Values Of Hydraulic Diffusivity Research Articles

Hydrology Drives Crustal Deformation and Modulates Seismicity in the Matese Massif (Italy)

Abstract We analyze the interplay between hydrology, deformation, and seismicity in the Matese massif, located in the Italian Southern Apennines. We find that this area is characterized by the concurrent action of two hydrologically driven processes: the first is the deformation detected by the Global Navigation Satellite Systems (GNSS) data in the shallowest part (above the elevation of the major springs) of the Earth crust, in phase with the hydrological forcing; the second is the triggering of seismicity at depth with a delay suggesting a downward diffusive process. We study the first process by applying a principal component analysis to the GNSS displacements time series, aiming to identify a common signal describing the largest data variance. We find that the maximum horizontal displacements associated with the first principal component (PC1) are larger than 1 cm in two GNSS sites, and the PC1 temporal evolution is well correlated and in phase with the flow of the largest spring of the region, which we consider as proxy of the water content of the massif. This suggests that the main source of horizontal deformation is the water content fluctuations in the shallow portion of the Matese aquifer, in particular within fractures located in correspondence of the main mapped faults. The deformation rates caused by this process are one order of magnitude larger than the tectonic ones. Finally, we infer the second process by observing the correlation between the background seismicity and the spring discharge with a time lag of 121 days. In our interpretation, downward diffusive processes, driven by aquifer water content variations, propagate pore-pressure waves that affect the fault’s strength favoring the occurrence of microearthquakes. This is supported by the values of hydraulic diffusivity (1.5 m2/s) and rock permeability (3.2–3.8×10−13 m2), which are compatible with what is observed in karstified limestones.

Determining hydraulic diffusivity from ambient noise in subsurface flow rate and temperature data

SUMMARY Determining the properties that control fluid flow and pressure migration through rocks is essential for understanding groundwater, energy reservoirs and fault zones. However, direct measurements of these properties underground generally require expensive and invasive processes such as pumping large volumes of water in or out of the ground. These conventional methods may change the structure that they are trying to measure and do not resolve variations in space without complex, multiple experiments. Here, we capitalize on the existing, natural fluctuations of pore pressure, flow and temperature that are always present underground, likely driven by tides, seismic waves and other noise in the Earth. We develop the theoretical framework to utilize these noises to determine hydrogeologic structure. We begin with prior work demonstrating a connection between cross-correlation and Green's functions for diffusive processes and illustrate how the existing theory could be used specifically for the case of inferring hydraulic diffusivity from closely spaced observations of pore pressure. We extend the current theory to include a capability for inferring Green's functions from closely spaced flow rate measurements, rather than pore pressure data. We then note that closely spaced temperature measurements are much more practical as an observational system, however, diffusive transport is likely not the dominant cause of high-frequency fluctuations of temperature in the Earth. Advective transport is a more plausible controlling factor and thus we need to incorporate advection into the theory and model the corresponding response functions. We therefore develop a semi-analytical method that includes advection and numerically compute the corresponding response function. We use these synthetics to illustrate how cross-correlation of ambient noise in subsurface temperature measurements carries information about hydraulic structure and show that it is possible to constrain hydraulic diffusivity values from cross-correlation of closely spaced continuous temperature measurements. This new method opens up the possibility of passive determination of reservoir properties with high spatial resolution from closely spaced, continuous temperature data, which is a realistic deployment strategy that could be used in a wide variety of settings.

Hydrogeological characterization of heterogeneous volcanic aquifers in the Canary Islands using recession analysis of deep water gallery discharge

