Karl E. Butler

Measurement of the seismoelectric response from a shallow boundary

Field experiments carried out at a site near Vancouver, Canada have shown that a shallow lithologic boundary can be mapped on the basis of its seismoelectric response. As seismic waves cross the boundary between organic-rich fill and impermeable glacial till, they induce electric fields that can be measured at the surface with grounded dipole receivers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conversions. Two types of seismoelectric signals are observed. The primary response is distinguished by near simultaneous arrivals at widely separated receivers. Its arrival time is equal to the time required for a seismic P-wave to travel from the shotpoint to the fill/till boundary. On the surface, its maximum amplitude (about 1 mV/m) is measured by dipoles located within a few meters of the shotpoint. At greater distances, the amplitude of the primary arrival decays rapidly with offset, and secondary seismoelectric arrivals become dominant. They differ from the primary response in that their arrival times increase with dipole offset, and they appear to be generated in the immediate vicinity of each dipole sensor. Our studies show that the responses cannot be attributed to piezoelectricity or to resistivity modulation in the presence of a uniform telluric current. We infer that seismically induced electrokinetic effects or streaming potentials are responsible for the seismoelectric conversion, and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experiments were small in scale, the results are significant in that they suggest that the seismoelectric method may be used to map the boundaries of permeable formations.

Seismoelectric imaging of the vadose zone of a sand aquifer

We have acquired a 300-m seismoelectric section over an unconfined aquifer to demonstrate the effectiveness of interfacial signals at imaging interfaces in shallow sedimentary environments. The seismoelectric data were acquired by using a 40-kg accelerated weight-drop source and a 24-channel seismoelectric recording system composed of grounded dipoles, preamplifiers, and seismographs. In the shot records, interfacial signals were remarkably clear; they arrived simultaneously at offsets as far as 40 m from the seismic source. The most prominent signal was generated at the water table at a depth of approximately 14 m and had peak amplitudes on the order of 1 μV∕m . A weaker response was generated at a shallower interface that is interpreted to be a water-retentive layer. The validity of these two laterally continuous events, and of other discontinuous events indicative of vadose-zone heterogeneity, is corroborated by the presence of reflections exhibiting similar characteristics in a ground-penetrating rada...

Subtraction of powerline harmonics from geophysical records

Harmonic noise generated by power lines and electric railways has plagued geophysicists for decades. The noise occurs as electric and magnetic fields at the fundamental frequency of power transmission (typically 60 Hz in North America) and its harmonics. It may be recorded directly during time‐domain measurements of electric and magnetic felds, or indirectly, by geophone cables during the acquisition of seismic data.

Cancellation of multiple harmonic noise series in geophysical records

We describe a procedure for the simultaneous estimation of multiple stationary sinusoidal contaminants in a time series and demonstrate its application to the cancellation of powerline noise in seismic and seismoelectric records. An estimate of the noise in each record is obtained by seeking a linear combination of sinusoids that are harmonics of one or more fundamental frequencies; this combination must fit the data in a least‐squares sense. The algorithm accommodates estimation windows of arbitrary length, including windows shorter than one fundamental period, without cross‐contamination between harmonic estimates.Provision is made for refining the fundamental frequencies, for which small errors (e.g., 0.01 Hz in 60 Hz) can result in significant residual noise near the ends of the record (beating). Our strategy is novel in that it uses all specified harmonics and iteratively searches frequency space for a best fit using numerical derivatives. This determination is very robust for at least two fundamenta...

Delineating Recharge to a River Valley Aquifer by Riverine Seismic and EM Methods

The City of Fredericton, New Brunswick, Canada operates a water well field adjacent to the Saint John River. Water is extracted from a semi-confined aquifer that consists of a buried sand and gravel esker ridge discontinuously covered by a clay∕silt aquitard. Geological, hydrogeochemical, and hydraulic data have long pointed to infiltration of river water as the main source of aquifer recharge but the shape and size of the recharge zone has, until recently, remained speculative. A combination of three geophysical surveys, including resistivity imaging along the shoreline, and seismic and electromagnetic profiling on the river itself, have now revealed the complex outline of the area beneath the river where the infiltration likely occurs. Broadband seismic reflections, with dominant frequencies of 2.5kHz were recorded from depths as great as 50m using a boomer source and 1KB Seistec line-in-cone receiver well suited to operation in the shallow water environment. EM31 and EM34 terrain conductivity meters, d...

