Subsurface Investigations Research Articles

Enhanced subsurface investigation utilizing electrical resistivity tomography and precise positioning tools (study case: Rongga landslide)

This study presents an enhanced approach for subsurface investigation using Electrical Resistivity Tomography (ERT) coupled with precise positioning tools, focusing on the Rongga Landslide in Indonesia. ERT is widely recognized for its ability to map subsurface conditions by detecting resistivity contrasts associated with geological structures and water content, especially in landslide-prone areas. The study combines GPS and digital waterpass altimeter with ERT to improve the precision of subsurface anomaly detection. Important information regarding moisture distribution, graben structures, and sliding surfaces was revealed by the generation of two ERT profiles. Our research indicates that a number of factors, including lithological characteristics, erosion along riverbanks, and tectonic activity—specifically, a nearby fault structure—may have contributed to the Rongga landslide These findings highlight the usefulness of ERT to evaluate the risk of landslides and offer useful data for geotechnical research in tectonically active areas.

Subsurface investigation for pre-foundation study utilizing integrated geophysical and geotechnical methods, UNILORIN campus

Subsurface investigation for pre-foundation study utilizing integrated geophysical and geotechnical methods, UNILORIN campus

Georadar Survey and Simulation for Subsurface Investigation at Historical Mosque of Sorghatmesh, Cairo, Egypt

Sorghatmesh mosque is a historical structure that was constructed in Cairo, Egypt, by Prince Saif El-Din Sorghatmesh in 1356. A dual-frequency ground-penetrating radar (GPR) with 250–700 MHz was used to investigate the subsurface of the Sorghatmesh mosque for restoration purposes. A total of 37 lines were surveyed on the ground floor of the mosque. The subsurface utilities were detected, and the status of the concrete base and the medium of the ground floor were assessed. A set of subsurface anomalies were detected and interpreted within the ground floor area of the mosque. In order to validate the interpretation, a trial pit was drilled on the ground floor, allowing for the visual inspection of the subsurface, and a Georadar numerical simulation was carried out to study the responses of the subsurface materials and conditions. For a better comprehension of the results, the ground floor area was categorized into five zones where the GPR interpretations between survey lines are almost similar. This work not only demonstrates the effectiveness of GPR as a non-invasive investigation tool but also highlights the potential of integrating advanced technologies into cultural heritage preservation by offering refined methodologies and insights for future research and restoration efforts.

Advanced predictive modelling of electrical resistivity for geotechnical and geo-environmental applications using machine learning techniques

Electrical Resistivity (ER) is one of the best geophysical methods for subsurface investigation, especially for geotechnical and geo-environmental studies. Being non-invasive, economical and rapid, this method is highly preferable to geotechnical engineers for continuous evaluation of soil properties along the resistivity profile. Numerous studies have been conducted to correlate the subsurface properties with the ER. However, most of the studies consider a single input variable, which is correlated with the resistivity values using some conventional regression analyses. Very few studies have been conducted to obtain the resistivity value with multiple input parameters, like unit weight, temperature, porosity, moisture content, etc. Since, the soil parameters have a combined effect on resistivity, hence, correlations between the resistivity and the multiple input parameters are urgently required for a better and more reliable result. Moreover, the non-linear properties of soil make the task more complicated. To fill up this research gap, in the present study, 2772 ER tests were conducted using seven different types of soil with different combinations of temperature, density, and water content. Using this database, a Support Vector Regression (SVR), Artificial Neural Network (ANN) model and Extreme Gradient Boosting (XGB) were developed for prediction of ER. It has been understood that all the models are acknowledged as trustworthy data modelling tools. However, the XGB model performs better with an R2 of 0.99 during the training and testing phase. Further, a parametric study was also done to determine, how each input parameter affects the ER. An error analysis was also performed to see the consistent discrepancy between the experimental and projected values of ER. The outcomes validate the robustness of the XGB model, indicating that it can serve as a substitute method for ER prediction.