Aquifers constitute the main freshwater supply of oceanic islands. Maintaining groundwater quantity and quality is of critical concern as demographic and climatic changes place additional pressures on already fragile water resources systems. Islands with heterogeneous volcanic aquifers pose additional difficulties in assessing their water resources. This work proposes an approach for improving the hydrogeological characterization of heterogeneous volcanic aquifer systems by making use of recession coefficients from deep water gallery discharge. To demonstrate the usefulness of this approach, hydrographs and recession curves from groundwater discharge of 30 water galleries on La Palma (Canary Islands) were evaluated. This analysis allowed us to obtain the main hydrogeological parameters of a volcanic aquifer system, in terms of hydraulic diffusivity. A Maillet-Boussinesq model with an exponential decay law was adopted, according to field observations of drainage discharge. The alpha coefficients of recession values ranged between 10−3 and 4·10−4 day−1 and showed significant spatial correlation with insular geology. Additionally, hydraulic diffusivity values of island hydrogeological domains were obtained from recession coefficients using the Rorabaugh-Singh method. Weighted storage coefficients for volcanic materials were in the range of 3% to 7%, with an average transmissivity in the range of 15 to 150 m2·day−1. The methodology proposed has demonstrated its usefulness in coping with local uncertainty in hydraulic characterization of insular aquifers associated with volcanic heterogeneity. This is an improvement compared to standard pumping tests, thus providing hydraulic parameters prior to numerical analysis for water management planning.

Electro-geohydraulic estimation of shallow aquifers of Owerri and environs, Southeastern Nigeria using multiple empirical resistivity equations

Estimates of the geohydraulic parameters of the aquifers of Owerri and environs were carried out using different empirical resistivity equations with the objective of evaluating the predictive capabilities of the various equations. Forty Vertical Electrical Sounding (VES) data were acquired with a maximum half-current electrode spacing of 500 m using the Schlumberger electrode configuration. Aquifer resistivity across the study area varies from 330 to 78,100 Ωm with a mean value of 9465.8 Ωm. Aquifer depth ranges from 22.5 to 91.8 m with a mean value of 61.79 m while aquifer thickness across the study area ranges from 10.8 to 77.0 m with an average value of 36.72 m. The Dar-Zarrouk parameters revealed that the transverse resistance ranges from 12,276 to 2,297,120 Ωm2 with a mean value of 330,297.01 Ωm2 while the longitudinal conductance ranges between 0.000214 and 0.529231 Ω−1 with a mean value of 0.01323Ω−1. Aquifer hydraulic conductivity values vary from about 2.97–58.55 m/day with a mean value of 15.55 m/day while aquifer transmissivity values vary from about 37.433–7004.61 m2/day with a mean value of 1007.18 m2/day. Aquifer storativity values across the study area vary from 0.0000324 to 0.000231 with a mean value of 0.00011 while aquifer hydraulic diffusivity values ranged between 335,425 and 79,383,443.33 with a mean value of 9,621,354.647. Model ranking using statistical analysis revealed that out of the three different equations used for estimating hydraulic conductivity in the study area, the New Model equation is the best alternative in the absence of pumping test data.

Hydrogeologic Property Estimation in Plate Boundary Observatory Boreholes Using Tidal Response Analysis

Because of the influence pore pressures have on effective stress, understanding hydrogeologic properties that control fluid flow and pressure distribution is important in characterizing earthquake and deformation processes. Here, we utilize borehole pressure changes in response to earth tides to determine hydrogeologic properties and their time variations for 17 boreholes within the NSF Earthscope’s Plate Boundary Observatory (PBO) network along the San Andreas fault and Cascadia subduction zone. Our analysis considers solutions for both confined and unconfined aquiares. Resulting permeability and hydraulic diffusivity values range from6.4×10−16–8.4×10−14 m2 and1×10−4–9×10−1 m2s−1, respectively, whereas specific storage values are generally~1×10−6 m−1. The values are fairly consistent through time, reasonable given lithology, and are comparable to other regional studies. For one borehole, values are also comparable to those determined with traditional aquifer test data. In contrast with previous determinations of the high-frequency poroelastic response to seismic waves, no obvious spatial trends in hydrogeologic properties determined from long-wavelength tidal perturbations are observed. Within the recurring time-series estimates, only one borehole exhibits clear permeability enhancement by earthquakes, whereas nearby boreholes with similar lithology and hydrogeologic property values do not. This highlights the variable susceptibility of rocks to permeability enhancement. Together, these results provide quantitative constraints useful for models of large-scale groundwater flow around large fault systems and the potential hydrologic influence on deformation and fault slip behavior.

