A. C. Hinnell

Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion

Improved extraction of hydrologic information from geophysical data through coupled hydrogeophysical inversion A.C. Hinnell 1 , T.P.A. Ferre 1 , J.A. Vrugt 2 , J.A. Huisman 3 , S. Moysey 4 , J Rings 3 , and M.B. Kowalsky 5 Hydrology and Water Resources, University of Arizona, Tucson, AZ, 85721-0011 Center for Nonlinear Studies (CNLS), Mail Stop B258, Los Alamos, NM 87545 ICG 4 Agrosphere, Forschungszentrum Julich, 52425 Julich, Germany Environmental Engineering and Earth Sciences, Clemson University, Clemson, S.C. 29634 Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 Abstract There is increasing interest in the use of multiple measurement types, including indirect (geophysical) methods, to constrain hydrologic interpretations. To date, most examples integrating geophysical measurements in hydrology have followed a three-step, uncoupled inverse approach. This approach begins with independent geophysical inversion to infer the spatial and/or temporal distribution of a geophysical property (e.g. electrical conductivity). The geophysical property is then converted to a hydrologic property (e.g. water content) through a petrophysical relation. The inferred hydrologic property is then used either independently or together with direct hydrologic observations to constrain a hydrologic inversion. We present an alternative approach, coupled inversion, which relies on direct coupling of hydrologic models and geophysical models during inversion. We compare the abilities of coupled and uncoupled

Practical limitations and applications of short dead time surface NMR

There is increasing interest in the unique measurement capabilities of nuclear magnetic resonance (NMR) for hydrologic applications. In particular, the ability to quantify water content (both bound and free) and to infer the permeability distribution are critical to hydrologists. As the method has gained in acceptance, there has been growing interest in extending its range to near-surface and vadose zone applications and to measurement in finer grained and magnetic soils. All of these applications require improved resolution of early-time signals, which requires shorter measurement dead times. This paper analyses three physical/electrical processes that limit the minimum achiev- able measurement dead times in surface NMR applications: 1) inherent characteristics of electro- mechanical and semiconductor switching devices, 2) the effective bandwidth of the receiver and signal processing chain, 3) transient signals associated with induced eddy currents in the ground. We then describe two applications of surface NMR that rely on reduced measurement dead time: detection and characterization of fast decaying NMR signals in silt and clay and the detection of fast decaying NMR signals in magnetic geology.

Neuro-Drip: estimation of subsurface wetting patterns for drip irrigation using neural networks

Design of efficient drip irrigation systems requires information about the subsurface water distribution of added water during and after infiltration. Further, this information should be readily accessible to design engineers and practitioners. Neuro-Drip combines an artificial neural network (ANN) with a statistical description of the spatio-temporal distribution of the added water from a single drip emitter to provide easily accessible, rapid illustrations of the spatial and temporal subsurface wetting patterns. In this approach, the ANN is an approximator of a flow system. The ANN is trained using close to 1,000 numerical simulations of infiltration. Moment analysis is used to encapsulate the spatial distribution of water content. In practice, the user provides soil hydraulic properties and discharge rate; the ANN is then used to estimate the depth to the center of mass of the added water, and the vertical and radial spreading around the center of mass; finally, this statistical description of the added water is used to visualize the fate of the added water during and after the infiltration event.

Basin-scale transmissivity and storativity estimation using hydraulic tomography

While tomographic inversion has been successfully applied to laboratory- and field-scale tests, here we address the new issue of scale that arises when extending the method to a basin. Specifically, we apply the hydraulic tomography (HT) concept to jointly interpret four multiwell aquifer tests in a synthetic basin to illustrate the superiority of this approach to a more traditional Theis analysis of the same tests. Transmissivity and storativity are estimated for each element of a regional numerical model using the geostatistically based sequential successive linear estimator (SSLE) inverse solution method. We find that HT inversion is an effective strategy for incorporating data from potentially disparate aquifer tests into a basin-wide aquifer property estimate. The robustness of the SSLE algorithm is investigated by considering the effects of noisy observations, changing the variance of the true aquifer parameters, and supplying incorrect initial and boundary conditions to the inverse model. Ground water flow velocities and total confined storage are used as metrics to compare true and estimated parameter fields; they quantify the effectiveness of HT and SSLE compared to a Theis solution methodology. We discuss alternative software that can be used for implementing tomography inversion.

Surface NMR instrumentation and methods for detecting and characterizing water in the vadose zone

A commercially available surface NMR instrument was modified to address the challenges of using earth’s field surface NMR to detect and characterize water in the unsaturated (or vadose) zone. The modified instrument incorporates faster switching electronics to achieve an instrument dead time of 2.8 ms, and higher output power electronics to enable a maximum coil voltage of 8000 volts and coil current of 800 amps. The instrument was used to collect and interpret surface NMR data at several active vadose zone investigation sites in the western US. A 6-week surface NMR experiment was conducted at a managed aquifer storage and recovery facility in Arizona, to explore the measurement capabilities and limitations of the instrument, during a managed infiltration event. The resulting time lapse surface NMR data were used to map zones of held water prior to the flood event, image the influx of water through the top 15 metres of the subsurface during and after the event, quantify the spatial and temporal distribution of infiltrating water throughout the event, and characterize the distribution of water in different relative pore sizes throughout the event. Data obtained at pseudo-static vadose zone investigation sites indicate that the surface NMR instrument can detect and image some forms of water held in unconsolidated vadose zone formations, at depths up to 30 metres. Complementary NMR logging data indicate that the surface NMR instrument does not detect all of the water held in these pseudo-static formations, but that the non-invasive surface NMR data may yield valuable information nonetheless.

Inferring hydraulic properties using surface-based electrical resistivity during infiltration

Hydraulic conductivity and sorptivity are rock properties of primary importance for subsurface hydrologic analysis. Standard methods to determine these properties, such as ring infiltrometers, are based on adding water under constant pressure and monitoring the water flux into the ground. Ring infiltrometers are tens of centimeters in diameter. However, the concept of inferring hydraulic properties by monitoring infiltration beneath ponded water could be applied to hydrologic studies at larger scales. Two such cases, for which it may be useful to infer hydraulic properties, are artificial recharge facilities and ephemeral streams (Figure 1).