Gradiometer Survey Research Articles

Addressing magnetometer inconsistencies in drone-borne total field gradiometer surveys: case study from Rånbogen, Arctic Norway

SUMMARY Inconsistencies between (lightweight optical pumped) magnetometers in total field gradiometer surveys can introduce instrument-dependent responses, complicating geological interpretation. Here, we present an equivalent source-based processing method that combines equivalent sources with kernel regression to eliminate these instrument-dependent responses. We evaluate the method using a drone-borne triangular gradiometer survey from Rånbogen, Arctic Norway. The Rånbogen survey exhibits instrument-dependent responses in the measured total field differences, likely resulting from differences in the magnetometer’s heading error and the complexities from the induced field from the system itself. A synthetic gradiometer survey was generated to test the method, where different nonlinear synthetic heading errors were added to the synthetic magnetometer readings. The equivalent source-based processing method effectively removed these heading errors while constructing an equivalent source model that fits the actual underlying synthetic model. Furthermore, applying the method to the Rånbogen survey removes the data’s correlation to its orientation, thereby minimizing the effect of the instrument-dependent response. The method condenses the gradiometer data to a single equivalent source surface where the derivatives of the total field can be calculated to facilitate interpretation. The results suggest that the equivalent source-based method removes the inconsistencies between the magnetometers in a total field gradiometer survey, allowing for drone-borne gradiometer systems with multiple lightweight magnetometers.

Integration of shallow geophysics, archaeology and archival photographs to reveal the past buried at Ingleside Plantation, Piedmont North Carolina (USA)

AbstractGround penetrating radar supported by electrical resistivity tomography and gradiometer surveys were used to map buildings and infrastructure documenting sequential property use by three generations of the Jacob Forney family who began as farmers in the backcountry of North Carolina and rose to prominence in government and industry within the Southeastern United States. At Ingleside, the antebellum plantation house has been preserved, and the adjacent property remains relatively undisturbed. Context for the geophysical surveys was provided by archival photographs, written accounts including monographs and newspaper articles, and an archaeological excavation of the stone hearth within the plantation's summer kitchen. The location of an early log home with a stone‐lined cellar with ties to the Piedmont Campaign of the American Revolution (in 1781) was newly discovered. In addition, a historic road, kitchen garden, and the postholes from an early post‐in‐the‐ground building were imaged within the subsurface. The external summer kitchen and privy are associated with the plantation house constructed in 1817. Several cesspool vaults of potential privies are ingrown with trees. The results of the geophysical surveys document the evolving land use within one family in the American South and can be connected to a specific event in history, a goal of historical archaeology.

Archaeogeophysical exploration in Neuss‐Norf, Germany using electrical resistivity tomography and magnetic data

ABSTRACTA combination of electrical resistivity tomography (ERT) and magnetic gradiometry was selected to examine a hypothesis concerning the presence of remains of one of the oldest archaeological churches in the Rhineland, located in Neuss‐Norf, Germany. The gradiometer survey was carried out to measure the vertical gradient of the magnetic field using a proton precession magnetometer along several profiles. The magnetic data were reduced to the magnetic pole; then analytic signal and power spectrum techniques were applied. The ERT survey was based on the magnetic results, and both Wenner and dipole–dipole configurations were employed to collect the apparent resistivity data along 12 ERT profiles. The field ERT data from these two arrays were merged into one dataset to form a non‐conventional mixed array. The robust (blocky) inversion technique was applied to the resistivity data in order to derive the two‐dimensional resistivity distribution of the subsurface. Despite the noisy surroundings, the magnetic survey was able to give an indication of potential walls of the ancient church in addition to several subsurface magnetic sources. Moreover, highly resistivity anomalies were observed within the first 1–2 m of the subsurface soil and were interpreted as possible remains of man‐made structures. This depth range was also confirmed by the spectral analysis of the magnetic data. A strong consistency between the two methods was observed in some locations of the site. In addition, the ERT measurements confirm and complement most of the magnetic results. We successfully detected anomalous zones that could be associated with the walls of at least one ancient church building in addition to several possible archaeological structures in the survey area.

