Hydrogeologic Investigations via Direct Push Technology
Background/Objectives
Nuclear magnetic resonance (NMR) technology is a powerful non-destructive and non-radiative methodology that is widely used in a variety of disciplines, such as chemistry (high field NMR spectrometers) and medicine (MRI scanners). NMR probes the response of hydrogen nuclei to a magnetic field perturbation. In the presence of magnetic field, the hydrogen nuclei are polarized creating a net nuclear magnetization parallel to the main magnetic field. Next, an oscillating field is applied to excite this magnetization into a perpendicular plane where it precesses about the main magnetic field at Larmor frequency generating a detectable NMR signal. In earth sciences, NMR allows direct detection of hydrogen in subsurface pore spaces. The detected NMR signal allows quantitative determination of the volumetric water content and saturated porosity and further indicates the pore size distribution, and hydraulic conductivity. Development has been undertaken to utilize NMR within common direct push casing sizes. Development is now moving to couple NMR with CPT within a single push only housing to provide mechanical soil properties of CPT along with the NMR aquifer measurements.
Approach/Activities
The NMR technology has recently been shown to be compatible with direct push (DP) drilling methods that are widely used to explore contaminated sites in the environmental remediation sector. DP NMR uses a high-resolution ultra-slim NMR logging tool having diameter of 1.4 inches and vertical resolution of 5 inches that is deployed through 2¼-inch DP rods for up-logging measurements. Recently, Vista Clara Inc. and Geoprobe have developed a high-resolution CPT+NMR logging tool capable of obtaining both mechanical soil properties and hydrogeological parameters (e.g., water content, porosity, hydraulic conductivity) of the subsurface with a very high vertical resolution of 2 inches. The CPT+NMR tool is advanced to depth using static force so that CPT measurement of soil mechanical properties can be made as the tool is advanced to depth followed by NMR measurements of water content, saturated porosity, and hydraulic conductivity as the tool is retracted from the ground.
Results/Lessons Learned
Deployment of the DPNMR tool through 2¼-inch casing is relatively simple. The challenge of developing the combined CPTNMR tool is all the sensor electronics are housed within the tool as it is advanced into the unconsolidated formation. NMR measurements cannot be performed through steel so a push rod casing needed to be developed that would include a nonmetallic, nonconductive “window” section where the NMR measurements could occur and would not interfere with those measurements.
Logs of CPT and NMR show common trends between the CPT tip resistance, and local friction values compared with the T2 Distribution and pore size distribution of the NMR which indicates the soil types. We will show CPT+NMR logs from multiple sites. We will soon be performing these logs in a near shore environment. Results from CPT + NMR tools are both useful and desired especially for geotechnical purposes.