Residual Anomalies Research Articles

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.

Identification Of Liquefaction-Potential Zones Using The Gravity Method In Lolu Village, Central Sulawesi

A seismic event of magnitude 7.5 struck the Palu region in Central Sulawesi on September 28, 2018, precipitating a subsequent calamity in the form of a tsunami measuring 4-7 meters in height. This catastrophe was further compounded by the occurrence of liquefaction, leading to extensive devastation and a significant loss of life. In order to identify areas susceptible to liquefaction, it is anticipated that the employment of the gravity method, renowned for its capacity to discern density fluctuations associated with the mass of voluminous materials over a considerable detection range, will prove instrumental. The investigation of parameters and the characterization of liquefaction phenomena in regions previously afflicted by liquefaction disasters can be instrumental in devising strategies for mapping zones that are predisposed to such occurrences. The present study seeks to employ geophysical methods, specifically the gravity method, to delineate zones with the potential for liquefaction within the Lolu Village at Palu City, Central Sulawesi. Through the application of techniques designed to isolate regional and residual anomalies, it is envisaged that a clearer understanding of anomalies situated in shallower regions can be attained, with a specific focus on residential areas. Notably, due to liquefaction, approximately half of the residential areas have shifted a considerable distance of around 132 meters from their original positions. To facilitate the interpretation of subsurface layers, two-dimensional cross-sections are modeled to intersect the displaced and stationary areas. The residual map reveals discernible variations in anomaly values, with lower values observed in the areas that experienced liquefaction-induced movement. Subsurface modeling further demonstrates the presence of three distinct rock layers, namely a sandy layer, a gravel layer, and a rock layer. Additionally, the modeling depicts the formation of canals composed of hard rock, exhibiting varying thicknesses within the surface layer as a consequence of the liquefaction event in 2018. The existence of these canals serves as an indicator that when the sandy layer becomes saturated with water, it will flow along the topographical gradient, following the path of the subterranean canals that have formed.

Geomagnetic Data Analysis to Identify Subsurface Condition of Gondang Area, Bojonegoro

The presence of manifestation in the Gondang indicates a specific geological condition. Those manifestations are Banyu Kuning Hot Spring, Selo Gajah Hot Spring, and Jari Mud Burst. The purpose of this study, which was carried out near Mount Pandan, was to identify the distribution of magnetic fields and magnetic anomalies, including the patterns of potential geothermal energy distribution in the region based on its magnetism. It also aimed to explain the rock structure in the manifestation area based on susceptibility values. The survey was conducted using 94 measuring points that were randomly distributed throughout the manifestation area. The data was corrected using IGRF and diurnal correction, and it was examined using Surfer 11 to construct a contour map and Oasis Montaj to visualize 3D models. According to the processed data, the residual anomaly’s value ranges from -232.1 nT until 159.4 nT. For the anomaly of residual, 2D and 3D models demonstrated low susceptibility dissemination values ranging from -0.029 until -0.0135 cgs on the manifestation zone. The high susceptibility value, which is considered an intrusion rock, has a value range of 0.011 to 0.046 cgs.

IDENTIFIKASI BATUAN PENYUSUN SISTEM PANAS BUMI NON-VULKANIK BANGKA TENGAH BERDASARKAN PENAMPANG 2D ANOMALI GAYA BERAT

The geothermal system in the Central Bangka area is a radiogenic non-volcanic geothermal system. This geothermal system is not associated with the volcanism process. The purpose of this research is to identify the rock components of the Non-Volcanic geothermal system in the Central Bangka geothermal field based on the 2D cross section of the gravity anomaly. This research was conducted by performing spectral analysis using Fast Fourier Transform (FFT) on the complete Bouger anomaly to obtain the residual anomaly. 2D modeling of the residual anomaly was done using inverse modeling with the smoothness constraint equation. The results show that the subsurface structure of the non-volcanic geothermal system in the Central Bangka geothermal area consists of cover rock in the form of sandstone (2.38 - 2.42 gr/cm3) at a depth of 3,000 m, reservoir rock consists of granite. (2.65 - 2.66 gr/cm3) at a depth of 0 - 9,000 m and the bedrock consists of diorite (2.87 - 2.9 gr/cm3) at a depth of 2,000 - 10,000 m.