Mobility, Thickness, and Hydraulic Diffusivity of the Slow‐Moving Monroe Landslide in California Revealed by L‐Band Satellite Radar Interferometry

AbstractActive landslides cause fatalities and property losses worldwide. Landslide behaviors can be enigmatic in natural landscapes and therefore require high‐quality observations of their kinematics to improve our ability to predict landslide behavior. Here we use geodetic interferometric synthetic aperture radar (InSAR) observations to characterize the geometry and the spatio‐temporal deformation of the slow‐moving, deep‐seated Monroe landslide in northern California between 2007 and 2017. InSAR phase discontinuities show that the landslide is bounded by discrete strike‐slip faults at the lateral margins and segregated into distinct kinematic elements by normal and thrust faults. We find that the Monroe landslide has been moving consistently, with a maximum rate of about 0.7 m/year in the narrowest longitudinal center of the transport zone. The thickest landslide mass is estimated to be in a zone located between subsiding and the uplifting kinematic elements at the lower transport zone. The timing and magnitude of the landslide displacement are modulated by the intensity and duration of precipitation, as well as the antecedent water content of the landslide mass. In addition, we use a one‐dimensional pore‐water pressure diffusion model to estimate the rainfall‐modulated pore‐water pressure changes and hydraulic diffusivity. We find hydraulic diffusivity values between m2/s, which agrees with ground‐based measurements in this region. Displacement measurements on the hillslopes obtained from geodetic InSAR observations allow us to characterize the mobility, depth, and hydraulic diffusivity of slow‐moving landslides.

Hydraulic diffusivity in a coastal aquifer: spectral analysis of groundwater level in responses to marine system

The hydraulic diffusivity gives a measure of diffusion speed of pressure disturbances in groundwater system; large values of hydraulic diffusivity lead to fast propagation of signals in aquifer. This research provides a novel design and derives spectral representation to determine hydraulic diffusivity using spectral analysis of groundwater levels coupled with time-dependent boundary adjacent to marine system and no flow boundary in aquifer system. To validate the proposed method, water levels of fluctuated boundary and groundwater well in a sandy confined aquifer were collected. The hydraulic diffusivity is then obtained by an inverse process in the non-linear complex form of spectral relationship. The method essentially is constructed on the conceptual design of natural forcing transmitted in large aquifer. It is unlike the conventional field pumping test which is only used to determine hydraulic properties of groundwater in small range around the well. Hydraulic diffusivity of the confined aquifer is determined using real observation and then checked by comparing to the published range. It suggests that without local aquifer test to estimate hydraulic diffusivity in a coastal aquifer using spectral representation with its relevant flow system and boundary has become feasible.

The puzzle of high heads beneath the West Cumbrian coast, UK: a possible solution

A region of high heads within the Borrowdale Volcanic Group (BVG; a fractured crystalline rock) beneath the coastal plain of West Cumbria, England (UK), is identified as a possible relic left over by the Late Devensian ice sheet. It was found during investigations in the 1990s. Contemporary modelling work failed to produce a satisfactory explanation of the high heads compatible with the ‘cold recharge’ isotopic signature of the groundwater. This study has reassessed the original hydraulic testing results. By plotting density-adjusted heads versus their depth below the water table in the immediate vicinity of the borehole in which they were measured, a depth profile resembling a ‘wave’ was revealed with a peak value located at 1,100 m depth. The possibility that this wave represents relic heads from the last major ice sheet has been assessed using one-dimensional mathematical analysis based on a poroelastic approach. It is found that a wet-based ice sheet above the West Cumbrian coast was probably thick enough and sufficiently long-lasting to leave such relic heads providing that the hydraulic diffusivity of the BVG is in the order of 10−6 m s−1. Initial assessment 20 years ago of the long-interval slug tests suggested that such low values are not likely. More recent interpretation argues for such low values of hydraulic diffusivity. It is concluded that ice sheet recharge is the most likely cause of the raised heads, that the BVG contains significant patches of very low conductivity rock, and that long-interval single-hole tests should be avoided in fractured crystalline rock.

