Geophysical Survey Methods Research Articles

3D characterization of the Mila 18 archaeological site in Warsaw, Poland: From imaging to excavation

Archaeological site investigations in urban environments are often beset with challenges such as (1) an absence of buried artifacts due to recent disturbance from infrastructure development or (2) community concerns about potential site impacts from excavations. Noninvasive geophysical surveys that use a combination of methods can help mitigate the risks of uncertain outcomes by identifying areas where culturally significant features are more likely to be uncovered. We show how new technology and traditional geophysical survey methods were used to characterize the subsurface of the Mila 18 Memorial site in Warsaw, Poland. This site is one of the most important places of remembrance for the Holocaust and coincides with the location of an underground bunker that was used by Jewish resistance groups during the 1943 Warsaw Ghetto Uprising. In this case study, we showcase the use of drone multispectral imaging and handheld lidar scanning in conjunction with other geophysical techniques including electrical resistivity tomography, ground-penetrating radar, magnetic gradiometer, twin-probe resistance, and fixed-frequency electromagnetic surveying. The geophysical results were included in an interactive 3D site model to help identify a suitable site for excavation. To document the excavation and to validate and further interrogate the geophysical survey results, we used lidar-based photo-textured scans of the excavation that were incorporated into the 3D site model.

Integrated geochemical and geophysical monitoring for the assessment of natural pollutant mitigation in heavy metal-contaminated areas of South Korea, Deagu.

The evaluation and monitoring of sites contaminated with heavy metals are essential for pollution remediation and prevention. In this study, we conducted geophysical and geochemical investigations at a site exhibiting heavy metal contamination downstream from an abandoned mine, with the aim of analyzing the extent of contamination and its temporal variation. We employed geophysical survey methods including electrical resistivity and induced polarization surveys of areas contaminated with heavy metals. Repeated surveys were conducted over time using the electrical resistivity method. Numerical simulations were employed to mitigate and eliminate electrical noise stemming from topography on the site. Additionally, time-lapse inversion was conducted on the resistivity data sets to analyze the changes in resistivity caused by variations in heavy metal contaminants. In the geochemical survey, soil samples were collected from the same locations as the geophysical survey, and chemical properties including pH, water content, electrical conductivity, and cation exchange capacity were analyzed. Our results showed that with the reduction of major sources of As and Zn contamination by 50%, the time-lapse electrical resistivity inversion results indicated that the resistivity of the subsurface materials increased by a factor of two. This paper demonstrated the natural reduction of the heavy metal contaminants at the site due to rainfall, aiming to comprehensively analyze the resultant alteration of both geochemical and geophysical properties.

Research on the Stability and Water Isolation of Waterproof Coal Pillars between Adjacent Working Faces under the Influence of Water Ponding Goaf—A Case Study

Retaining a waterproof coal pillar is an important measure to defend against water inrush accidents in mining areas and guarantee the safe mining of the next working face. In this paper, the mechanical model of the coal pillar is established and the calculation formula of the waterproof coal pillar width is derived. Then, the development of the water-conducting fracture zone of the overlying rock layer under different coal pillar widths is analyzed using numerical simulation and finally, the integrity of the coal pillar is detected using the geophysical survey method. The main conclusions are as follows: (1) According to the mechanical failure characteristics of the coal pillar, it can be divided into the plastic zone, elastic zone, and water pressure damage zone. The mechanical calculation model for each zone was established, and the formula for calculating the width of the waterproof coal pillar was obtained. (2) Numerical simulation was employed to investigate the development condition of the water conducting fracture zone in the overlying rock strata under the actual width of the waterproof coal pillar; the simulation results indicated that the water conducting fracture zone of two working faces was not connected, which can effectively prevent the accumulation of water in the 2303 goaf. (3) On-site geophysical surveys determined that the influence of water-logged goaf on the coal pillar is between 5 to 15 m; the integrity of the waterproof coal pillar is good, which effectively prevents water accumulation in the previous working face goaf and ensures safe mining in the next working face.