Application of Seismicity and Gravity Observation-Based Filtering Model for Mapping a Fault Structure in Weh Island, Indonesia

Aceh Province is one of the Indonesian regions prone to earthquakes beneath the Great Sumatran Fault, which has been focused mapped on the land. At the same time, the continuity of the Great Sumatran Fault to the marine and islands is rarely investigated. We apply the Gravity Model Plus which has a resolution of 200 m/px, to map a detailed structure of the the Great Sumatran Fault in Weh Island. The Bouguer anomaly varied between 32 – 78 mGal, which reveals the response of the rocks comprising Weh Island, such as those on the east side with a high-value reaction corresponding to highly dense basalts 2.84 g/cm3 and on the west side with a low-value response representing the density of andesites 2.56 g/cm3. The residual anomaly indicated several local faults, e.g., Sabang and Seuke faults located on the east side, Lhok Jeumpa on the central side, and Pria Laot on the west side. Meanwhile, the derivative data from the Bouguer anomaly could explain the fault structures corresponding to regional geological maps, such as the Sabang fault in Balohan in the northwest-southeast direction and several others controlling the Jaboi volcano, i.e., Ceunohot and Leumomate. The focal mechanism from seismicity analysis figured the fault orientation from the Seulimeum segment in the direction of NW-SE, which is essential to assess an active fault. Based on data processing, we concluded the the Gravity Model Plus data is effective for first mapping the fault structure in developing countries, such as Indonesia.

Identifikasi Sesar dengan Menggunakan Metode Gaya Berat (Studi Kasus: Sulawesi Tengah)

The study of the geological structure of Sulawesi Island, especially the Central Sulawesi region, is very interesting because it has high complexity as a result of the dynamic interactions of the world's three main plates. Indications of high seismic activity and deformation processes in the Central Sulawesi region can be found from the morphological appearance and activity of several existing faults such as the Palu-Koro fault, the Lawanopo fault, and the Matano fault. The data used in this study are secondary data measuring the distribution of gravitational field anomalies (FAA) and topographical data using the Topex application. At CBA values ranging from 31 mGal to 56 mGal. The height anomaly on the CBA contour map is shown in red to light purple which has a value range of 40.7 mGal to 62.7mGal. Low anomaly is shown in light green to blue which has a value range of 27.3 mGal to 3.2 mGal. The regional gravity anomaly values range from 5.1 mGal to 62.8 mGal with a distribution of gravity anomalies from south to north. While in this study it can only be explained that the residual anomaly value is influenced by rock type and density. The high anomaly found in the southern part is thought to be caused by rocks having high density and being in a sedimentary rock environment which has low density.

An aeromagnetic denoising-decomposition-3D inversion approach for mineral exploration

Reduction of aeromagnetic noise and extraction of mineralization-related residual anomalies are critical for aeromagnetic data processing in mineral exploration. This study introduced a multifractal singular value decomposition (MSVD) method to remove the noise and improved the bi-dimensional empirical mode decomposition (BEMD) algorithm to extract residual magnetic anomalies. It is shown that MSVD and improved BEMD could effectively reduce the noise and extract residual magnetic anomalies. Then, a wavenumber–domain iterative approach is applied in 3D imaging of magnetic anomalies and gradients with depth constraints, which is a rapid tool for qualitative and quantitative interpretation of magnetic data and is suitable for rapidly imaging large-scale data. The 3D inversion result is verified by four geological sections along the regional tectonic directions and some drilling holes on the deposits. It is revealed that this proposed approach is practical and effective in dealing with aeromagnetic data interpretation and inversion for mineral exploration.

Subsurface Model Of Mt. Sinabung Using The GGM-Plus Satellite Gravity Data And Deconvolution Euler

Mt. Sinabung in Karo Regency, North Sumatra Province, with an altitude of 2450 meters above sea level, became active again in 2010 after a break in volcanic activity for ± 300 years. Since the eruption in 2010 until now, eruptions are still ongoing periodically. The aim of this research is to obtain a model of the subsurface density distribution of Mt. Sinabung. Modeling was carried out using GGM-Plus 2013 satellite gravity data and ERTM2160 topographical data. Bouguer correction and terrain processing use an average density of 2.67 g/cc. After that, the anomaly was separated by using the moving average. Residual anomalies were then analyzed using Euler deconvolution. The results obtained are the existence of a fault structure on the west and three layers of rock obtained which consist of the basement with a density of 2.8 g/cc 3 – 3.3 g/cc, then Toba pyroclastic deposits with a density 1.8 g/cc – 2.3 g/cc and limestone deposits with a density of 2.4 g/cc – 2.7 g/cc.

The northeastern Algeria hydrothermal system: gravimetric data and structural implication

The Tell Atlas of Algeria has a huge potential for hydrothermal energy from over 240 thermal springs with temperatures up to 98^circ C in the Guelma area. The most exciting region is situated in the northeastern part which is known to have the hottest hydrothermal systems. In this work, we use a high-resolution gravity study to identify the location and origin of the hot water, and how it reaches the surface. Gravimetric data analysis shows the shapes of the anomalies arising due to structures at different subsurface depths. The calculation of the energy spectrum for the data also showcases the depths of the bodies causing anomalies. 3D-Euler deconvolution is applied to estimate the depths of preexisting tectonic structures (faults). These preprocessing steps assist with assessing signal attenuation that impacts the Bouguer anomaly map. The residual anomaly is used in a three-dimensional inversion to provide a subsurface density distribution model that illustrates the locations of the origin of the dominant subsurface thermal systems. Overall, the combination of these standard processing steps applied to the measurements of gravity data at the surface provides new insights about the sources of the hydrothermal systems in the Hammam Debagh and Hammam Ouled Ali regions. Faults that are key to the water infiltrating from depth to the surface are also identified. These represent the pathway of the hot water in the study area.