Trends in waterborne electrical and EM induction methods for high resolution sub-bottom imaging

Shallow water applications of electrical and electromagnetic geophysical methods have grown in recent years with recognition of the information these methods can provide regarding groundwater-surface water interaction, geotechnical engineering, exploration, marine geology and other fields. In many applications, spatial variations in resistivity are useful as a proxy for variations in another bulk material property such as pore water salinity, clay content, porosity, or temperature. Applications of galvanic resistivity methods have been buoyed by the development of marine configurations that are now commercially available. In contrast, with two notable exceptions, most applications of EM induction methods have involved experimental adaptations of instruments originally designed for use on land. Methods for shallow water resistivity and EM induction surveys are at an exciting stage of development where several promising applications have been demonstrated but the suite of tools and components commercially available and widely tested remains relatively small.

Influence of fracture anisotropy and lithological heterogeneity on wellfield response in a fluvial sandstone aquifer of the Carboniferous Moncton Subbasin, Canada

Pump tests and geophysical logs acquired in a fluvial sandstone aquifer within the resource-rich Moncton Subbasin of New Brunswick, Canada, have been used to characterize fracture patterns and flow directions for purposes of developing a water-wellfield protection plan. Fracture patterns consist of three high-angle sets, and a low-angle set parallel to bedding. NW-trending high-angle fractures, predominantly in fluvial sandstone units, appear to be most important in controlling groundwater flow directions. This contrasts with an earlier regional hydrogeological study that attributed most flow to sub-horizontal bedding-plane fractures. Water levels monitored during a 72-h pump test revealed drawdown extension parallel to the NW-trending fracture set. Drawdown curves indicate that the aquifer is laterally constrained—likely reflecting differences in fracturing observed between the channelized sandstone and surrounding shale units. As a result, groundwater flow induced by pumping is influenced by both fracture anisotropy and by the heterogeneity of the fluvial depositional environment. Relationships observed between fracture patterns, regional geological structure and lithology provide a basis for extrapolating the conceptual model to other nearby areas in the region, where potential impacts of geological resource development on groundwater are attracting public concern.RésuméLes données des essais de pompage et des diagraphies géophysiques acquises dans un aquifère de grès fluviaux au sein du sous bassin Moncton de New Brunswick (Canada) riche en ressource, ont été utilisées pour caractériser l’organisation des fractures et les directions d’écoulement avec pour objectif de développer un plan de protection de la ressource en eau au niveau de forages. L’organisation des fractures se fait selon trois familles avec un angle élevé et une famille parallèle au plan de stratification. Les fractures avec un angle élevé de direction NW, prédominantes dans les unités de grès fluviaux, apparaissent comme étant celles qui contrôlent de manière significative les directions d’écoulement souterrain. Ceci est en opposition avec une étude hydrogéologique régionale antérieure qui associait l’essentiel des écoulements souterrains aux fractures sub-horizontales dans le plan plan de stratification. Les niveaux d’eau enregistrés au cours des 72 heures, correspondant à la durée des essais de pompage, ont révélé une extension du cône de rabattement parallèle à la famille des fractures d’orientation NW. Les courbes du rabattement indiquent que l’aquifère est contraint latéralement—probablement reflétant des différences dans la fracturation observée entre le grès avec des chenaux et les unités de schistes environnantes. Par conséquent, les écoulements d’eau souterraine induits par le pompage sont influencés aussi bien par l’anisotropie des fractures que par l’hétérogénéité