INVESTIGATION OF MINERAL DEPOSITION IN IJEDA, OSUN STATE SOUTHWESTERN NIGERIA USING GROUND MAGNETIC DATA

A geophysical ground magnetic survey method was used to investigate the presence and depths of magnetic minerals at Ijeda, Oriade Local Government area of Osun-State Nigeria. A G856AX Proton precession magnetometer was used to acquire 122 magnetic data points partitioned into 11 profiles. The station intervals and spacing were 10 m and 5 m respectively between each profile. Regional gradient and the effect of a geomagnetic reference field were removed from the observed survey data, the residual anomalies obtained were plotted against distance using Microsoft excel. Analytic Signal Method (ASM) and Peter’s Half Slope Method (PHSM) were utilized in estimating average Magnetic Source Depths. The Obtained magnetic data were plotted as 1-grid vector map, 2-D contour map and 3-D surface distribution map using surfer application to produce the qualitative analysis. The maximum and maximum magnetic intensity values obtained were 25.0 nT and -41.8 nT respectively, which suggested the area of magnetic mineral occurrences. The depths to the magnetic sources were calculated for 11 profiles and the average depth values ranged from 3.93 m ± 1.22 to 7.34 m ± 1.55 using both the ASM and the PHSM. The results showed that the study area holds a very good potential for magnetic mineral exploration.

Processing GPR Surveys in Civil Engineering to Locate Buried Structures in Highly Conductive Subsoils

Many studies have illustrated the great benefit of ground-penetrating radar (GPR) in civil engineering. However, in some cases, this geophysical survey method does not produce the desired results due to the electromagnetic characteristics of the subsoil. This study presents the results obtained in two locations near Linares (southern Spain), evaluating the detection of structures buried in conductive host materials (0.02 S/m in site 1 and 0.015 S/m in site 2) characterized by strong signal attenuation. Accounting for the study depth, which was 1.5 m, a 500 MHz shielded GPR antenna was used at both sites. At the first site, a controlled experiment was planned, and it consisted of burying three linear elements. An iron pipe, a PVC pipe, and a series of precast blocks were buried at a depth of 0.5 m in a subsoil composed of highly conductive clayey facies. To eliminate additional multiples caused by other superficial structures and increasing the high-frequency content, the predictive deconvolution flow was applied. In the 3D processing, the cover surfaces technique was used. Once the acquired GPR signals was analyzed and the optimal processing flow established, a second site in which different infrastructures in a conductive host medium formed by marly facies was explored. The 2D flow and 3D processing applied in this work allows to detect and see the continuity of some structures not visible for the default processing.

Penentuan Kedalaman dan Sudut Kemiringan Sesar Grindulu di Pacitan Menggunakan Metode Crow Search Algorithm (CSA) pada Data Self-Potential (SP)

Self-Potential (SP) is a geophysical survey method that is relatively easy and inexpensive. Interpretation of SP data can be used for various purposes such as the detection of landslide-prone areas, exploration of various types of minerals, and identification of the parameters of a fault or crack. In this study, SP data acquisition was carried out in Tambakrejo Village, Pacitan District with a total of 102 measurement data which aims to determine the depth and dip of the Grindulu Fault. SP data acquired in the field needs to be corrected for reference, namely corrections caused by a displacement of the starting point of measurement. This data is then filtered to increase the signal-to-noise ratio (SNR) and sharpen the resulting anomalies. This filtering process is carried out using the ICEEMD (Improved Complete Ensemble Empirical Mode Decomposition) method which is a development of the EMD method. Furthermore, the SP data inversion process to obtain model parameters is carried out by utilizing the CSA (Crow Search Algorithm) method. Based on the anomaly model generated from the SP data inversion process, it can be concluded that the Grindulu Fault was identified at a distance of 803,8 meters from the starting point of measurement with depths ranging from 11,06 to 102,74 meters. Furthermore, based on distance, depth, and anomaly shape data, the dip value can be calculated. The calculation results show that the dip of the Grindulu Fault in the study area is 75.58o. Identification of the Grindulu Fault in the form of depth and dip is very important in efforts to model the fault comprehensively.

Application of Geophysical Survey Methods in Projects for the Elimination of Objects of Accumulated Environmental Damage

The analysis is carried out and the relevance of the application of geophysical methods for the elimination of objects of accumulated environmental damage is shown. The conclusion is made about the effectiveness of the proposed methodology, which provides a reduction in the duration of engineering surveys, as well as the cost of PIR by 1.5 times compared with traditional methods.

