Measuring Groundwater Flow Velocities near Drinking Water Extraction Wells in Unconsolidated Sediments

Groundwater is an important source of drinking water in coastal regions with predominantly unconsolidated sediments. To protect and manage drinking water extraction wells in these regions, reliable estimates of groundwater flow velocities around well fields are of paramount importance. Such measurements help to identify the dynamics of the groundwater flow and its response to stresses, to optimize water resources management, and to calibrate groundwater flow models. In this article, we review approaches for measuring the relatively high groundwater flow velocity measurements near these wells. We discuss and review their potential and limitations for use in this environment. Environmental tracer measurements are found to be useful for regional scale estimates of groundwater flow velocities and directions, but their use is limited near drinking water extraction wells. Surface-based hydrogeophysical measurements can potentially provide insight into groundwater flow velocity patterns, although the depth is limited in large-scale measurement setups. Active-heating distributed temperature sensing (AH-DTS) provides direct measurements of in situ groundwater flow velocities and can monitor fluctuations in the high groundwater flow velocities near drinking water extraction wells. Combining geoelectrical measurements with AH-DTS shows the potential to estimate a 3D groundwater flow velocity distribution to fully identify groundwater flow towards drinking water extraction wells

Self-Potential Signals Generated by the Corrosion of Buried Metallic Objects with Application to Contaminant Plumes

Large-amplitude (\u3e100 mV) negative electric (self)-potential anomalies are often observed in the vicinity of buried metallic objects and ore bodies or over groundwater plumes associated with organic contaminants. To explain the physical and chemical mechanisms that generate such electrical signals, a controlled laboratory experiment was carried out involving two metallic cylinders buried with vertical and horizontal orientations and centered through and in the capillary fringe within a sandbox. The 2D and 3D self-potential (SP) data were collected at several time steps along with collocated pH and redox potential measurements. Large dipolar SP and redox potential anomalies developed in association with the progressive corrosion of the vertical pipe, although no anomalies were observed in the vicinity of the horizontal pipe. This discrepancy was due to the orientation of the pipes with the vertical pipe subjected to a significantly larger EH gradient. Accounting for the electrical conductivity distribution, the SP data were inverted to recover the source current density vector field using a deterministic least-squares 4D (time-lapse) finite-element modeling approach. These results were then used to retrieve the 3D distribution of the redox potential along the vertical metallic cylinder. The results of the inversion were found to be in excellent agreement with the measured distribution of the redox potential. This experiment indicated that passively recorded electrical signals can be used to nonintrusively monitor corrosion processes. In addition, vertical electrical potential profiles measured through a mature hydrocarbon contaminated site were consistent with the sandbox observations, lending support to the geobattery model over organic contaminant plumes

Drone-Borne Electromagnetic (DR-EM) Surveying in The Netherlands: Lab and Field Validation Results

In the past decade, drones have become available and affordable for civil applications, including mapping and monitoring the Earth with geophysical sensors. In 2017 and 2019, the feasibility of executing frequency domain electromagnetic (FDEM) surveys using an off-the-shelf drone was investigated at Deltares Institute. This paper reports firstly the preparatory tests executed to determine the optimal instrumental configuration, flight path, data processing and inversion schemes and secondly the three field validation tests executed to demonstrate the feasibility of the drone-borne electromagnetic survey in real-scale applications. At several test sites, the optimal configuration of the drone and electromagnetic instruments, such as the mounting device and distance of the electromagnetic (EM) sensor with respect to the drone, the flight altitude, the coil separation and frequency of the EM source, efficiency and safety, and the assemblage of instrument and drone data were investigated. This has resulted in a robust method to acquire accurate and repeatable in-phase, quadrature and apparent resistivity data, and a workflow for data correction, processing and inversion scheme was developed. During those tests, three EM instruments were tested. The drone-borne electromagnetic (DR-EM) system has the ability and efficacy to fly over inaccessible areas and surface water. Compared to helicopter-borne electromagnetic surveys, the spatial resolution is much higher, which allows very detailed 3D mapping of subsurface targets, and the survey costs are relatively low. Repeated drone-borne electromagnetic (DR-EM) surveys allow low-cost monitoring of local changes in water saturation and salinity

Characterisation of ground motion recording stations in the Groningen gas field

The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (VS). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ VS values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed VS profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent VS information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative VS profiles at the accelerograph station sites. The measured VS profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the VS profile and the observed amplification from vertical array stations is also excellent

Spatial monitoring of groundwater drawdown and rebound associated with quarry dewatering using automated time-lapse electrical resistivity tomography and distribution guided clustering

