Using water-level fluctuations in response to Earth-tide and barometric-pressure changes to measure the in-situ hydrogeological properties of an overburden aquifer in a coalfield

Abstract

For the quantitative evaluation of the impact of mining on a groundwater system, it is necessary to constrain the hydrogeological and mechanical properties. However, the in situ estimation of the mechanical properties of rock such as compressibility and porosity, is often difficult. Additionally, determining the hydraulic properties such as hydraulic conductivity, of rock by conventional methods is often expensive. The response of the groundwater level to external loading such as Earth tides and barometric pressure, couples the hydrogeological and mechanical processes of rocks, thus providing a way to infer these properties in the field. This study compared aquifer parameters inferred from tidal and barometric responses with those inferred from conventional hydraulic tests and rock mechanics tests in three groundwater monitoring wells at a site in China. The results show that the hydraulic conductivity inferred from a tidal response is similar to that of a pum** test. The compressibility values calculated for the three wells are all higher than those determined by experiment, and the porosity values calculated are all lower than those determined by experiment, but the differences between the calculated and experimentally measured values are lower than one order of magnitude. Considering the costs and convenience of the water-level response method, this method is a good choice for obtaining the properties of an aquifer, especially those in areas of tectonic activity and those affected by anthropogenic perturbations.

Résumé

Pour l’évaluation quantitative de l’impact de l’exploitation minière sur un système hydrogéologique, il est nécessaire de contraindre les propriétés hydrogéologiques et mécaniques. Cependant, l’estimation in situ des propriétés mécaniques de la roche (telles que la compressibilité et la porosité) est souvent difficile. En outre, la détermination des propriétés hydrauliques (telles que la conductivité hydraulique) de la roche par des méthodes conventionnelles est souvent coûteuse. La réponse du niveau piézométrique aux chargements externes (telles que les marées terrestres et la pression barométrique) couple les processus hydrogéologiques et mécaniques des roches, fournissant ainsi un moyen de déduire ces propriétés sur le terrain. Cette étude a comparé les paramètres de l’aquifère déduits des réponses aux marées et à la pression barométrique avec ceux déduits d’essais hydrauliques et des essais de mécanique des roches classiques dans trois piézomètres sur un site en Chine. Les résultats montrent que la conductivité hydraulique déduite d’une réponse à la marée est similaire à celle d’un essai de pompage. Les valeurs de compressibilité calculées pour les trois puits sont toutes supérieures à celles déterminées in-situ tandis que les valeurs de porosité calculées sont toutes inférieures à celles déterminées in-situ, mais les différences entre les valeurs calculées et celles mesurées expérimentalement sont inférieures à un ordre de grandeur. Compte tenu des coûts et de la commodité de la méthode de réponse des niveaux piézométriques, cette méthode est un bon choix pour obtenir les propriétés d’un aquifère, en particulier celles des zones d’activité tectonique et celles affectées par des perturbations anthropiques.

Resumen

Para la evaluación cuantitativa del impacto de la minería en un sistema de aguas subterráneas, es necesario limitar las propiedades hidrogeológicas y mecánicas. Sin embargo, la estimación in situ de las propiedades mecánicas de la roca (como la compresibilidad y la porosidad) suele ser difícil. Además, la determinación de las propiedades hidráulicas (como la conductividad hidráulica) de la roca mediante métodos convencionales suele ser costosa. La respuesta del nivel de las aguas subterráneas a la carga externa (como las mareas terrestres y la presión barométrica) acopla los procesos hidrogeológicos y mecánicos de las rocas, lo que permite inferir esas propiedades sobre el terreno. En este estudio se compararon los parámetros del acuífero inferidos a partir de las respuestas de las mareas y la presión barométrica con los inferidos a partir de las pruebas hidráulicas convencionales y las pruebas de mecánica de rocas en tres pozos de monitoreo de aguas subterráneas en un sitio de China. Los resultados muestran que la conductividad hidráulica inferida a partir de una respuesta de marea es similar a la de una prueba de bombeo. Los valores de compresibilidad calculados para los tres pozos son todos más altos que los determinados por el experimento, y los valores de porosidad calculados son todos más bajos que los determinados por el experimento, pero las diferencias entre los valores calculados y los medidos experimentalmente son inferiores a un orden de magnitud. Teniendo en cuenta los costos y la conveniencia del método de respuesta al nivel del agua, este método es una buena opción para obtener las propiedades de un acuífero, especialmente las que se encuentran en zonas de actividad tectónica y las afectadas por perturbaciones antropogénicas.

摘要

为了定量评估采矿对地下水系统的影响, 有必要确定水文地质和力学参数。但是, 通常难以对岩石的力学参数(例如可压缩性和孔隙度)进行原位估计。另外, 通过常规方法确定岩石的水力性质(例如渗透系数)通常是昂贵的。地下水位对外部负荷(例如潮汐和大气压力)的响应将岩石的水文地质和力学过程联系了起来, 从而提供了一种估算野外这些参数的方法。本研究将潮汐和气压响应估算的含水层参数与常规水力测试和岩石力学测试推断出的参数, 在**某地的三口地下水监测井中进行了比较。结果表明,由潮汐响应推断出的水力传导率与抽水试验结果相似。从三口井计算的可压缩性值均高于通过实验确定的可压缩性值,并且所计算的孔隙率值均低于通过实验确定的结果,但计算出的值与实验测量值之间的差异小于一个数量级。考虑到水位响应方法的成本和便利性,该方法是获得含水层参数一种好方法,特别是在构造活动区域和受人为扰动影响地区。