Application of genetic algorithm technique to inverse modeling of tide–aquifer interaction

Modeling of tide–aquifer interaction plays a vital role in the management of coastal aquifer systems. A novel and robust methodology is presented in this paper for estimating aquifer parameters of coastal aquifers from tide–aquifer interaction data using tide–aquifer interaction model and genetic algorithm (GA). Two stand-alone computer programs were developed to optimize hydraulic diffusivities of unconfined and confined coastal aquifers at multiple sites using GA technique and tide–aquifer interaction model and considering two approaches (‘lumped tidal component approach’ and ‘multi-tidal component approach’). Five sets of real-world tide–aquifer interaction data at two sites of an unconfined aquifer and one set of tide–aquifer interaction data at three sites of a confined aquifer were used to demonstrate the efficacy of the methodology. The analysis of the GA-based inverse modeling results indicated that the ‘multi-tidal component approach’ yields more accurate and reliable hydraulic diffusivities for the unconfined aquifer (RMSE = 0.0129–0.0521 m, NSE = 0.70–0.97, and d1 = 0.91–0.99) as well as for the confined aquifer (RMSE = 0.0204–0.0545 m, NSE = 0.95–0.97, and d1 = 0.99) compared with the ‘lumped tidal component approach’. A comparative evaluation of data-size revealed that the short-duration datasets of the unconfined aquifer provide more reliable estimate of hydraulic diffusivity than the long-duration datasets. Further, it was found that the spring and neap tidal data yield unreasonable values of hydraulic diffusivity with considerably high values of RMSE and very low values of r 2, NSE, and d1, thereby suggesting that spring and neap tidal data are not suitable for aquifer parameter estimation. Overall, it is concluded that the GA-based tide–aquifer interaction model following ‘multi-tidal component approach’ is the most efficient tool for estimating aquifer parameters of unconfined and confined aquifers from tide–aquifer interaction data. The developed methodology is also applicable to other coastal basins of the world irrespective of hydrogeological settings.

A Matlab based GUI Application in Hydro seismicity of the Koyna Warna Region, India

Hydroseismicity was proposed to explain the occurrence of intraplate earthquakes in regions. Koyna - Warna region one of the reservoir triggered seismicity zone in India, which is an example of intraplate earthquake region. The seismicity is continuing after 45 years of impoundment of the Koyna reservoir in 1961 and 25 years of impoundment of Warna reservoir in 1985. The excess of previous maximum water level, the annual rise of water level in the reservoirs and filling rate of these reservoirs are some of the important factors in increasing the pore pressure. In this study, we used the daily changes in the water levels in both the reservoirs and calculated the pore pressure from starting of the impoundment of the reservoir up to 2008. We also studied the pore pressure changes with depth for various values of hydraulic diffusivity. An automatic formulation has been developed in Matlab for computing and plotting the pore pressure history.

Locally triggered seismicity in the central Swiss Alps following the large rainfall event of August 2005

In 2005 August, an unusual series of 47 earthquakes occurred over a 12-hr period in central Switzerland. The earthquakes occurred at the end of 3-d period of intensive rainfall, with over 300 mm of precipitation. The highest seismicity occurred as two distinct clusters in the region of Muotatal and Riemenstalden, Switzerland, a well-known Karst area that received a particularly large amount of rainfall. The large increase in seismicity, compared to the background, and the short time delay between the onset of the intense rainfall and the seismicity strongly suggest that earthquakes were triggered by rainfall. In our preferred model, an increase in fluid pressure at the surface due to a large amount of rain leads to a local increase in pore fluid pressure at depth. The increase in pore fluid pressure will reduce the shear strength of a porous medium by counteracting normal stress and, at the end, provoke failure. The series of triggered earthquakes in central Switzerland occurred in regions that have been seismically active in the past, showing similar hypocentre locations and magnitudes. This suggests that these earthquakes occurred on existing faults that were critically stressed. We modelled the intense rainfall as a step increase in fluid pressure at the surface that migrates to greater depths following the solution of the one-dimensional diffusion equation in a homogeneous half space. This allowed us to estimate the hydraulic diffusivity by plotting triggered seismicity in a time–depth plot. We found values of hydraulic diffusivity in the range from 0.01 to 0.5 m2 s−1 for our study area. These values are in good agreement with previous studies on earthquakes that were triggered by fluids, supporting the idea that the observed earthquake series was triggered by the large amount of rainfall.