Advancements and applications of GMS3: a surface and subsurface unified database system

Abstract GPR Mobile Mapping System 3D (GMS3) combines three-dimensional ground-penetrating radar (GPR) subsurface data with omnidirectional camera surface mobile mapping system (MMS) imagery to create a high-precision unified surface-subsurface database where underground features are easily localizable from the surface. In the context of Japan, the system is generally able to detect cavities as deep as 1.5 m (depending on soil water content) with a maximum horizontal positioning error of a few cm. The main application of GMS3 in Japan is road subsurface cavity investigations aimed at sinkhole prevention, detecting an average of 0.53 cavities per surveyed km in Japan’s national roads, and 4.76 cavities per km in municipal roads in the case of Matsuyama City. We also evaluate the feasibility of 3D mapping of buried pipes (necessary before undergrounding works) at the Johoku Campus of Ehime University. Since both services show increasing demand in Japan due to the country’s deteriorating infrastructure and its susceptibility to natural disasters, GMS3 is continuously being developed to stand out as a vital tool in subsurface surveying both in the Japanese and the international context.

DL-GPRd: Deep Learning for Ground Penetrating Radar Deconvolution

Abstract Ground-penetrating radar (GPR) is a vital tool for subsurface investigation, but traditional deconvolution methods have limitations. This study explores the integration of Deep Learning (DL) techniques to enhance GPR data processing. The model utilizes Bi-Directional LSTM neurons and synthetic data generation for training. Results show improved resolution and reduced noise compared to classical methods. Computational analysis highlights the efficiency of the approach, with faster prediction times. This DL-based deconvolution offers promising advancements for GPR imaging, enhancing subsurface characterization and exploration capabilities.

Comparison of torsional and SH-waves in isotropic and anisotropic media based on laboratory measurements

Abstract Utilization of the torsional wave signal has been reported in medical testing and engineering detection. A torsional wave is a shear-wave train whose vibrating direction (polarization) forms a torsional motion. However, there is a lack of understanding of its characteristics and potential for subsurface investigations. Here, compressional (P), shear (S), and torsional (T) transducers are constructed using different piezoelectric-transformer (PZT) chips in the laboratory, and the characteristics of the torsional (T) wave propagating in isotropic and fracture-induced anisotropic media are analyzed in line with conventional P- and S-waves, covering waveform, amplitude, arrival time, and velocity at different propagating angles. The results indicate that the velocity and anisotropy values measured using the T transducer are almost the same as the SH-wave. For propagation in isotropic and anisotropic media, the wavefield generated by the T transducer is very simple with no mode conversions and source-generated interferences, which is similar to the SH-wavefield, while the SV-wavefield comprises a series of complex events from mode conversions and interferences. For propagating in fluid-saturated fractured rocks, the torsional wavefield also comprises a fast and slow wave train, which is referred to as torsional wave splitting, and is similar to the shear-wave splitting from S transducers. Furthermore, the anisotropy parameters measured using P, S, and T transducers show a good correlation with the fracture density, and the P-wave anisotropy is higher in air-saturated rocks than water-saturated rocks, while the shear-wave anisotropy measured by the S and T transducers is sensitive to fracture density. Like the P transducer, the T transducer is polarization-independent, or directional invariant. Therefore, utilizing the T transducer may simplify the field logistics during 3D shear-wave surveys. However, field torsional wave generation is a major issue that needs further study.

Subsurface influences on watershed nutrient concentrations and loading in a clay dominated agricultural system

Water quality issues in the Great Lakes Basin persist despite management and monitoring efforts. With changing land use and climatic conditions, the need for improved understanding of nutrient loss from agricultural field to surface water is imperative to reduce consequences on human and aquatic life, such as perpetual algal blooms and contaminated drinking water. There remains a gap in holistic rural water management studies that investigate the understanding of the water cycle in rural settings and the interconnections between different components of the water cycle, including artificial drainage. In the current study, a year-round paired investigation of subsurface and nutrient dynamics was conducted in a small, intensively managed agricultural watershed within southwestern Ontario, Canada. Nutrients (Nitrate and Phosphorus) and stable isotopes of water (δ2H and δ18O) were sampled at varying scales (i.e., field and watershed), from multiple hydrologic components (e.g., groundwater, surface water, tile flow, and soil water), from 2020 to 2022. Results indicate that nutrient concentrations in surface water were elevated during the growing season, while nutrient loading was greater during the non-growing season in surface water. Monthly nitrate loads were found to be significantly (p < 0.01) related to groundwater elevation, soil moisture and tile flow, whereas total phosphorus loading was found to be significantly (p < 0.01) related to soil moisture and tile flow. δ2H and δ18O signatures revealed that during the growing season nutrient movement was predominantly through interflow pathways (including tile drainage) comprised of event-water. In the non-growing season, there was an increased subsurface influence on nutrient transport. This study highlights the importance of including subsurface characterization in nutrient management studies. A comprehensive understanding of varying seasonal hydrologic mechanisms that control nutrient movement within an agricultural watershed is essential to sustain growing rural communities.

