WEBVTT Kind: captions Language: en 00:00:12.490 --> 00:00:16.570 [Narrator] The goal of hydrogeophysics is to use geophysical methods to find, detect, and assess properties 00:00:16.570 --> 00:00:18.220 relevant to hydrologists. 00:00:18.220 --> 00:00:22.830 Accurate porosity, saturation, and hydraulic conductivity estimates are critical for understanding 00:00:22.830 --> 00:00:26.780 - aquifer heterogeneity, - fate and transport of contaminants in the 00:00:26.780 --> 00:00:29.510 environment, and - water availability. 00:00:29.510 --> 00:00:33.860 Whereas most geophysical methods are only indirectly sensitive to hydrologic properties, 00:00:33.860 --> 00:00:38.950 Nuclear Magnetic Resonance (or "NMR") is unique in that it directly senses the presence 00:00:38.950 --> 00:00:40.320 of water. 00:00:40.320 --> 00:00:45.449 As a result, borehole NMR can provide in-situ estimates of hydrologic parameters that scientists 00:00:45.449 --> 00:00:50.140 and project managers need for environmental site and water resources assessment. 00:00:50.140 --> 00:00:54.269 Borehole NMR contains information on the overall water content as well as the distribution 00:00:54.269 --> 00:00:56.740 of the water in porous material. 00:00:56.740 --> 00:00:59.350 Using borehole NMR logs, practitioners can estimate 00:00:59.350 --> 00:01:01.990 - vertical distributions of water content, - porosity, and 00:01:01.990 --> 00:01:04.960 - even pore size distribution. 00:01:04.960 --> 00:01:08.760 Using established textbook formulas, it is possible to estimate hydraulic conductivity 00:01:08.760 --> 00:01:10.590 from borehole NMR measurements. 00:01:10.590 --> 00:01:15.060 Furthermore, the sensitivity to pore size distribution may allow discrimination between 00:01:15.060 --> 00:01:19.299 bound and unbound water, which controls back diffusion. 00:01:19.299 --> 00:01:23.010 Project managers can use this information to develop site conceptual models and construct 00:01:23.010 --> 00:01:27.350 predictive models to improve environmental management. 00:01:27.350 --> 00:01:31.999 Borehole NMR is extensively used in oil and gas reservoir characterization and can replace 00:01:31.999 --> 00:01:35.909 traditional logging methods for estimating porosity that use radioactive sources that 00:01:35.909 --> 00:01:38.200 have safety concerns. 00:01:38.200 --> 00:01:42.229 Recent technological advancements have led to cost-effective and smaller borehole tools 00:01:42.229 --> 00:01:45.210 suitable for environmental applications. 00:01:45.210 --> 00:01:49.609 With safer, more economical, and more portable tools, borehole NMR logs produced by these 00:01:49.609 --> 00:01:54.420 tools can be easily repeated to monitor changes in porosity and hydraulic conductivity over 00:01:54.420 --> 00:02:00.180 time, such as those caused by biogeochemical processes associated with contaminant transformations. 00:02:00.180 --> 00:02:06.259 NMR is based on the same physical principles as MRI in medicine. 00:02:06.259 --> 00:02:10.039 In the NMR measurement, hydrogen atoms in water molecules preferentially align with 00:02:10.039 --> 00:02:14.120 strong magnets inside the tool establishing a known background state. 00:02:14.120 --> 00:02:19.110 Then, the tool pulses a radio frequency, pushing the water molecules out of alignment with 00:02:19.110 --> 00:02:20.670 this background state. 00:02:20.670 --> 00:02:25.420 This perturbation causes a measurable magnetization, as the atoms precess and decay back to the 00:02:25.420 --> 00:02:27.080 background state. 00:02:27.080 --> 00:02:31.310 The total amount of water present is derived from the magnetization strength. 00:02:31.310 --> 00:02:37.090 The pore-size distribution is calculated from the timing of the decay. 00:02:37.090 --> 00:02:40.730 Borehole NMR tools use these principles to generate logs presented in terms of water 00:02:40.730 --> 00:02:43.420 content and pore size distribution. 00:02:43.420 --> 00:02:47.590 Although borehole NMR can provide valuable site data, site conditions and other factors 00:02:47.590 --> 00:02:49.070 may limit its use. 00:02:49.070 --> 00:02:54.810 For example, because of the strong magnet, the tool cannot log in steel casings. 00:02:54.810 --> 00:02:58.810 Different borehole NMR tools are sensitive to different radial distances from the tool, 00:02:58.810 --> 00:03:03.390 therefore different diameter boreholes require different tools. 00:03:03.390 --> 00:03:07.700 Each tool is sensitive to a specific distance from the center of the tool. 00:03:07.700 --> 00:03:12.740 Before you install your borehole, you have the undisturbed subsurface formation. 00:03:12.740 --> 00:03:18.110 When a borehole is drilled, the area immediately surrounding the borehole is also disturbed. 00:03:18.110 --> 00:03:22.440 When we conduct borehole NMR logging, each tool is sensitive to a different radial distance 00:03:22.440 --> 00:03:26.410 in order to take measurements of the undisturbed formation. 00:03:26.410 --> 00:03:30.370 Relative to conventional logging tools, the measurements may be more time consuming. 00:03:30.370 --> 00:03:34.370 As a result, cost and time constraints may necessitate discrete measurements rather than 00:03:34.370 --> 00:03:36.590 continuous logging. 00:03:36.590 --> 00:03:40.590 Like all geophysical tools, borehole NMR is not a silver bullet. 00:03:40.590 --> 00:03:44.770 Rather, borehole NMR should be viewed as one component of a toolbox of integrated geophysical 00:03:44.770 --> 00:03:48.120 methods for environmental site management. 00:03:48.120 --> 00:03:51.920 To learn more about borehole NMR, visit our web site for a list of references for this 00:03:51.920 --> 00:03:57.290 video or explore the USGS Fractured Rock Geophysical Toolbox Method Selection Tool.