Dewatering systems used for mining and quarrying operations often result in highly artificial and complex groundwater conditions, which can be difficult to characterise and monitor using borehole point sampling approaches. Here automated time-lapse electrical resistivity tomography (ALERT) is considered as a means of monitoring subsurface groundwater dynamics associated with changes in the dewatering regime in an operational sand and gravel quarry. We considered two scenarios: the first was unplanned interruption to dewatering due to a pump failure for a period of several days, which involved comparing ALERT monitoring results before and after groundwater rebound; the second involved a planned interruption to pumping over a period of 6 h, for which near-continuous ALERT monitoring of groundwater rebound and drawdown was undertaken. The results of the second test were analysed using distribution guided clustering (DGC) to provide a more quantitative and objective assessment of changes in the subsurface over time. ALERT successfully identified groundwater level changes during both monitoring scenarios. It provided a more useful indication of the rate of water level rise and maximum water levels than piezometer monitoring results. This was due to the piezometers rapidly responding to pressure changes at depth, whilst ALERT/DGC provided information of slower changes associated with the storage and delayed drainage of water within the sediment. By applying DGC we were able to automatically and quantitatively define changes in the resistivity sections, which correlated well with the direct observations of groundwater at site. For ERT monitoring applications that generate numerous time series, the use of DGC could significantly enhance the efficiency of data interpretation, and provide a means of automating groundwater monitoring through assigning alarm thresholds associated with rapid changes in groundwater conditions

Complex conductivity of soils

The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m21, NaCl, 258C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in-phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent

Συνεχής έλεγχος και ο ρόλος της ανάλυσης δεδομένων- εφαρμογή σε διάφορους τομείς επιχειρήσεων

Η εργασία αφορά στον τρόπο με τον οποίο οι διάφορες επιχειρήσεις μπορούν να κάνουν χρήση της ανάλυσης δεδομένων και να βελτιώσουν την θέση τους.The dissertation deals with how companies can use the data analysis for improving their position