Application of Experimental Configurations of Seismic and Electric Tomographic Techniques to the Investigation of Complex Geological Structures

The main purpose of this study is the subsurface investigation of two complex geological environments focusing on the improvement of data acquisition and processing parameters regarding electric and seismic tomographic techniques. Two different study areas, in central–east Peloponnese and SE Attica, were selected, where detailed geological mapping and surface geophysical survey were carried out. The applied geophysical survey included the application of electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and ground penetrating radar (GPR). The geoelectrical measurements were acquired with different arrays and electrode configurations. Moreover, various types of seismic sources were used at seventeen shot locations along the seismic arrays. For the processing of geoelectrical data, clustered datasets were created, increasing the depth of investigation and discriminatory capability. The seismic data processing included the following: (a) the creation of synthetic models and seismic records to determine the effectiveness and capabilities of the technique, (b) spectral analysis of the seismic records to determine the optimal seismic source type and (c) inversion of the field data to create representative subsurface velocity models. The results of the two techniques successfully delineated the complex subsurface structure that characterizes these two geological environments. The application of the ERT combined with the SRT are the two dominant, high-resolution techniques for the elucidation of complex subsurface structures.

Estimation of top depth to underground targets of Karous-Hjelt and Fraser filtering of VLF-EM measurements: The Thumb's rule approach

Prediction of the exact location and depth of underground targets with the very low-frequency electromagnetic (VLF-EM) technique is one of the most important and difficult tasks in geophysical investigations. This study examined and compared the conventional 2D KIFFILT inversion pseudo-section and the use of Thumb's rule technique in the Fraser filter plot to estimate the top depth of underground targets. The VLF-EM measurement was performed over several empirical buried target models to identify anomalies or geophysical responses corresponding to subsurface targets. The Karous-Hjelt and Fraser filtering techniques were applied to estimate the depth of the identified anomalies using Thumb's rule and the conventional 2D KIFFILT inversion. The signal behaviours of the VLF-EM current density pseudo-sections and the application of Thumb's rule effectiveness in delineating empirical buried target models were examined. Thumb's rule shows 65 % accuracy with the actual depth of the empirical buried target models, while the conventional 2D KHFFILT inversion shows 30 % accuracy with the actual depth of burial. Thumb's rule is more effective and precise in predicting the accurate depth of underground targets. The influences of conductive and resistive materials on VLF-EM signals and the challenges of VLF-EM surveys were discussed. Thumb's rule is suggested as a substantial technique for estimating top depth to the underground target where depth estimation is of prime interest due to its large degree of accuracy. In addition, the total depth of the current density distribution was noted to be increased when the distance between measuring points increased. This means that VLF-EM signals with longer wavelengths indicate deeper depth penetration into the ground than signals with shorter wavelengths. The accuracy of Thumb's rule regarding top depth estimation of the anomalies has been successfully tested and validated, which can be used for VLF-EM investigation where accurate depth estimation is required. The VLF-EM technique can be considered reliable for depth estimation using Thumb's rule approach, which applies to a wide range of subsurface investigations.

Effect of laser energy on the fretting wear resistance of femtosecond laser shock peened Ti6Al4V