A Massive, Late Neolithic Pit Structure associated with Durrington Walls Henge

A series of massive geophysical anomalies, located south of the Durrington Walls henge monument, were identified during fluxgate gradiometer survey undertaken by the Stonehenge Hidden Landscapes Project (SHLP). Initially interpreted as dewponds, these data have been re-evaluated, along with information on similar features revealed by archaeological contractors undertaking survey and excavation to the north of the Durrington Walls henge. Analysis of the available data identified a total of 20 comparable features, which align within a series of arcs adjacent to Durrington Walls. Further geophysical survey, supported by mechanical coring, was undertaken on several geophysical anomalies to assess their nature, and to provide dating and environmental evidence. The results of fieldwork demonstrate that some of these features, at least, were massive, circular pits with a surface diameter of 20m or more and a depth of at least 5m. Struck flint and bone were recovered from primary silts and radiocarbon dating indicates a Late Neolithic date for the lower silts of one pit. The degree of similarity across the 20 features identified suggests that they could have formed part of a circuit of large pits around Durrington Walls, and this may also have incorporated the recently discovered Larkhill causewayed enclosure. The diameter of the circuit of pits exceeds 2km and there is some evidence that an intermittent, inner post alignment may have existed within the circuit of pits. One pit may provide evidence for a recut; suggesting that some of these features could have been maintained through to the Middle Bronze Age. Together, these features represent a unique group of features related to the henge at Durrington Walls, executed at a scale not previously recorded.

Application of magnetic method for mapping buried structures around archaeological site of Masjid Tuha Indrapuri

Magnetic gradiometer survey has been conducted on the yard of the archaeological site of Masjid Tuha Indrapuri, Aceh Besar Regency, Aceh Province. The site is one of the oldest mosques erected during the Aceh Sultanate period. Magnetic method was applied for mapping archaeological structures buried beneath the surface. Total magnetic field data were measured using Proton Precession Magnetometer with grid spacing of 2 meters between stations covering the entire area of the site. Diurnal and international geomagnetic reference field data were corrected to the measured data in order to calculate total magnetic field anomalies that were influenced by the buried magnetic objects. The total magnetic field anomalies distribution shows two elongated structures with U-shaped patterns surrounding the mosque. The patterns are also revealed in reduction to the pole and derivative vertical filters of the total field anomaly data. The anomaly patterns are considered a response from the rest of the buried fences that were built around the mosque in the past.

Evaluation of mineral exploration targets defined by airborne gravity gradiometry through gravity and magnetic modelling: vicinity of the Iron Range Fault, Purcell anticlinorium, southern Canadian Cordillera

An airborne gravity gradiometer survey was recently flown over the Iron Range Fault in the Purcell anticlinorium, southern Canadian Cordillera. The fault is commonly associated with iron oxide mineralization having characteristics similar to those of iron oxide Au ± Cu deposits. Drilling near the fault has revealed Au ± Cu–Pb–Zn mineralization. Prominent positive vertical gravity gradient (VGG) anomalies defined by the survey were identified as targets for follow-up exploration. Possible sources of the target anomalies were investigated by modelling gravity, VGG, and magnetic data along several profiles. Modelling of regional-scale profiles of the vertical component of gravity crossing exploration targets provides a regional perspective on the regional geological setting, dominated by the broad Goat River anticline, whose axis closely follows the Iron Range Fault. Modelling indicates that several VGG anomalies are related to Moyie sills, although one anomaly is modelled as a narrow vertical body (120 m wide, 1000 m vertical extent, 40 m deep) just west of the Iron Range Fault. Its apparent high density of 3500 kg/m3 suggests metallic content, making it a choice candidate for follow-up investigation. Drilling at the southern end of this geophysical target intersected a Moyie intrusion, but untested geochemical anomalies in the vicinity encourage follow-up exploration. The densities of modelled units derived from VGG profiles across two other specific targets indicate that Moyie sills represent one target and iron oxide mineralization the other, as supported by magnetic modelling, which also delineated vertical zones of significantly magnetic material along the Iron Range Fault.