présente au sein de l’environnement de dépôt fluvial. Des relations observées entre l’organisation des fractures, la structure géologique régionale et la lithologie fournissent une base pour l’extrapolation du modèle conceptuel pour d’autres zones proches dans la région, où les impacts potentiels de l’exploitation de ressource géologique sur les eaux souterraines constituent des inquiétudes pour le public.ResumenSe han utilizado ensayos de bombeo y los perfilajes geofísicos tomados en un acuífero de areniscas fluviales dentro del valioso recurso de la Moncton Subbasin of New Brunswick, Canadá, para caracterizar los esquemas de fracturas y direcciones de flujo con el propósito de desarrollar un plan de protección de agua de un campo de pozos. Los esquemas de las fracturas consisten de tres conjuntos de ángulo alto, y un conjunto de ángulo bajo paralelo a la estratificación. Las fracturas de ángulo alto con tendencia NW, predominantes en las unidades de areniscas fluviales, aparecen como las más importantes en el control de las direcciones de flujo de agua subterránea. Esto contrasta con un estudio previo hidrogeológico regional que atribuía la mayor parte del flujo de las fracturas subhorizontales del plano de estratificación. Los niveles de agua monitoreados durante 72 horas de ensayos de bombeo revelaron la depresión en una extensión paralela al conjunto de fracturas cuya tendencia es NW. Las curvas de depresión indican que el acuífero está limitado lateralmente, muy probablemente reflejando diferencias en la fracturación observada entre la arenisca canalizada y las unidades circundantes de esquistos. Como resultado, el flujo de agua subterránea inducido por el bombeo está influenciado tanto por la anisotropía de la fractura como por la heterogeneidad del ambiente depositacional fluvial. Las relaciones observadas entre los esquemas de las fracturas, la estructura geológica regional y la litología proveen una base para extrapolar el modelo conceptual a otras áreas próximas de la región, donde los impactos potenciales del desarrollo de los recursos geológicos sobre el agua subterránea están acaparando la preocupación pública.摘要在加拿大新布伦兹维克省资源丰富的蒙克顿海盆河流相砂岩含水层中采用抽水试验和地球物理测井来刻画裂隙模式和地下水流向的特征,进而用于水源地保护方案的建立。断裂模式由三组高角度和一组平行于层理的低角度的断裂组成。在河流相砂岩层中占主导地位的北西向高角度断裂是控制地下水流向的最主要因素。这与早先的认为亚水平层理面断裂是主要控制因素的区域水文地质研究形成对比。在72个小时抽水试验期间监测得到的水位数据发现降深外延平行于北西向的裂隙组。降深曲线表明含水层侧向受到了约束—这可能反映了水道砂岩和周围页岩层之间断裂的差异。因此,抽水引起的地下水流受到了断裂的各向异性和河流相沉积环境的非均质性的影响。断裂模式、区域地质构造和岩性之间的关系为把这个概念模型外推到这个地区中的其它附近区域提供了依据,在这个地区,地质资源的开发对地下水的潜在影响正引起公众的关注。ResumoEnsaios de bombagem e perfis geofísicos obtidos num aquífero arenítico fluvial localizado na sub-bacia Moncton em New Brunswick, Canadá, rica em recursos geológicos, foram usados para caraterizar padrões de fracturação e direções de escoamento com o objetivo de desenvolver um plano de proteção de um campo de captação. Os padrões de fratura consistem em três sistemas de ângulo elevado e um sistema de baixo ângulo paralelo à estratificação. As fraturas de ângulo elevado com orientação NW, predominantemente nas unidades areníticas fluviais, parecem ser as mais importantes no controlo das direções de escoamento da água subterrânea. Esta ideia contrasta com os resultados de um estudo hidrogeológico regional anterior que atribuiu o escoamento maioritário a fraturas sub-horizontais no plano de estratificação. Os níveis da água monitorizados num ensaio de bombagem, com a duração de 72 horas, revelaram rebaixamento paralelo ao conjunto de fraturas com orientação NW. As curvas de rebaixamento indicam que o aquífero é limitado lateralmente—provavelmente refletindo diferenças de fracturação observadas entre o arenito canalizado e as unidades de xistos adjacentes. Em resultado disto, o escoamento subterrâneo induzido pela bombagem é influenciado tanto pela anisotropia da fracturação como pela heterogeneidade resultante do ambiente de sedimentação fluvial. As relações observadas entre os padrões de fracturação, a estrutura geológica regional e a litologia são uma base para a extrapolação de um modelo conceptual para outras áreas próximas onde os impactos potenciais do desenvolvimento de recursos geológicos sobre a água subterrânea estão a suscitar o interesse público.