Испытание пластов на кабеле как метод оперативного уточнения геологогеофизических моделей и оценки запасов углеводородов на этапе геологоразведочных работ

The paper considers benefits of Wireline Formation Testing (WFT) enhanced applications to increase the field geophysical survey methods information content by using advanced pressure survey, downhole fluid analyses with sampling and interval testing including the assessment of reservoir hydrodynamic parameters (permeability, productivity, skin factor, etc.). The results present a comprehensive analysis of well logging data augmented withWFT to determine fluid level contacts and reservoirs saturation in the Lower Cretaceous deposits for a well drilled on Caspian Sea shelf. It was shown the possibility to identify potentially productive gas-saturated intervals by using well logging and WFT in conditions of reduced porosity and permeability properties. Discussed approach is also presented the way how to optimize well testing program through productivity evaluation using the WFT methods in orderto reduce the time for conducting hydrodynamic studies without loss of information content.

Continuous Automatic Estimation of Volumetric Water Content Profile During Infiltration Using Sparse Multi-Offset GPR Data

Ground-penetrating radar (GPR) is a non-destructive and non-invasive geophysical survey method that has been used to characterize soil volumetric water content (VWC) dynamics. An array antenna GPR system was used to collect nearly seamless, time-lapse multi-offset GPR data during an in-situ infiltration test on sand dunes with limited traces. Because the data volume was significant, an approach was utilized to automatically determine electromagnetic wave velocities from sparse common midpoint (CMP) data using standard velocity analysis, such as semblance analysis. The objective of this study was to develop a methodology that allows one to automatically perform velocity analysis by interpolating sparse CMP data obtained with the array GPR system. The proposed method determined the optimal normal moveout velocity values and the removal range of the F-K zone pass filter that minimized errors between the original and interpolated CMP data using leave-one-out cross-validation (LOOCV). After interpolating the sparse CMP data with the F-K zone pass filter, semblance analysis was used to determine the time-lapse velocity structure of the soil profile during water infiltration. The velocity data were converted to VWC data based on the Topp equation, which relates the soil VWC to the soil dielectric constant. The proposed method was tested using CMP data obtained via numerical simulation and experiments. The VWC profile from the proposed approach matched well with the independently observed VWC profiles obtained from an invasive probe-type soil moisture sensor.

Regional patterns, local techniques. Remote sensing and archaeology at prehispanic site of San Isidro, El Salvador

The archaeological site of San Isidro, Sonsonate, El Salvador, is located at a geographically advantageous position within the transitional zone between large study areas of Mesoamerica and Central America. It became the focus of the Polish-Salvadoran Proyecto Arqueológico San Isidro (PASI) investigation in 2018. The overarching research question of the Project is to understand the prehispanic cultural and social dynamics within this zone, and recover identities of its ancient inhabitants. A number of non-invasive research methods have been employed to accommodate the wishes of the host community to avoid disturbance of the sugarcane plantation. Drone-based mapping, carried out between 2018 and 2021, demonstrated an exceptional size of the ancient settlement, consisting of more than 50 still-visible remains of architectural structures dispersed over an area of 6 square kilometers. Three remote-sensing geophysical survey methods were used in the presumed center of the site. Ground Penetrating Radar (GPR), magnetometry, and electric resistivity analyses have all detected anomalies related to the eruption of the Ilopango volcano. Initial archaeological excavations permit tentative dating of the prehispanic San Isidro lifespan to the Middle and Late Preclassic periods (1000–400 BCE, and 400 BCE - AD 250, respectively), with possible extension into Early Classic (AD 250 – 600). Construction techniques of the investigated mounds prove to be rare in the general area, but have close analogies in the relative vicinity of San Isidro.

A Pilot Study in Archaeological Metal Detector Geophysical Survey

Abstract Metal detection (MD) has traditionally been viewed as a limited geophysical survey method for the identification of metal objects below the surface. However, this pilot study examines techniques utilizing the “ground balance” function, common to most modern metal detectors, to identify subsurface magnetic anomalies. The results of surveys have yielded inconclusive results on the use of metal detectors for feature identification. However, the results of this study suggest a high potential for more efficient and more productive archaeological reconnaissance surveys. The ground balance function, when combined with systematic sampling and geographic information systems interpolation methods, yields low-resolution subsoil magnetic susceptibility maps. Compared to other geophysical methods, such as gradiometry or electrical resistivity, the depth range of MD is limited. However, this technique, in upland contexts with shallow subsoils or sites with high potential for recent ground disturbing activities, can reveal subtle changes in the subsurface that traditional MD techniques would miss. Further studies are recommended to explore the many situations in which a metal detector can provide an informative alternative, though not a replacement, for other geophysical survey methods. This pilot study was funded by the National Geographic Society’s Early Career Grant program.