Requirements for airborne gravity gradient terrain corrections

Accurate terrain corrections are important for all gravity surveying. In airborne surveys, a digital model of the terrain is constructed and terrain corrections are calculated at each airborne measurement point. Airborne gravity gradiometry is of high spatial resolution and is particularly sensitive to nearby topographic variations, placing particular requirements on the terrain corrections. A combination of mathematical analysis and simulation studies has led to quantification of the requirements: current on-shore, low-level gradiometer surveys require sub-metre accuracy in navigation and in digital terrain model heights; cell sizes (and therefore also topographic sampling) in the terrain model should be about one-third of the ground clearance. The choice of terrain correction density depends on the application and it is important that the interpreter of the corrected gravity data has the ability to test the impact of changes in this density. Accurate calculation of the gravity gradient field at the airborne sampling points may be achieved by a wide variety of either spatial or harmonic domain methods. Calculation in the harmonic domain is fast but assumes the data represent a periodic function on a planar surface. Padding methods for periodic extension and piecewise continuation away from a plane both add error and slow the calculation. Spatial domain methods are slower but can be sped up by the use of various approximations. In both cases, a clear understanding of accuracy requirements is essential for making an appropriate tradeoff between accuracy and speed.

Q130: surveys op het terrein van een hellenistische en Romeinse boerderij in zuid-Italië

Q130: surveys over the site of a Hellenistic-Roman farm in southern Italy. 
 This article reports on the results of survey experiments conducted by a GIA team in 2010 over the site of a Hellenistic-Roman farm, Q130, located in the foothills enclosing the Sibaritide coastal plain in southern Italy. The experiments show that a ‘quick scan’ surface survey method achieves good results in locating the main structures, matching the results of a gradiometer survey conducted simultaneously, and that further useful chronological and functional detail was obtained by ‘total’ survey and in-field classification of the finds. The socio-economic context of this farm site must obviously be sought in the nearby Hellenistic colony of Thourioi and its Roman successor Copiae, but further questions about the character and chronology of the site can only be answered by excavation.

Airborne Gravity Gradiometer Survey over the Pelarang Anticline, Onshore Kutai Basin, Indonesia

The Pelarang Anticline is part of the NNE-SSW oriented Samarinda Anticlinorium, a detached thrust-and-fold belt in the Tertiary Kutai Basin. Results from an airborne gravity gradiometer survey over the Pelarang Anticline are presented herein. The Pelarang Anticline is interpreted as a detachment fold ~30km long with steeply dipping (70°-80°) flanks. However, seismic imaging on existing 2D data is poor.In October 2016 Cue Energy acquired airborne gravity gradiometer survey data over the anticline. The survey revealed a large (~10mGal) gravity signal range, and that the anticline is associated with a strong, positive gravity anomaly. Subsequent application of potential field enhancement filters clearly delineated the crest and the flanks of the feature.2D modelling of selected profiles across the anticline suggests that it can be modelled as a 1,500m-2,000m wide, by ~2,000m high shale body that is close to breaching the surface in places. This is in alignment with an interpretation that the feature is cored by high-pressure shales, resulting in un-prospective areas.However, 3D modelling has revealed significant along-strike variations in the depths to the crest of the anticline, suggesting the presence of several anomalous structural lows. Further investigation suggests these features are pull-apart mini-grabens, formed in response to localized shear movements. At least two commercial hydrocarbon accumulations, Sambutan and Mutiara, appear to be genetically related to the newly recognized structural anomalies.This survey has led to the recognition of a new exploration play in the region, and provided a tool to pursue it.

Tales from the outer limits: Archaeological geophysical prospection in lowland peat environments in the British Isles

AbstractIn order to systematically investigate the potential of conventional near surface geophysical techniques to locate waterlogged archaeological targets in peatlands, the authors applied four conventional geophysical methods – earth resistance, ground‐penetrating radar (GPR), magnetic gradiometry and frequency domain electromagnetics (FDEM) – to four lowland peat archaeological test sites in Great Britain. In this article we demonstrate that a Neolithic trackway was identified in the GPR data in Somerset, with likely ‘proxy’ detections of chemical changes showing up in both electrical and magnetic surveys. This was determined by a coring programme and inductively coupled plasma optical emission spectrometry (ICP‐OES) multi‐element analysis of peat samples to determine the relative concentrations of geophysically relevant chemical elements. Though no Bronze Age timbers were detected at Flag Fen, a post‐Bronze Age agricultural landscape was identified in both the GPR and gradiometer surveys. We conclude that GPR has the greatest potential for archaeological geophysical prospection in peatland environments, but that electrical and magnetic methods can usefully be employed as secondary sources of information and should not be discounted from future research. Further, this article argues that better understandings must be developed of the impacts of geochemistry on geophysical data if we are going to realistically pursue ‘whole landscape’ surveys.