Developing algorithms for time-lapse goelectrical data

In this thesis, a comparative study between the existing time-lapse inversion algorithms of geoelectrical data is taken place and based on the results a new improved algorithm is proposed. Each algorithms has its advantages and disadvantages. While ratio of independent inversion elevates areas with great changes between the phases, allowing rough changes, many artifacts are observed making the final interpretation hard. While difference inversion 1in some cases reduces those artifacts, it does not eliminates them. 4D inversion 2succeed to reduce the artifacts allowing to elevate the area of great changes, but at the same time it smoothes this change. As a results sometimes, especially in spatial changes, cannot represent the actual position and shape of this change. By using active time constrained inversion results best results are achieved since: • It smoothes areas without actual change, reducing this way the artifacts, like 4D inversion. • The same time elevates the area with the actual change since in that area the constrained are minimal. So most of the times, it represents the shape and position of the change in the best way.. It is crucial for the active time constrained inversion to pre-predict the area of the actual change. By using synthetic data, it was found that the use of the difference matrices is able to pre-predict the area with great accuracy. But even in cases that the difference matrices are not able to pre-predict the actual change, the produced ratio images are similar to the images produced with the other algorithms. Finally, in cases that independent inversion represents the actual change best, the quantity of the artifacts is making the distinguish between actual changes and artifacts hard. Generally, active time constrained inversion can be applied in all cases, making safely predictions of the area with the actual change. Comparing active time constrained inversion with the other algorithms the following advantages can be extracted: • It is the optimum choice, since it represents the actual change best. • The same time it reduces artifacts. • One solution to invert all time-lapse data simultaneous.Στα πλαίσια της παρούσας διδακτορικής διατριβής μελετώνται οι τεχνικές επεξεργασίας διαχρονικών γεωηλεκτρικών δεδομένων και βάσει των αποτελεσμάτων της μελέτης προτείνεται μια νέα βελτιωμένη τεχνική διαχρονικής αντιστροφής ηλεκτρικών δεδομένων. Κάθε τεχνική παρουσιάζει πλεονεκτήματα και μειονεκτήματα. Έτσι ενώ η αντιστροφή διαφορών αναδεικνύει τις περιοχές με τη μεγαλύτερη αλλαγή από φάση σε φάση, επιτρέποντας το μοντέλο να έχει απότομες αλλαγές, το πλήθος των τεχνουργημάτων είναι σημαντικό με αποτέλεσμα πολλές φορές να επηρεάζει την τελική εικόνα δυσκολεύοντας την ερμηνεία. Με την τεχνική της αντιστροφής διαφοράς 1σε κάποιες περιπτώσεις παρατηρείται μια ελάττωση των τεχνουργημάτων, αλλά δεν εξαλείφονται. Η τεχνική της σύγχρονης χρονοεξαρτημένης αντιστροφής 2πετυχαίνει σε πολύ μεγάλο βαθμό την εξάλειψη των τεχνουργημάτων αναδεικνύοντας την περιοχή της πραγματικής αλλαγής, αλλά όμως οδηγεί σε μεγάλη εξομάλυνση της εικόνας. Αποτέλεσμα είναι πολλές φορές, ιδίως σε μοντέλα που έχουμε και χωρική αλλαγή μεταξύ των φάσεων, να αδυνατεί να αναπαραστήσει την πραγματική θέση και μορφή της αλλαγής. Με την τεχνική της ενεργά χρονομεταβλητής αντιστροφής πετυχαίνεται ένα καλύτερο αποτέλεσμα καθώς: • υπάρχει μεγάλη εξομάλυνση σε περιοχές που δεν υπάρχουν αλλαγές, εξαλείφοντας έτσι τα τεχνουργήματα όπως συμβαίνει και με την τεχνική 4D. • Αναδεικνύονται έντονα οι περιοχές με μεγάλη αλλαγή, καθώς σε αυτές ο αλγόριθμος δεν επιτρέπει εξομάλυνση. Έτσι τις περισσότερες φορές αναπαριστάνεται καλύτερα η θέση, η μορφή και η αλλαγή μεταξύ των φάσεων. Απαραίτητο στάδιο για τη χρήση της ενεργά χρονομεταβλητής αντιστροφής είναι η προεκτίμηση της περιοχής μεγάλων αλλαγών. Από το σύνολο των συνθετικών μοντέλων που δοκιμάστηκαν, φάνηκε ότι η χρήση των πινάκων διαφοράς είναι σε θέση να αναπαραστήσει με ακρίβεια την περιοχή των αλλαγών, με αποτέλεσμα οι λόγοι που παράγονται να είναι πλησιέστερα στους πραγματικούς. Αλλά ακόμα και σε περιπτώσεις που οι πίνακες διαφορών αδυνατούν να απεικονίσουν τις πραγματικές αλλαγές, οι παραγόμενοι λόγοι είναι άμεσα συγκρίσιμοι με αυτούς των άλλων τεχνικών. Τέλος, στους λόγος που η διαφορά αντιστροφών πετυχαίνει να αναπαραστήσει την πραγματική αλλαγή, το επίπεδο θορύβου των τεχνουργημάτων είναι τέτοιο που δεν είναι ξεκάθαρη η πραγματική αλλαγή από τον θόρυβο. Γενικά, η τεχνική της ενεργά χρονομεταβλητής αντιστροφής μπορεί να εφαρμοστεί σε όλων των ειδών τα δεδομένα, ώστε να γίνει μια ασφαλής εκτίμηση των περιοχών αλλαγών. Συγκριτικά, από τις γνωστές τεχνικές, η τεχνική της ενεργής χρονομεταβλητής αντιστροφής παρουσιάζει τα ακόλουθα πλεονεκτήματα Αποτελεί τη βέλτιστη επιλογή, καθώς πετυχαίνει ρεαλιστικότερη απεικόνιση της αλλαγής. Συγχρόνως επιτυγχάνει ελαχιστοποίηση των τεχνουργημάτων. Λύνει το πρόβλημα για όλα τα διαχρονικά δεδομένα συγχρόνως

Thermoelectric self-potential and resistivity data localize the burning front of underground coal fires

Self-potential signals and resistivity data can be jointly inverted or analyzed to track the position of the burning front of an underground coal-seam fire. We first investigate the magnitude of the thermoelectric coupling associated with the presence of a thermal anomaly (thermoelectric current associated with a thermal gradient). A sandbox experiment is developed and modeled to show that in presence of a heat source, a negative self-potential anomaly is expected at the ground surface. The expected sensitivity coefficient is typically on the order of -0.5 mV degrees C-1 in a silica sand saturated by demineralized water. Geophysical field measurements gathered at Marshall (near Boulder, CO) show clearly the position of the burning front in the electrical resistivity tomogram and in the self-potential data gathered at the ground surface with a negative self-potential anomaly of about -50 mV. To localize more accurately the position of the burning front, we developed a strategy based on two steps: (1) We first jointly invert resistivity and self-potential data using a cross-gradient approach, and (2) a joint interpretation of the resistivity and self-potential data is made using a normalized burning front index (NBI). The value of the NBI ranges from 0 to 1 with 1 indicating a high probability to find the burning front (strictly speaking, the NBI is, however, not a probably density). We validate first this strategy using synthetic data and then we apply it to the field data. A clear source is localized at the expected position of the burning front of the coal-seam fire. The NBI determined from the joint inversion is only slightly better than the value determined from independent inversion of the two geophysical data sets