Femtosecond laser shock peening (FsLSP) is performed on engineering alloys to improve their wear resistance in dry and oil-lubricated sliding conditions. In this study, the influence of FsLSP on the fretting wear behavior of Ti6Al4V alloy is studied. For this purpose, FsLSP treatment, with laser energies of 50, 100, 150, and 200 μJ, is performed on a Ti6Al4V sample which mainly consists of α-phase. Topographical investigations using white light interferometry reveal that with increase in the laser energy, the coverage area of laser-induced periodic surface structures (LIPSSs) decreases, and the extent of surface pitting damage increases, leading to higher surface roughness. While surface hardness also initially increases with increasing laser energy, it remains invariant when the energy is increased beyond 150 μJ. Sub-surface microstructural investigations and kernel average misorientation maps obtained from electron backscatter diffraction reveal that FsLSP treatment leads to the formation of severely deformed and mildly deformed layers along the depth of the alloy surface. Surface hardening due to FsLSP is attributed to the activation of prismatic <a>, basal <a> slip systems, and 101¯2, 112¯3 tensile twins in the severely deformed zone along with grain refinement, which is an outcome of dynamic recrystallization. Fretting wear tests indicate that the coefficient of friction (CoF) of FsLSP treated alloys is consistently lesser than that of its as-received counterpart, whose CoF is 0.38. In contrast, the wear resistance, quantified by the wear rate, wear volume and wear depth, is highest in the sample treated with laser energy of 100 μJ but lower for the as-received as well as 150 and 200 μJ laser treated samples. These results are explained on the basis of the differences in the contact area, formation of surface hardening layer and the size and integrity of asperities on the surface, which in turn influences the dominant fretting wear mechanism.

Assessment of piles’ resistance driven sequentially in fine-grained soils using pile load tests

This paper presents a field pile load test program conducted on four 0.36 m closed-end steel pipe (CEP) piles with lengths ranging between 11 to 13 m installed in fine-grained soils. Subsurface investigations with standard penetration tests (SPT) and cone penetration tests (CPT) with pore pressure measurements were performed at the site. Three pushed-in piezometers at incremental offsets from the piles were also installed to monitor pore water pressure changes during and after the installation of piles. Several dynamic load tests (DLT) were performed at different times to observe the change in pile resistance. A static load test (SLT) was also performed on one of the piles. Some load test results showed an unexpected decrease in the resistances of some piles with time. The study showed that construction activities, e.g. installation of other piles, disturbs the soil and groundwater conditions which can significantly affect the pile resistance measured during load tests. This investigation revealed that pile driving and restrikes should be scheduled such that the effect of construction activities on load tests results will be avoided or minimized.

Radiocarbon dating of samples connected to rock-cut features at Myrina Kastro (Lemnos Island, Greece) and calculation of the regional marine reservoir effect in the Late Bronze Age in the Aegean Sea

AbstractThe Kastro peninsula constitutes the extension towards the West of Myrina, the Lemnos capital, on the western coast of the island, in the North Aegean Sea. The ongoing research project on rock-cut features and rock-art of this complex site included a five-year (2002–2007) subsurface investigation, during which, among other mobile finds, charcoal and seashell samples were also collected, associated in situ to rock-cut features. Subsequently, in an attempt to bring about information on the dating of the rock-cut site, an investigation based on 14C has also been undertaken. Therefore, the purpose of the present paper is the AMS dating of the unearthed anthropogenic deposits and the calculation of the regional marine reservoir effect during the end of the Late Bronze Age. Our results show that the age of the deposits is spanning from the 13th century BC till the 6th century AD. Moreover, the 14C ages of two pairs of charcoal-seashell samples showed that the mean marine reservoir age R(t) in this region from the 13th to the 10th centuries BC is 175 ± 59 14C yrs and the mean local sea surface reservoir deviation ΔR is found to be –288 ± 108 14C yrs (within 1σ).

The Estimation of Shear Wave Velocity for Shallow Underground Structures in the Central Himalaya Region of Nepal

A subsurface investigation was conducted to assess the suitability of a site for potential tunnel construction, focusing on the determination of shear wave velocities (Vs) in subsurface materials. This study employed three distinct methods to analyze Vs in weathered soft rock: drilling mechanism, multichannel analysis of surface waves (MASW), and microtremor array measurement (MAM). Through the utilization of MASW and MAM, empirical relationships were established, enabling the determination of Vs based solely on soil type and depth, offering a practical alternative to the limitations of SPT N-Value, particularly when exceeding 50 blows. The comparison of Vs values obtained from these methods revealed a close alignment between empirical techniques and MASW/MAM, which proved to be cost-effective and an efficient alternative to drilling for comprehensive underground structure assessments. The reliability of MASW was further underscored through its comparison with existing empirical methods. Moreover, the empirical approach demonstrated its efficacy in predicting velocities in weathered soft rock within the Central Himalayan region of Nepal, thus enhancing the feasibility study of underground structures. Lastly, this study proposed a Vs-Depth correlation specifically tailored for highly weathered meta-sandstone bedrock resulting in clay and sandy soils.