Geoarchaeological Core Prospection as a Tool to Validate Archaeological Interpretation Based on Geophysical Data at the Roman Settlement of Auritz/Burguete and Aurizberri/Espinal (Navarre)

Geophysical survey methods are broadly used to delimit and characterize archaeological sites, but the archaeological interpretation of geophysical data remains one of the challenges. Indeed, many scenarios can generate a similar geophysical response, and often interpretations can not be validated without access to the subsoil. In large geophysical surveys many anomalies are detected and validation through archaeological trenches can not be afforded. This paper analyses the validity of geoarchaeological core survey to check the archaeological interpretations based on geophysical results. The Roman site located at Auritz/Burguete and Aurizberri/Espinal (Navarre), provides a great case of study as many investigations have been carried out. After the gradiometer survey performed in 2013 a sediment core survey was designed. 132 cores were drilled using a hand-held coring machine and the sediments were analysed in situ. Site delimitation and archaeological interpretations based on magnetic data could be improved or corrected. In this regard, the core survey proved to be an useful methodology as many anomalies could be checked within reasonable time and resources. However, further geophysical investigations trough GPR revealed unexpected remains in areas where no archaeological deposits were identified through coring. Excavations showed poor conservation level in some of those areas, leading to thin archaeological deposits hard to identify at the cores. The sediment core survey, therefore, was proved to be inconclusive to delimit the archaeological site.

Removal of sensor tilt noise in fluxgate gradiometer survey data by applying one‐dimensional wavelet filtering

AbstractArchaeological prospection with magnetometer instruments is performed in a wide range of field configurations, ranging from single probe setups to mobile arrays that allow combining multiple sensors. The latter type, whereby instruments are mounted onto a cart system, are particularly prone to motion‐induced noise. Sensor tilt, for example, causes in‐line noise that can obscure magnetic variations present. To remediate these effects, image processing techniques are the most frequently applied. However, while efficient in producing more levelled data plots, these procedures are often associated with a smoothing penalty whereby low‐intensity or small‐scale anomalies are masked. We propose a one‐dimensional signal processing approach, based on discrete wavelet analysis. By selecting wavelets that correspond to the motion‐induced noise patterns, such effects can be targeted more precisely, reducing the risk of feature masking or artefact creation. Evaluation of the proposed procedure on three fluxgate gradiometer datasets collected with a hand‐propelled push‐cart system, proved it a valid and more dedicated method to reduce the impact of motion induced noise in magnetometry data collected with cart mounted array setups.

Chinook salmon and green sturgeon migrate through San Francisco Estuary despite large distortions in the local magnetic field produced by bridges.

Empirical evidence exists that some marine animals perceive and orient to local distortions in the earth’s main static geomagnetic field. The magnetic fields produced by undersea electric power cables that carry electricity from hydrokinetic energy sources to shore-based power stations may produce similar local distortions in the earth’s main field. Concerns exist that animals migrating along the continental shelves might orient to the magnetic field from the cables, and move either inshore or offshore away from their normal path. We have studied the effect of the Trans Bay Cable (TBC), an 85-km long, high voltage, direct current (DC) transmission line leading underwater from Pittsburg, CA to San Francisco, CA, on fishes migrating through the San Francisco Estuary. These included Chinook salmon smolts (Oncorhynchus tshawytscha) that migrate downstream through the San Francisco Estuary to the Pacific Ocean and adult green sturgeon (Acipenser medirostris), which migrate upstream from the ocean through the estuary to their spawning habitat in the upper Sacramento River and return to the ocean after spawning occurs. Based on a detailed gradiometer survey, we found that the distortions in the earth’s main field produced by bridges across the estuary were much greater than those from the Trans Bay Cable. The former anomalies exceeded the latter by an order of magnitude or more. Significant numbers of tagged Chinook salmon smolts migrated past bridges, which produced strong magnetic anomalies, to the Golden Gate Bridge, where they were recorded by dual arrays of acoustic tag-detecting monitors moored in lines across the mouth of the bay. In addition, adult green sturgeon successfully swam upstream and downstream through the estuary on the way to and from their spawning grounds. Hence, the large anomalies produced by the bridges do not appear to present a strong barrier to the natural seasonal movement patterns of salmonid smolts and adult green sturgeon.