Field tests of piezoelectric exploration for quartz

Electromagnetic signals have been measured in response to seismic excitation of different geophysical targets, including mineral bodies such as quark and sulphides. The signals can be explained by conversions of seismic energy to electromagnetic energy by seismoelectric phenomena, such as piezoelectricity. In a typical experiment, explosives or repetitive mechanical sources are initiated at various distances from a target and arrival times of EM responses used to image the target. Employing hardware and software techniques to remove powerline harmonics and other environmental noise, we have measured seismoelectric fields as small as tens of microvolts per metre at frequencies below 1 kilohertz. Several experiments at surface sites have given encouraging results for exploration for quartz. At the Humboldt Site, Australia, we have measured piezoelectric responses from a massive quartz structure. A dipping surface, presumably the quartz body, was imaged using the EM arrival times and measured seismic velocity. Seismoelectric signals were produced during work at the Paymaster Site where a quartz-ankerite vein is hosted by basalts. There was encouraging evidence that an anomaly exists over the vein.

The seismoelectric response of a sandy aquifer - surface profiling experiment

E046 The Seismoelectric Response of a Sandy Aquifer – Surface Profiling Experiment J.C. Dupuis (University of New Brunswick) K.E. Butler (University of New Brunswick) & A.W. Kepic* (Curtin University of Technology) SUMMARY In October 2006 an experimental multi-channel seismoelectric survey was acquired over the sandy superficial aquifer of the Gnangara Mound which is an important water supply area serving the city of Perth Australia. The survey was motivated by the results of borehole experiments conducted during the previous two months that identified clear interfacial seismoelectric effects originating from the water table. This article presents the methods we used to acquire

Enhancement and modelling of signals from natural piezoelectric targets

Electromagnetic signals from piezoelectric targets in the earth can be generated using seismic sources and measured with electric or magnetic field receivers. The signals are typically small compared to the ambient electromagnetic noise and are difficult to identify in unprocessed records. Three data processing algorithms involving stacking, low-pass filtering, and sinusoid subtraction have been developed to enhance the signal-to-noise ratio of piezoelectric data acquired during field experiments. In addition, an analytic modelling technique has been developed to investigate the relationship between seismic and piezoelectric signals. The stacking technique, designed for use with repetitive seismic sources, employs a robust triggering algorithm that enables it to be used effectively even when the trigger signal is poor. High frequency noise is attenuated using a zero phase frequency domain low-pass filter with variable cut-off frequency and slope. The sinusoid subtraction technique is used to remove powerline noise which occurs at frequencies of 60 Hz and its harmonics. The amplitude and phase of each harmonic are estimated by calculating the Fourier series coefficients for that frequency. A sinusoid having the estimated amplitude and phase is then subtracted from the data to remove the harmonic. Remarkable improvements in the signalto-noise ratio have been achieved by sinusoid subtraction as powerline noise levels typically exceed piezoelectric signal amplitudes by factors ranging from five to a few hundred times. For purposes of analytic modelling, a piezoelectric target is represented by a number of spheres each of which become independently polarized during the passage of a spherical elastic wave. The electric potential at a point in a uniform conductive medium surrounding the target is estimated by summing the potentials due to each of the polarized spheres. It is shown that the form of the electric potential time series generated by a uniformly polarized target is approximately proportional to the first derivative of the elastic wave particle velocity. In contrast, the potentials generated by non-uniformly polarized targets, representing quartz veins, are approximately proportional to the particle velocity itself. A distinct pulse of electric potential is also generated when the elastic wave reaches the outer limits of a vein-like target; such signals could yield important information on target dimensions if they could be identified in real piezoelectric data.