Unraveling stratigraphic complexities of transgressive surfaces with trace fossil omission suites and juxtaposed softground suites

Recognizing stratigraphically important bounding surfaces relies on assessing the facies that juxtapose these surfaces as well as the trace fossil omission suite associated with the surface. The ability to differentiate between purely transgressive surfaces and those that conceal earlier stratigraphic surfaces that allude to a more complex history of development regarding the identification of allogenic surfaces of sequence stratigraphic significance that might have otherwise been overlooked. This study analyzes four examples of transgressive surfaces of erosion from Cretaceous-aged strata, with three of the examples representing a TSE/SB (amalgamated transgressive surface of erosion with a sequence boundary) that mask earlier base level falls. Consideration of variable omission suites along depositional strike and dip highlights the influence of substrate rheology on the nature of omission suites. In this study we demonstrate marked changes in the omission suite along bounding surfaces along proximal to distal trends that arise due to changes in underlying rheology. Finally, detailed core analysis is shown to be an imperative asset to subsurface investigation to address the limitations of using wireline well data alone in identifying stratigraphically important surfaces for more accurate regional mapping and characterization of transgressive surfaces. This study highlights that many stratigraphically important surfaces such as those that correspond to a TSE/SB may go unnoticed if utilizing wireline data alone. Overall, this study provides a comprehensive approach to further understanding and identifying transgressive surfaces in the rock record, especially those that demarcate a more complex history other than simple transgressive ravinement.

Combined onshore and offshore wide-scale seismic data acquisition and imaging for carbon capture and storage exploration in Havnsø, Denmark

Strong global actions for climate change include carbon capture and storage (CCS) as a feasible solution to reach carbon neutrality and raise opportunities for detailed subsurface investigations. An acquisition set-up designed for onshore-offshore zones was maximized for wide-scale high-resolution structural imaging and implemented to cover a domal structure of interest for CCS utilization close to the town of Havnsø (Denmark). The challenges of the combined acquisition and processing of land and marine multisensor data along a 42 km seismic profile are analyzed, the suggested solutions are applied, and the limitations are discussed. On the onshore side, a nodal array and a seismic landstreamer system were simultaneously used, whereas along the transition zone, a marine seismic streamer and ocean-bottom seismometers were added to record the seismic response generated by two seismic vibrator sources. The adopted sensing domains (velocity, acceleration, and pressure) were studied, and different processing steps were evaluated to enable their processing and subsequent data set merging. Results suggest, as the best approach, a separate prestack processing of the different data sets and the computation of new geometries and new surface-consistent residual static correction after their merging. The data acquired in the transition zone illuminate, for the first time, the subsurface geology of the region, delineating an expected domal closure. The final seismic section shows high continuity of the reflections with good resolution along the entire profile, identifying the main reservoir structure and the surrounding areas, which are important to ensure reservoir integrity and safe exploitation over longer time scales. Shallow and deep reflections are consistent with the stratigraphic column from a well log near the profile. The presented study shows a comprehensive workflow for processing such a multisensor data set in onshore and transition zone settings.

Engineering and environmental assessment of soilbag-based slope stabilisation for sustainable landslide mitigation in mountainous area

Changes in land use significantly impact landslide occurrence, particularly in mountainous areas in northern Thailand, where human activities such as urbanization, deforestation, and slope modifications alter natural slope angles, increasing susceptibility to landslides. To address this issue, an appropriate method using soilbags has been widely used for slope stabilisation in northern Thailand, but their effectiveness and sustainability require assessment. This research highlights the need to evaluate the stability of the soilbag-based method. In this study, a case study was conducted in northern Thailand, focusing on an area characterised by high-risk landslide potential. This research focuses on numerical evaluation the slope stability of soilbag-reinforced structures and discusses environmental sustainability. The study includes site investigations using an unmanned aerial photogrammetric survey for slope geometry evaluation and employing the microtremor survey technique for subsurface investigation. Soil and soilbag material parameters are obtained from existing literatures. Modelling incorporates hydrological data, slope geometry, subsurface conditions, and material parameters. Afterwards, the pore-water pressure results and safety factors are analysed. Finally, the sustainability of soilbags is discussed based on the Sustainable Development Goals (SDGs). The results demonstrate that soilbags effectively mitigate pore-water pressures, improve stability, and align with several SDGs objectives. This study enhances understanding of soilbags in slope stabilisation and introduces a sustainable landslide mitigation approach for landslide-prone regions.