Novel insights into prehistoric land use at Stonehenge by combining electromagnetic and invasive methods with a semi-automated interpretation scheme

Geophysical survey methods have led to high-resolution mapping of subsurface remnants of ancient landscapes at continually expanding spatial scales. Yet, particularly when applied across entire archaeological landscapes, spanning hundreds of hectares, resultant datasets provide little direct information about inhabitation, environments or change over time.Focusing on a 2.5 km2 area around Stonehenge, we show how geophysical soil survey, when combined with targeted sampling and excavation, can enable reliable empirically-grounded identification of complex activity traces. Particular focus lies on anthropogenic dug pits, identification and interpretation of which are vital in European earlier prehistoric archaeology due to their close connection with inhabitation and ceremonial practices. By integrating frequency domain electromagnetic and invasive datasets, and using a semi-automated interpretation scheme, we identified previously unknown concentrations of large pits (with diameters >2.4 m) among several thousand smaller pits and natural features across the Stonehenge landscape. Excavations of a subset of identified features demonstrate that, in this area, our investigative methodology is 66% accurate for identifying large anthropogenic pits. Our results have significant implications for understanding Stonehenge and its landscape setting, revealing elusive forms of Mesolithic to later Bronze Age land use that - even within the world's most intensively researched archaeological landscape - have gone unrecognized until now.These findings underscore both the crucial role of archaeological excavation as an essential basis for reliable interpretation of geophysical data, as well as the perils of inductive visual interpretation of features’ morphologies and their spatial configuration in non-invasive survey data.

Archaeological survey in the Machili Valley, Zambia: a report on the 2019 preliminary field season

ABSTRACT After c. AD 700 the Machili Valley in Western Province, Zambia, is exemplary of the type of ‘in-between’ places that made up large portions of the African continent where states did not develop, but that were anything but isolated and undifferentiated. Limited archaeological surveys in the 1950s to 1970s situate the Machili Valley into a larger context of Iron Age life in Zambia in particular and in south-central Africa more broadly. This paper details early results from survey work in Machili conducted in 2019 that employed a combination of geophysical and shovel test survey methods to re-survey previously documented sites, identify new ones and study localised variations in iron production practices in the valley. Results suggest geographic and temporal changes in settlement patterns and iron production practices, as well as in the spatial relationships between domestic areas and iron smelting and smithing locations among Early Farming Communities in Zambia.

The Use of Multi-Geophysical Methods to Determine the Geothermal Potential: A Case Study from the Humenné Unit (The Eastern Slovak Basin)

The exploration of the geothermal potential of a geological unit has multiple aspects. The most important elements are the geological structure, the hydrogeological conditions, and heat flow. The analysis of the above-mentioned elements attempts to help maximize the use of a geothermal aquifer’s potential. When choosing the most appropriate geophysical method, it is important to consider various factors, such as basic structural parameters as well as the total cost of exploration. This is especially true for low-thermal areas. Comparison of multi-geophysical exploration methods used in the study of the carbonate aquifer of the Humenné Unit identified the advantages and weaknesses of each method. The Humenné Unit is the north basement unit of the Eastern Slovak Basin (with a heat flow value ranging from 100 to 125 mW·m−2), which is part of the Pannonian Basin (with a high heat flow ranging from 50 to 130 mW·m−2). The calculation of the geothermal potential was based on the results of several methods. Some important geophysical survey methods resulted in: (a) deep seismic cross-sections which clarified the position and overall internal structure of the aquifer (b) gravimetric measurements in the form of a map of the Complete Bouguer Anomalies (calculated for density 2.67 g·cm−3) which confirmed the presence of structural elevations and depressions. These elevations and depressions intensified the water yield, heat flow and raised the overall temperature (c) the use of geoelectric resistivity profiling, which is a fast and cheap method, but is limited by depth. The similar resistivities ρ: 80–360 Ωm of carbonates and andesites was a serious problem. The specific heat-energy potential has a wide range of 0.337–19.533 GJ/m2. The highest values above 15 GJ/m2 are reached in areas where the temperature on the surface of the Triassic carbonates exceeds 90 °C.

