Geophysics provides a multidimensional suite of investigative methods that are transforming our ability to see into the very fabric of the subsurface environment, and monitor the dynamics of its fluids and the biogeochemical reactions that occur within it. Here we document how geophysical methods have emerged as valuable tools for investigating shallow subsurface processes over the past two decades and offer a vision for future developments relevant to hydrology and also ecosystem science. The field of “hydrogeophysics” arose in the late 1990s, prompted, in part, by the wealth of studies on stochastic subsurface hydrology that argued for better field‐based investigative techniques. These new hydrogeophysical approaches benefited from the emergence of practical and robust data inversion techniques, in many cases with a view to quantify shallow subsurface heterogeneity and the associated dynamics of subsurface fluids. Furthermore, the need for quantitative characterization stimulated a wealth of new investigations into petrophysical relationships that link hydrologically relevant properties to measurable geophysical parameters. Development of time‐lapse approaches provided a new suite of tools for hydrological investigation, enhanced further with the realization that some geophysical properties may be sensitive to biogeochemical transformations in the subsurface environment, thus opening up the new field of “biogeophysics.” Early hydrogeophysical studies often concentrated on relatively small “plot‐scale” experiments. More recently, however, the translation to larger‐scale characterization has been the focus of a number of studies. Geophysical technologies continue to develop, driven, in part, by the increasing need to understand and quantify key processes controlling sustainable water resources and ecosystem services.

[1] There is growing interest in the use of geophysical methods for hydrological model parameterization. Empirical induced polarization (IP)-hydraulic conductivity (K) relationships have been developed, but these are only applicable to sediments in which the IP response shows limited variation with electrical current frequency. Here we examine the spectral IP response of samples taken from a UK sandstone aquifer and compare measured parameters with physical and hydraulic properties. We demonstrate the limited value of existing IP-K models due to the inherent IP frequency dependence of these samples. Our results show how the mean relaxation time, t, is a more appropriate measure of IP response for these sediments. A significant inverse correlation between the surface area to pore volume ratio and t is observed, suggesting that t is a measure of a characteristic hydraulic length scale. This is supported by a measured strong positive correlation between log t and log K. Our measurements also reveal evidence of a relationship between t and a dominant pore throat size, which leads to postulations about the parallelism between the spectral IP behavior and unsaturated hydraulic characteristics. Additional experiments show how the relaxation time is affected by degree of fluid saturation, indicating that saturation levels must be accounted for if our empirical relationships are applied to vadose zone studies. Our results show clear evidence of the potential value of frequency-based IP measurements for parameterization of groundwater flow models.

[1] We use complex conductivity measurements to predict the hydraulic conductivity (K) of unconsolidated materials. The samples include natural sediments and artificial sand/clay mixtures. We apply the Börner et al. [1996] model, which is based on the Kozeny-Carman equation and incorporates electrical estimates of formation factor (F) and specific surface area per unit volume-to-porosity ratio (S por ), from the real (s 0 ) and imaginary (s 00 ) conductivity components respectively. We find that K correlates with s 00 but shows no correlation with F, which we attribute to the wide range in grain size for these materials. The Börner model appears primarily dependent on the K -s 00 relation. The relationship between s 00 and S por is nonlinear and appears to depend upon material type. Further examination shows that s 00 is well correlated with effective grain size (d 10 ) and is relatively independent of the material type. We propose a simple Hazen-type equation in which the effective grain size is estimated from s 00 . This simple model provides order of magnitude estimates of K for a range of unconsolidated sediments.