Black Swan airborne geophysical survey structural interpretation for hydrocarbons targeting in the Perth Basin

The integrated interpretation of the airborne FALCON® Airborne Gravity Gradiometer (AGG) survey was designed to assist oil and gas producer Empire Oil and Gas in identifying target areas for hydrocarbon exploration in the Perth Basin. It was developed as a synthesis of the geological structure, tectonic evolution and principles of gravity and magnetic data behaviour. The survey identified areas containing large structural leads and trends as the target of future gas exploration activities, including infill 2D seismic acquisition. The prospectivity of some identified leads has been significantly increased by the gas discovery at wells Red Gully North-1 and Gingin-1, located in EP 389 in central Perth Basin.

Applying advanced gravity and magnetic inversion methods to expand the Platreef PGE-Ni-Cu resource in the Bushveld Complex

There are many potential field inversion algorithms available, and all are sufficiently capable of generating a model that explains supplied geophysical observations. The challenge is extracting a model that provides real geological insight. Here we present applications of two different styles of advanced inversions to a deep exploration program at the Platreef PGE-Ni-Cu deposit in the Bushveld Igneous Complex of South Africa. The initial approach was to apply generalised focussing constraints to a 3D magnetic vector inversion, an approach chosen to manage the effect of expected strong remanent magnetisation. This resulted in successful prediction and drill definition of inferred resources within a deep, west-dipping extension to the shallow-dipping “Flatreef” deposit. Later, a detailed 3D model of geological constraints based on drilling and mapping was constructed and used to tightly constrain inversions of gravity data derived from a FALCON airborne gravity gradiometer survey. The resulting 3D density model accurately predicted a continuation of the Flatreef host rocks to shallower levels than previously anticipated. This facilitated further drill-definition of additional inferred resources within a southern extension of the Flatreef deposit. Key to the success of the inversions at accurately targeting mineralisation at depths of 700–1300 m depth, was the inclusion and integration of all available information to ensure that predictions were consistent with prior observations.

Under the Park. Recent Geophysical Surveys at Verulamium (St Albans, Hertfordshire, UK)

AbstractThis paper presents the first results of the geophysical surveys – principally a large scale gradiometer survey – of Verulamium Park, St Albans, Hertfordshire, UK, under which lies approximately half of the Roman city. Verulamium was the third largest Roman city in the province of Britannia – covering some 81 ha – and the largest which is currently available for survey. Approximately 65 ha lies under parkland or pasture. The 30 ha available under the Park was the subject of a magnetometry survey in 2013–2014, along with smaller areas of ground penetrating radar (GPR), earth resistance and magnetic susceptibility. These surveys were undertaken as part of a community archaeology project funded by the Arts and Humanities Research Council (AHRC). The surveys have detected a wide variety of features including stone buildings of varying size and complexity, pottery kilns, roads, pits and ditches. The results so far suggest the town can be divided into an area of largely elite housing, an area characterised by smaller structures and industrial features, and a somewhat enigmatic magnetically quiet area. Challenges in the interpretation of the results are discussed, as are potential solutions and planned future work. Copyright © 2016 John Wiley & Sons, Ltd.

Multi‐technique Geophysical Survey in and around the Hillfort Lossow – a Bronze and Iron Age Central Site in Brandenburg, Germany

AbstractAt the hillfort Lossow, integrated geophysical surveys were applied as part of an interdisciplinary research project. The importance of the prehistoric and Slavic site rests on various shafts within the hillfort, which are dated to the Early Iron Age and contain findings of offering processes associated with cult practices. At present more than 60 shafts with a depth of 5 to 7.5 m are known from test trenches and excavations inside the hillfort. In the settlement in front of the hillfort, large‐scale magnetic gradiometer surveys and electrical resistivity mapping were applied to detect new archaeological features, verified by field surveys and targeted excavation. Inside the hillfort, a combination of magnetic gradiometer surveys, electrical resistivity imaging, ground‐penetrating radar, and pulse‐induction metal detection were used to reveal the complexity of structures like archaeological pits, shafts, pit houses and modern disturbances in varying soil types. These measurements resulted in a very complex signal of the geophysical data. We used an integrated approach for the interpretation of data from magnetic gradiometer surveys, pulse‐induction metal detection and ground‐penetrating radar surveys. Thus it became possible to interpret most of the detected anomalies as archaeological features from the Slavic period or modern metal artefacts in the topsoil. The detection of the Iron Age shafts covered by complex settlement structures only became possible using high resolution electrical resistivity tomography and low frequency ground‐penetrating radar. Copyright © 2016 John Wiley & Sons, Ltd.