A thermal pressurization model for the spontaneous dynamic rupture propagation on a three‐dimensional fault: 2. Traction evolution and dynamic parameters

We investigate the dynamic traction evolution during the spontaneous propagation of a 3‐D earthquake rupture governed by slip‐weakening or rate‐ and state‐dependent constitutive laws and accounting for thermal pressurization effects. The analytical solutions as well as temperature and pore pressure evolutions are discussed in the companion paper by Bizzarri and Cocco. Our numerical experiments reveal that frictional heating and thermal pressurization modify traction evolution. The breakdown stress drop, the characteristic slip‐weakening distance, and the fracture energy depend on the slipping zone thickness (2w) and hydraulic diffusivity (ω). Thermally activated pore pressure changes caused by frictional heating yield temporal variations of the effective normal stress acting on the fault plane. In the framework of rate‐ and state‐dependent friction, these thermal perturbations modify both the effective normal stress and the friction coefficient. Breakdown stress drop, slip‐weakening distance, and specific fracture energy (J/m2) increase for decreasing values of hydraulic diffusivity and slipping zone thickness. We propose scaling relations to evaluate the effect of w and ω on these physical parameters. We have also investigated the effects of choosing different evolution laws for the state variable. We have performed simulations accounting for the porosity evolution during the breakdown time. Our results point out that thermal pressurization modifies the shape of the slip‐weakening curves. For particular configurations, the traction versus slip curves display a gradual and continuous weakening for increasing slip: in these cases, the definitions of a minimum residual stress and the slip‐weakening distance become meaningless.

Determination of hydraulic parameters of an unconfined alluvial aquifer by the floodwave-response technique

Groundwater constitutes a vital component of the water-resource system. Adequate knowledge of aquifer parameters is of utmost importance for proper groundwater management. In the present study, an attempt has been made to critically analyze the river–aquifer interaction and to explore the practicability and reliability of the floodwave-response technique in estimating the hydraulic parameters of the Konan aquifer, Japan. The analysis of the pertinent hydrologic and hydrogeologic data revealed a strong relationship between the Monobe River stage and the groundwater levels over a major portion of the Konan basin. Inverse modeling, using the floodwave-response model at four sites, yielded high values of hydraulic diffusivity that ranged from 16 to 194 m2/s. However, the overall average aquifer diffusivity was found to be reasonable (0.7–3.5 m2/s). All the selected flood events were not found suitable for determining reasonable values of hydraulic diffusivity at a given site. Using the optimal diffusivities at two sites and the aquifer transmissivity from the pumping tests at these sites, the storage coefficient values were estimated as those of truly confined aquifers. In contrast, reasonable values of storage coefficient were obtained based on the average aquifer diffusivity estimates. The sensitivity analysis of the floodwave-response model indicated that the optimal hydraulic diffusivity is very sensitive to the distance parameters of the model, which necessitate precise measurement of these parameters. It is concluded that a judicious application of the floodwave-response technique is essential, especially in the case of unconfined aquifers.

Mutual relationship between microseismicity and seismic reflectivity: Case study at the German Continental Deep Drilling Site (KTB)

Two fluid injection experiments were conducted at the German Continental Deep Drilling Site (KTB) in 1994 and 2000. Microseismicity occurred at different depth intervals. Hypocenter locations were precisely localized. Here, microseismicity is analyzed in terms of its spatio‐temporal evolution characteristics. An approach is applied which assumes microseismicity to be triggered by a diffusive process of pore pressure relaxation. The method yields estimates of hydraulic parameters of rocks on large spatial scales. At the KTB site the method enables us to study hydraulic diffusivity at two different depth intervals. We observe significant variations in the evolution characteristics of the seismic activity at different depths. Estimates of hydraulic diffusivity for shallower parts of the crust seem to be much smaller than for deeper regions. To understand reasons for this, we have analyzed spatial relations of hypocenter locations to the distribution of intensities of seismic reflections. Low values of hydraulic diffusivity correlate with low reflection intensities and high diffusivities with large intensities, respectively. The analysis confirms the hypothesis that the process of pore pressure relaxation along pre‐existing and critically stressed natural fractures is an important triggering factor for induced microseismicity.