Review and Future Perspective of Geophysical Methods Applied in Nearshore Site Characterization

Seabed surveying is the basis of engineering development in shallow waters. At present, geophysical survey methods mainly utilize sonars for qualitative surveying, which requires the calibration of the results found through in situ drilling and sampling. Among them, the parameters required for engineering designs are obtained from either in situ tests or laboratory experiments of soil samples retrieved from drilling. However, the experience from onshore applications shows that the physical quantities obtained through quantitative geophysical survey methods for shallow waters can be indirectly used to estimate engineering parameters or directly as parameters for engineering evaluation, which has high application potential. This review analyzes various geophysical survey methods for nearshore site characterization (i.e., side-scan sonar, single/multi- beam sonar, sub-bottom profiler, seismic reflection method, and underwater magnetometer) and challenges in their application, and introduces quantitative geophysical survey methods (including the underwater seismic refraction method, seismic surface wave method and underwater electrical resistivity tomography) that are worth focusing on for future development. Three application difficulties have been identified, namely, the lack of operational efficiency, appropriate operational equipment and systems, and sufficient guidance for experimental shallow sea applications. It is hoped that comprehensive discussion of these challenges will increase awareness leading to engineering improvements in the surveying and measuring capabilities in shallow waters, further reducing the risk of geotechnical hazards.

PRELIMINARY GEOLOGICAL INVESTIGATION INTO THE CAUSES OF FOUNDATION FAILURE IN JALINGO, NORTHEAST NIGERIA

Post foundation study for the preliminary causes of cracks on buildings at the Central Bank of Nigeria (old site) Jalingo branch, Northeast Nigeria, was carried out using a geophysical approach and groundwater level monitoring. The geophysical survey method employed was the electrical resistivity method. Vertical electrical sounding (VES) was carried out using a Schlumberger electrode array with a maximum half current electrode separation of 20m, vertical electrical sounding (VES) was carried out. The VES results revealed the presence of three (3) geoelectric subsurface layers in the area, namely clayey-sand, fractured/weathered basement, and crystalline basement. The resistivity of the clayey-sand varies from 45-210 Ωm, while that of fractured/weathered basement varies from 305 - 5858 Ωm and that of the crystalline basement varies from 1000-10075 Ω-m with corresponding thicknesses ranging from 0.5 - 2.3 m, 0.2 - 6 m, 2.6 – 4 m to an infinite depth respectively. The study area's field curves were analysed; H and Q-curve types with 35.7% and 64.3%, respectively. Iso-resistivity delineation at 0.5 m, 1.5 m and 3 m showed that the region with high resistivity corresponding to the crystalline basement is found in the northeastern part of the study area and is most suitable for the foundation. The results show that the investigated building was sited within the incompetent clayey-sand layer with resistivity generally less than 200 ῼm.

Forensic Study on Federal Road Pavement Failures at Rengit and Semerah, Batu Pahat

Forensic investigations in engineering term may be conducted to identify the causes of failure to facilitate the design of proper repairs, or to improve the performance or lifespan of a component, assembly, or structure. This paper presents the combination of geotechnical investigation and geophysical survey method as a forensic tool to predict the causes of pavement failure occurred along the coastal area of federal road FT005. The number and type of field testing are varying on each selected study area at Rengit and Semerah, Batu Pahat as this location constructed on soft soil area. Non-destructive testing (NDT) method using electrical resistivity tomography (ERT) were chosen to be applied at the worst severity of the road failure. Three number of resistivity lines of 100m and 200m were laid out using ABEM Terrameter LS2 with gradient method of electrode arrays covering up to 40-meters depth. Then, further testing of destructive testing (DT) method using borehole drilling conducted near the ERT lines to obtain soil profile with SPT N-value measurement. The disturbed and undisturbed samples were obtained to carried out laboratory test for soil classification. After that, inspection of road pavement layers were implemented at five number of trial pit tests which excavated up to subgrade layer to determine the thickness of the materials used in road construction. Three number of mackintosh probe test were also conducted on top of the subgrade layer after the trenching to obtain the strength of the subgrade layer. The results presented showed that severe longitudinal cracking were the predominant premature failures on the roads studied due to settlement effect of soft soil. The analysis obtained from RES2DINV program stated that the subsurface profiling was dominantly in saturated condition which the resistivity value was less than 100 ohm.m. From the SPT N-value it is observed that, the very soft soil layer is up to 8 m followed by the soft to stiff clays soil. Another cause of failure was resulted from the differential settlement due to the effect of different design of road construction with varies material used. The reliability and efficiency of the instruments used were also discussed in this study.