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Petrophysical interpretation of resistivity measurements is often hindered by the dependence of resistivity on the interconnected pore fluids and the interconnected pore surfaces. Induced polarization (IP) measurements yield parameters that are only controlled by the interconnected pore surfaces, thereby offering the opportunity to constrain interpretation of resistivity measurements. Using a database composed of 63 sandstone and unconsolidated sediment samples covering nine independent investigations, we identified a strong linear relationship between the real part of surface conductivity ([Formula: see text]) determined from multisalinity ([Formula: see text]) resistivity measurements and the imaginary conductivity ([Formula: see text]) measured with IP at a frequency of about 1 Hz. We found [Formula: see text] with a coefficient of determination ([Formula: see text]) of 0.911 and a standard deviation of [Formula: see text] of 0.022. We found a similar relation when the normalized chargeability (from Debye decomposition) of the frequency dependence of the IP response is used instead of [Formula: see text]. By estimating the true formation factor ([Formula: see text]) recorded at high salinity, we solved for [Formula: see text] and found that it parallels the salinity dependency of the imaginary conductivity, [Formula: see text], as reported in recent studies. We also found that the value of the [Formula: see text] determined from this experimental study was generally consistent with predictions of the POLARIS model when the mobility of the ions in the Stern layer was assumed to be [Formula: see text] of the mobility of the ions in the diffuse layer (considered equal to the mobility of the ions in the bulk solution). We discovered how the identified relationship can be used to significantly improve (1) the estimation of the true formation factor and (2) the groundwater conductivity, from a single salinity resistivity measurement when an IP measurement is also made. The approach offers an opportunity to improve estimation of porosity, formation factor, and salinity in well logging and hydrogeophysical investigations.

This paper reviews the recent geophysical literature addressing the estimation of saturated hydraulic conductivity (K) from static low frequency electrical measurements (electrical resistivity, induced polarization (IP) and spectral induced polarization (SIP)). In the first part of this paper, research describing how petrophysical relations between electrical properties and effective (i.e. controlling fluid transport) properties of (a) the interconnected pore volumes and interconnected pore surfaces, have been exploited to estimate K at both the core and field scale is reviewed. We start with electrical resistivity measurements, which are shown to be inherently limited in K estimation as, although resistivity is sensitive to both pore volume and pore surface area properties, the two contributions cannot be separated. Efforts to utilize the unique sensitivity of IP and SIP measurements to physical parameters that describe the interconnected pore surface area are subsequently introduced and the incorporation of such data into electrical based Kozeny-Carman type models of K estimation is reviewed. In the second part of this review, efforts to invert geophysical datasets for spatial patterns of K variability (e.g. aquifer geometries) at the field-scale are considered. Inversions, based on the conversion of an image of a geophysical property to a hydrological property assuming a valid petrophysical relationship, as well as joint/constrained inversion methods, whereby multiple geophysical and hydrological data are inverted simultaneously, are briefly covered. This review demonstrates that there currently exists an opportunity to link, (1) the petrophysics relating low frequency electrical measurements to effective hydraulic properties, with (2) the joint inversion strategies developed in recent years, in order to obtain more meaningful estimates of spatial patterns of K variability than previously reported.

Over the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IP data for imaging purposes, and the increasing application of the method in near-surface investigations. We here summarized the current state of the science of the SIP method for near-surface applications and describe which aspects still represent open issues and should be the focus of future research efforts.Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving the different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing database of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has been made also in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IP data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3-D, time-3 lapse and multi-frequency IP data, which defines a 5-D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.

Abstract:We want to develop a dialogue between geophysicists and hydrologists interested in synergistically advancing process based watershed research. We identify recent advances in geophysical instrumentation, and provide a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology. The focus of the paper is to identify instrumentation that could significantly advance this vision for geophysics and hydrology during the next 3-5 years. We acknowledge that this is one of a number of possible ways forward and seek only to offer a relatively narrow and achievable vision. The vision focuses on the measurement of geological structure and identification of flow paths using electrical and magnetic methods. The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved. Of specific interest are the airborne systems that can cover large areas and are appropriate for watershed studies. Although airborne geophysics has been around for some time, only in the last few years have systems designed exclusively for hydrological applications begun to emerge. These systems, such as airborne electromagnetic (EM) and transient electromagnetic (TEM), could revolutionize hydrogeological interpretations. Our vision centers on developing nested and cross scale electrical and magnetic measurements that can be used to construct a three-dimensional (3D) electrical or magnetic model of the subsurface in watersheds. The methodological framework assumes a 'top down' approach using airborne methods to identify the large scale, dominant architecture of the subsurface. We recognize that the integration of geophysical measurement methods, and data, into watershed process characterization and modelling can only be achieved through dialogue. Especially, through the development of partnerships between geophysicists and hydrologists, partnerships that explore how the application of geophysics can answer critical hydrological science questions, and conversely provide an understanding of the limitations of geophysical measurements and interpretation.