Geophysical Responses over the Cannington Ag-Zn-Pb Deposit-Queensland

SUMMARY The Cannington deposit is a high grade Ag-Zn-Pb deposit found in 1990 by BHP Minerals drilling an isolated 1,000 nT aeromagnetic feature. Following the discovery of Cannington, numerous airborne, ground and borehole surveys have been carried out which overall, provided some assistance at better defining the ore system but did not lead to the discovery of new major deposit in the area. While Cannington possessed a clear magnetic response, the presence of a thick conductive cover made the use of EM and electrical techniques challenging. BHP used Cannington as a test ground for a variety of new techniques including a ground SQUID EM sensor, modified airborne EM technology (higher power and lower base frequency) and over 10 years after discovery, the first ever Falcon airborne gravity gradiometer survey in Australia.

Integrated Archaeogeophysical Approach for the Study of a Medieval Monastic Settlement in Basilicata

AbstractThe paper deals with the results of an archaeo-geophysical approach adopted for the study and the reconstruction of the architectural plan of the medieval monastery of San Pietro a Cellaria in Calvello (Basilicata, Southern Italy). The monastery is a remarkable witness to Benedectine architecture of the 12-13th century in Basilicata, built by monks of the Congregation of S. Maria di Pulsano, who were active mainly in southern Italy. The historical data and the diachronic architectural study, based on the analysis of building techniques, provide evidence for a long and intense history, during which the monastery underwent several architectural changes, including the demolition of buildings and the superposition of other constructional elements. The only preserved medieval remains are a church with a nave; the adjacent structures are more recent. This preliminary data prompted a research project to shed new light on the as yet unknown history of the medieval monastery. Specifically, a remote sensing approach around the monastery including aerial survey by unmanned aerial vehicle (UAV), ground-penetrating radar (GPR) and geomagnetic survey in gradiometric configuration (MAG), was adopted in order to verify the possible existence of buried masonry structures and other possible features of archaeological interest, including channels and aqueducts. The GPR time slices were constructed from closely spaced parallel profiles. The time slices, computed by averaging radar reflections over vertical time windows several nanoseconds thick, are used to map subsoil features associated with the structures, probably of anthropogenic origin. To facilitate the interpretation of the results, a threedimensional image was constructed using closely spaced parallel profiles, which are linearly interpolated. The MAG survey was carried in gradiometer configuration, in order to study magnetic properties of the shallow subsoil. Ground-penetrating radar gives details about archaeological structures in a limited area where survey was possible, while gradiometer survey confirms GPR results and improves archaeological knowledge in the areas where GPR survey was impossible. This multi-sensor remote sensing program revealed a wide variety of archaeological features of interest, which may be targeted accurately with excavations in the future.

Geophysical methods used in the discovery of the Kitumba iron oxide copper gold deposit

A range of geophysical techniques has been used at various stages of the discovery and delineation of the Kitumba deposit in Central Zambia. Early era magnetics, geologic mapping, artisanal Cu plays, and the application of an iron oxide copper gold (IOCG) exploration model led explorers to the area in the 1990s. An airborne gravity gradiometer (AGG) survey was flown in 2004, and it highlighted key regional elements considered to be prerequisite for prospective IOCG mineralization. The AGG survey accurately delineated the spatial extents of two target areas referred to as the Kitumba and Mutoya systems. Gravity, radiometric, and magnetic data sets acquired as part of the AGG survey have mapped geologic and structural information as well as the extent of the IOCG alteration system. Significant uranium anomalism in the radiometric data was identified at Kitumba upon which the discovery hole S36-001 was sited. In 2012, a 3D direct current resistivity and induced polarization survey was conducted over Kitumba. The survey results provided 3D models of induced polarization chargeability anomalism and allowed successful delineation of sulfide material within the known deposit. The survey also provided an enhanced understanding of the 3D geometry of the mineralization. This improved understanding allowed a refocusing of drilling activities to best target extensions to existing mineralization.