Application of magnetometry, georadar (GPR) and geoelectrical methods in archaeo-geophysical investigation of a Napoleonic battlefield with fortification at Pressburg (Bratislava, Slovakia)

This article presents the successful application of several geophysical methods in archaeological prospection with integration of LiDAR data and examination of historical documents. The locality of interest is situated in an urban area of Bratislava city on the Danube River where no previous archaeological research has been undertaken. It comprises the oldest public park in Central Europe which is surrounded by a fortification system dating to the Napoleonic siege of Pressburg (Bratislava) in 1809. Urban areas are often challenging for conducting archaeo-geophysical investigation due to limited access and existing obstacles of modern origin.For the purposes of this investigation shallow geophysical survey methods were applied including magnetometry, ground penetrating radar (GPR), 2D electrical resistivity tomography (ERT) and electrical resistance survey. Magnetometry was used to obtain a general view of the survey areas and to help to target the following surveys. ERT and GPR were used to explore detected features in more detail and obtain information about their shape and depth. The results of the survey show the importance of the combined approach using multiple geophysical methods while also highlighting their benefits and limitations.The survey was successful in investigating the basic elements of the fortification system which includes a rampart, a ditch and a bastion. The results have revealed information about the current state of the ramparts as well as their original construction. Also, the previously unrecorded remains of a bastion were identified along with previous phases of the fortification. The results obtained reveal the archaeological potential of this historical locality and help to better understand its extension, which is essential for its future protection and development.

Geophysical Investigations and Aquifer Characteristics of the Benin Formation at Agbarho, Delta State, Nigeria

Groundwater investigations were carried out at Agbarho using geophysical survey method to delineate the aquifer and subsurface lithology. Three (3) vertical electrical sounding were conducted with a maximum current electrode spacing of 300m. The data acquired were subjected to partial curve matching and computer processing. The results delineated five geoelectric layers for the area, which include; topsoil, silty sand, silty clay, fine sand, medium grain sand and coarse sand. The Dar-Zarrouk parameter revealed an aquifer system that has a resistivity values ranging from 503.9-966.9ohm-m, an average hydraulic conductivity of 7.73 m/day, and transmissivity in the range of 156.24-181.44m2/day. The depth to the aquifer range from 13.2-42.7m and the aquifer thickness is between 7-36.7m. Results from seven down-hole logs revealed the presence of silty sand, fine sand and fine-medium grained sand. The transmissivity values from pumping test range from 162.29-286.56m2/day and storativity is from 0.001-0.00838. The aquifer is capable of yielding useful quantity of water to the wells and sustainable groundwater management plan can be develop for the area.

GEOPHYSICAL SURVEY AS A TOOL TO REVEAL SUBSURFACE STRATIFICATION AT A SMALL AGRICULTURAL HEADWATER CATCHMENT: A CASE STUDY

Catchment drainage area is a basic spatial unit in landscape hydrology within which the authorities estimate a water balance and manage water resources. The catchment drainage area is commonly delineated based on the surface topography, which is determined using a digital elevation model. Therefore, only a flow over the surface is implicitly considered. However, a substantial portion of the rainfall water infiltrates and percolates through the soil profile to the groundwater, where geological structures control the drainage area instead of the topography of the soil surface. The discrepancy between the surface topography-based and bedrock-based drainage area can cause large discrepancies in water balance calculation. It this paper we present an investigation of the subsurface media stratification in a headwater catchment in the central part of the Czech Republic using a geophysical survey method - electrical resistivity tomography (ERT). Results indicate that the complexity of the subsurface geological layers cannot be estimated solely from the land surface topography. Although shallow layers copy the shape of the surface, the deeper layers do not. This finding has a strong implication on the water transport regime since it suggests that the deep drainage may follow different pathways and flow in other directions then the water in shallow soil profile or shallow subsurface structures.