Weighted Compressive Sensing Applied to Seismic Interferometry: Wavefield Reconstruction Using Prior Information

Abstract Seismic interferometry is widely used for passive subsurface investigation using seismic noise. The technique requires much storage for long noise records to suppress interferometric noise, which consists of spurious arrivals that do not correspond to the inter-receiver surface waves. Such long recordings may not be available in practice. Compressive sensing (CS), which is a wavefield reconstruction technique operating on incomplete data, may increase the availability, and reduce storage limitations of long noise time series. Using a numerical example of a linear array surrounded by sources and the Fourier basis for a sparse transform, we show that inter-receiver wavefields can be recovered at the locations where seismometers are unavailable, reducing the storage required for interferometry. We propose and develop a weighted CS algorithm that helps suppress the spurious arrivals by incorporating a priori information about the arrivals of surface waves that can be expected.

An Evaluation of ASTM Standards for Implementation of Ground Penetrating Radar for Pavement and Bridge Deck Evaluations

Abstract Ground penetrating radar (GPR) is a common nondestructive evaluation technique that operates with electromagnetic energy (EM) to detect objects and anomalies in a material’s subsurface. GPR is an effective method for civil engineering applications for the assessment of structures and roadways specifically for pavement and bridge deck condition assessment. While surveying with GPR is straightforward, requirements differ depending on the testing objective, and interpretation of the results can pose some challenges. Fortunately, the American Society for Testing Materials (ASTM) provides a standard set of suggestions for the utilization of GPR, which include general information and recommendations for operating GPR systems and analyzing the output for different purposes. However, reaching a full comprehensive understanding of each standard may require some additional knowledge. This paper focuses on three separate ASTM standards that are related to GPR implementations. These standards include the following: ASTM D6432-19, Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation; ASTM D4748-10(2020), Standard Test Method for Determining the Thickness of Bound Pavement Layers Using Short-Pulse Radar; and ASTM D6087-22, Standard Test Method for Evaluating Asphalt-Covered Concrete Bridge Decks Using Ground Penetrating Radar. After a review and assessment of these three standards, an analysis is presented to discuss the applications, contemplations, and limitations of the standards.

Monitoring of the Groundwater Level using GRACE with GLDAS Satellite Data in Ganga Plain, India to Understand the Challenges of Groundwater, Depletion, Problems, and Strategies for Mitigation

A sub-surface hydrogeological investigation conducted in the Gangetic Plain of India would involve studying the groundwater resources, aquifer characteristics, and overall hydrogeological conditions in the region. The Gangetic Plain is a vast alluvial plain formed by the sediments deposited by the Ganges and its tributaries, making it a significant groundwater (GW) bearing region, which is surrounded by a thick stratum of unconsolidated Alluvial deposits and comprises rich aquifer systems. This study describes a procedure for estimation of the subsurface GW levels for pre-monsoon, 2021 which has been determined by JPL, RL05, temporal datasets of Gravity Recovery and Climate Experiment (GRACE), and Global Land Data Assimilation System (GLDAS). The water level observed ranged from 2.3 m to 13 m in the Bhabhar and Terai Belt, 14 to 23 m in the Central Alluvial Plain and Khadar Belt, and 24 m to 31 m in the Marginal Alluvial Plain. Two types of variation are seen: local and geological variation. The GW storage justifies the remote sensing perspective and further validates our research study that regional terristrial water storage mapping can be expeditiously and accurately cognizable by the GRACE/GLDAS mission. This method is used to estimate changes in regional GW, it is a very accurate and time-limited method rather than the traditional method of GW estimation. The use of this technology will help in knowing the discharge status of GW in the state and make policies for its over-exploitation. Drinking water can be conserved to a great extent and monitoring of GW can also be done in the time frame. And attention can be given to those districts where water is being exploited more. In view of the increasing impact of climate change on water resources, this modeling will prove vital for understanding future scenarios and planning sustainable management strategies that will aid in the development of adaptation and mitigation measures.