Hydrodynamic characterization of a Sahelian coastal aquifer using the ocean tide effect (Dridrate Aquifer, Morocco)

In wells tapping coastal aquifers, piezometric fluctuations can be observed in response to the ocean tide. Simultaneous recordings of the ocean tide and of the piezometric variations may provide a basis for characterizing the hydrodynamics of the aquifer. This approach was attempted to characterize the Dridrate aquifer, located on the Atlantic coast of Morocco. This aquifer accounts for most of the regional drinking water resources. However, its hydrodynamic characteristics are very poorly known. The study compares observed and simulated piezometric fluctuations, under various assumptions (confined, semi-confined aquifer). The model, which best explains the hydrodynamic behaviour of this aquifer is a semi-confined and strongly heterogeneous aquifer model (calculated hydraulic diffusivity values vary over several orders of magnitude). This result is new and rather surprising, since to date this aquifer was considered confined in view of its geological setting. Consequently, new questions are raised regarding the protection and management of the groundwater resources of this aquifer.

A physical explanation for the positioning of the depth to the top of overpressure in shale‐dominated sequences in the Gulf Coast basin, United States

A one‐dimensional model of fluid pressure and porosity evolution is used to investigate the physical processes that control the development and maintenance of overpressure in a compacting sedimentary basin. We show that for shale‐dominated sequences the variation of the hydraulic diffusivity in both space and time is such that it produces a minimum between 2 and 4 km depth, consistent with observations from the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a “bottleneck” and thus determines the shallowest position of the depth to the top of overpressure. Above this region of bottleneck, overpressure does not develop because the porosity is sufficiently large to maintain high values of hydraulic diffusivity that are conducive to the rapid dissipation of excess fluid pressure. Within the overpressured shales, compaction propagates downward through the section, releasing fluids from the upper part of the section while continuing to restrain the upward flow of fluids from deeper within the section. As such, overpressures are predicted to be maintained within the deeper regions of a basin for tens to hundreds of millions of years. Further, fluid viscosity plays an important role in defining the depth behavior of hydraulic diffusivity as a function of time. Assuming a temperature‐dependent fluid viscosity guarantees that the hydraulic diffusivity minimum will always exist during the development of the basin. On the basis of our results, we find that the depth at which the porosity equals 14±4% correlates with the depth to the local hydraulic diffusivity minimum and thus the depth to the top of overpressure. Moreover, we interpret that the 14±4% represents the threshold porosity for which a shale actually begins to act as a seal. Within the Gulf Coast basin, the gross sediment facies consists of lower massive shales across which deltaic systems have prograded allowing the deposition of an alternating series of sandstones and shales that grade vertically into massive sandstones. The massive sandstones are highly permeable and are connected hydrologically to the surface. We conclude that these sandstones play little role in the development of overpressure because of their high permeability except to the extent that the base of the massive sandstones marks the minimum depth possible for the top of overpressure. In contrast, overpressuring is observed to develop within either the shale‐dominated sequence or the region of interspersed/interfingering sands and clays. The clay‐encompassed sands play only a passive role in the development and maintenance of overpressure because it is the low‐permeability clays that control the movement of fluids into and out of the sands.

Interpretation of the transient flow rate obtained from constant-head tests conducted in situ in clays

Hydraulic tests conducted in situ using a source condition of constant head are frequently employed to determine the hydraulic properties of low-permeability clays. In this paper, an analytical model is developed for analyzing the results of a constant-head test conducted under conditions where the influence of finite-thickness skin and partial penetration are present at the source well. The analytical model is derived by application of the Laplace transform method with respect to time and the finite Fourier cosine transform with respect to the vertical coordinate. The solution is used to produce type curves of dimensionless flow rate versus dimensionless time so as to investigate the influence of finite thickness skin and partial penetration on the results of constant-head tests. Results show that the presence of a skin zone of finite thickness having permeability less than the formation produces an inflection point in the type curves at which point the dimensionless flow rate tends asymptotically towards a steady value. For the case where the skin and formation permeabilities are similar, these type curves can be used to uniquely define both the skin and formation properties. Where the skin zone is very thin, the shape of the type curves mimics the curve for a uniform, fully confined medium, and thus only the properties of the skin are measured using type-curve analysis. Conversely, the type curves for the case where the skin zone is of greater permeability than the formation show a unique and interpretable shape for each skin thickness and ratio of formation to skin permeability over most practical values of hydraulic diffusivity. Additional effects due to partial penetration were found to be pronounced in piezometers completed with small screen lengths. Particular conditions where the conventional steady approximation can be used are also discussed. Key words : constant-head test, flow rate, skin zone, partial penetration.