Resumo

Para a avaliação quantitativa do impacto da mineração em um sistema de águas subterrâneas, é necessário obter as propriedades hidrogeológicas e mecânicas. No entanto, a estimativa in situ das propriedades mecânicas da rocha (como compressibilidade e porosidade) é muitas vezes difícil. Além disso, determinar as propriedades hidráulicas (como a condutividade hidráulica) da rocha por métodos convencionais é muitas vezes custoso. A resposta do nível das águas subterrâneas à carga externa (como marés terrestres e pressão barométrica) acopla-se aos processos hidrogeológicos e mecânicos de rochas; fornecendo, assim, uma maneira de inferir essas propriedades a campo. Este estudo comparou parâmetros de aquífero inferidos, a partir de respostas barométricas e de marés, com os inferidos, a partir de testes hidráulicos convencionais e testes de mecânica de rochas em três poços de monitoramento de águas subterrâneas em uma área na China. Os resultados mostram que a condutividade hidráulica inferida a partir de uma resposta das marés é semelhante à de um teste de bombeamento. Os valores de compressibilidade calculados para os três poços são todos maiores do que os determinados pelo experimento, e os valores de porosidade calculados são todos inferiores aos determinados pelo experimento, mas as diferenças entre os valores calculados e medidos experimentalmente são inferiores a uma ordem de magnitude. Considerando os custos e conveniência do método de resposta ao nível da água, este método é uma boa escolha para a obtenção das propriedades de um aquífero, especialmente aqueles em áreas de atividade tectônica e as afetadas por perturbações antropogênicas.

Appendices

Appendix 1: notation

△h_i:

Change in the sample height during the shearing process in the ith period, m

△φ:

Phase shift between the Earth tide and atmospheric drivers, degree

A :

Amplitude response, m/strain

A _v :

Volumetric strain sensitivity, m/strain

A _s :

Areal strain sensitivity, m/strain

A ₀ :

Initial cross-sectional area of the sample, m²

a _k :

Amplitude of the kth tidal constituent, m

B :

Skempton coefficient

b :

Aquifer thickness, m

B′ :

Leakage factor, m

BE:

Barometric efficiency

D :

Hydraulic diffusivity, m²/s

h ₀ :

Initial height of the sample, m

K :

Hydraulic conductivity, m/day

LE:

Barometric loading efficiency

M₂^ET:

M₂ constituents of the Earth tides

M₂^GW:

M₂ constituents of the hydraulic head

P :

Axial load, Pa

r _c :

Radius of the casing, m

S :

Storativity

S₂^AT:

S₂ constituent of the atmospheric pressure

S₂^ET:

S₂ constituent of the Earth tide

S₂^GW:

S₂ constituent of the hydraulic head

S _s :

Specific storage, m⁻¹

T :

Transmissivity, m²/s

y _i :

Observed groundwater level at data point i, m

β :

Aquifer matrix compressibility, Pa⁻¹

β _f :

Water compressibility, Pa⁻¹

β _s :

Solids compressibility, Pa⁻¹

ε ₁ :

Axial strain

ε _a :

Areal strain

ε _v :

Volumetric strain

η :

Phase shift between water level and Earth tide, degree

ν :

POISSION ratio

ρ :

Density of water, kg/m³

σ ₁ :

Maximum principal stress, Pa

σ ₃ :

Minimum principal stress, Pa

Φ :

Porosity

ω _k :

Angular frequency of the kth tidal constituent, cycles per day (cpd)

Appendix 2: the processes of data treatment by different methods

1. Estimating hydraulic conductivity

Filtering of groundwater level data. In order to remove the disturbance of long-term trend and the other periodic loads, the data of high frequency (>3 cpd) and low frequency (<0.8 cpd) should be filtered by program TSOFT.

Removing barometric effects from groundwater level data. The horizontal and vertical flow models are based on water level in response to Earth tide. In this study the barometric effects are removed by the program Baytap-G (Zhang et al. 2019).

Tidal response analysis. The calculation of amplitude response A (i.e. A_v) and phase shift η (Eq. 3) is an essential premise to estimate aquifer parameters, so it is necessary to correct the barometric pressure effect by tidal response analysis using program Baytap-G.

Estimating hydraulic conductivity K. According to the results of tidal response analysis, T and S can be fitted and calculated by the horizontal (Eqs. 4–5) or vertical flow model (Eqs. 7–8) decided by the sign of η. Consequently, hydraulic conductivity K can be calculated.

2. Estimating compressibility β and porosity Φ

Filtering of groundwater level data. In order to remove the disturbance of long-term trend and the other periodic loads, the data of high frequency (>3 cpd) and low frequency (<0.8 cpd) should be filtered by program TSOFT.

Estimating A_sand LE.A_s is calculated by proportional relation (Eq. 14), and BE is calculated by program MATLAB based on Eq. (15), and according to the relation between BE and LE (Eq. 16), the consequent LE can be derived.

Estimating compressibility β and porosity ϕ. According to the calculated A_s and LE, compressibility β and porosity ϕ (Eqs. 10–11) can be calculated.