Earthquakes related to hydraulic mining and natural seismic activity in western New York State

We have monitored the seismic activity of western New York since 1970 using a combination of up to eight permanent seismograph stations and several portable stations. Our investigation centered on the Attica-Dale area, the site of several damaging earthquakes in this century. Although the background level of seismicity was found to be extremely low, less than one event per month in the first 12 months of our study, this near quiescence was broken in 1971 by a sharp increase in seismicity at our site near Dale following the initiation of fluid injection under high pressure (120 bars tophole) at a hydraulic mining operation nearby. This facility, which mines salt from the Vernon Formation of Silurian age, is centered near the Clarendon-Linden Fault, a major north-south trending system of high-angle thrust faults that extends for over 100 km from Lake Ontario to Allegheny County, New York. Although the seismic events were small (none were recorded at our station 30 km to the northwest), as many as 80 occurred per day, and many were felt locally. The marked increase in seismic activity after attaining high pressures, the closeness of these events to the bottom of the injection well, and the near cessation of activity within 48 hours of the shutdown of injection strongly suggest that this activity was caused by the triggering of tectonic strain on or near the Clarendon-Linden Fault by the high fluid pressures of the mining operation. The minimum pressure (41–48 bars) at which seismic activity occurred is consistent with predictions made by applying the Hubbert and Rubey theory of effective stress to hydrofracturing stress measurements from Alma, New York. Since 1972, five other wells at Dale have been hydrofractured at pressures of about 110 bars, but none had abrupt changes in seismic activity associated with them during the high-pressure phases of pumping. The well used in 1971 (0.43 km deep) is the closest (about 50 m) of the six to the Clarendon-Linden Fault. It was hydrofractured near the base of the salt layer, whereas the others were hydrofractured well within the salt layer. Thus fluids under high pressure appear to have reached the Clarendon-Linden Fault in 1971 but not in the other five cases. A focal mechanism involving thrust faulting on a plane nearly parallel to the Clarendon-Linden Fault was obtained from events that continue to occur but at a much reduced level in the brine field in 1974 and 1975. An earthquake of magnitude 2.7, which occurred about 7 km to the west of the brine field in 1973, is apparently unrelated to the injection operation and is of natural origin. This and other nearby natural events appear to be associated with the western and southwestern branches of the Clarendon-Linden Fault. Hydraulic diffusivity values calculated from the space-time relationship of earthquakes triggered by fluid injection at Denver and Rangely, Colorado, Matsushiro, Japan, and Dale are about 104 and 105 cm2/s and are similar to those obtained from precursory anomalies of earthquakes. The similarity of the diffusivity values suggests that the precursory changes do involve the movement of water. In each of these four places the wells used for fluid injection bottomed into or very close to major fault zones.

Field and laboratory determination of the hydraulic diffusivity of a confining bed

The field and laboratory evaluations of the hydraulic diffusivity of a confining bed are presented. The field approach consisted of measuring the head in the confining layer by placing piezometers equipped with differential pressure transducers at different depths in the confining layer. Pumping from the overlying aquifer resulted in an observed head change at the aquifer confining bed interface which for long periods of time could be approximated by a step drawdown. The best fit of observed data to the set of theoretical type curves for the step drawdown solution yields a hydraulic diffusivity, K'/S'S, of 2.9 × 10−2 cm2/sec. Hydraulic diffusivity values for the natural effective load, as obtained in the laboratory using consolidation techniques, are in general agreement with this result. Hydraulic conductivity values as determined in the laboratory by consolidation and constant flow methods are generally similar for the same sample. Particle size distribution and the clay mineralogy are also given for each sample.