Boundaries Determination in Potential Field Anomaly Utilizing Analytical Signal Filtering and its Vertical Derivative in Qeshm Island SE Iran

The analytical signal method and its derivatives can be explicitly calculated and assessed for analyzing and estimating the boundary of geological structures, such as faults or dikes, from potential field anomalies. Gravity field of the east of Qeshm Island and its north coast from Sarkhun to Latidan in SE Iran has been measured in one-kilometer regular network. The regional anomaly effects are deducted from the Bouguer anomaly, where it can get the residual anomalies which is the considered dataset used in this study. The Bouguer range is -297 to -330 milligals (mGal) applying filters and modeling. It was revealed that there are five negative anomalies caused by salt penetration in the region. Regional anomalies expectedly have a low spatial frequency and long wavelengths, while the residual anomalies associated with shallow sub-surface potentials have high spatial frequencies and short-wave wavelengths.Two different fault trends were observed comparing the artificial model using MATLAB coding, the real one, and the filtered residual anomaly map of the Qeshm region. Also, another fault trend is suggested in the east height of Oman, which is a possible place of salt penetration. Due to the limited dimensions (120 * 100 km) and the information obtained from the existing drillings, the depth of the anomaly resources can be examined up to 8 km. Due to the gravitational condition between the two domes of Qeshm saline and the bridge located in the Hara saline area, it is suggested to be considered as an area for further oil exploration routines.

Real-time online prediction of surface settlement considering spatiotemporal characteristics during foundation excavation

The accurate prediction of surface settlement induced by foundation excavation is challenging owing to complex spatiotemporal characteristics. To address this challenge, a real-time online prediction model comprising input, offline, and online modules was proposed. Grey relation analysis was employed to extract effective spatial coupling variables to improve the quality of the data in the input module; a long short-term memory network was trained to capture temporal nonlinearity in the offline module; and a statistical process control cloud platform was embedded to realize online updating when predicting residual anomalies in the online module. The model was verified using a time series dataset from a metro foundation project and outperformed other existing models with high accuracy. Our results could help managers to control settlement in a timely manner to prevent disasters. Clarifying the spatial coupling scale to improve the quality of input data of the model is left for future work.

Determination of buried active faults and earthquake potential for Izmir and its surroundings (western Turkey) using aeromagnetic anomalies and seismological data

The paper aims to delineate buried faults in Izmir city and its surroundings, western Turkey using aeromagnetic and seismological data. In this context, the geophysical data processing techniques including reduced‑to‑pole transform (RTP), power spectrum analysis, high-pass filter and second vertical derivative method (SVD) have been applied to the total field aeromagnetic data of the study area. First, to remove the undesirable effects caused by the dipolar nature of the Earth field, RTP transform has been applied to those data. Second, the average depths of the regional and residual sources in the region have been calculated as 16.84 km and 3.75 km, respectively using power spectrum analysis. After, to emphasize the effects of the residual anomalies, the high-pass filter has been applied to the RTP data. Finally, the second vertical derivative method (SVD) has applied to the filtered data for delineating the uncovering buried faults and their lineaments in the eastern part of the Aegean extension. The results from those methods show five major fault zones that could cause devastating earthquakes in the area. Especially, the study reveals for the first time that one of these faults which lies from Doganbey to the city center of Izmir in the literature actually reaches out to Manisa city in the NE direction. As a result, these lineaments can be evaluated as traces of buried faults could be an important clue in predicting earthquake potential. A comparison of seismicity map and the heat flow map shows that the region (between Cesme and Seferihisar) represented with a low b‑value has a high potential earthquake. The spatial distribution of the earthquakes, b‑values and heat flow values in the depths may be related to the existence thin lithospheric mantle. Furthermore, the region represented by strong aeromagnetic anomalies may be considered to be magmatic material arising from the magma filling inside the strike-slip faults. The fault structure observed on the SVD map are also important for the geothermal energy potential of the region as well.

Carbonate-rock aquifers in arid environments (central Tunisia, Western Mediterranean province): gravity and time domain electromagnetic methods investigations

Gravity and time domain electromagnetic (TDEM) methods were used in this study to investigate the subsurface hydrogeology of the carbonate-rock aquifer in arid environments (central Tunisia, Western Mediterranean province). The El Houdh basin can be considered an appropriate case for evaluating the vulnerability of groundwater resources under present-day climate change. The identification of structural context and the discontinuities affecting the Eocene or Campanian to Maastrichtian carbonates is an important key to recognize water recharge and discharge pathways and to develop effective and long-term groundwater exploration strategies. First, we have produced an available residual gravity field using a specific Gaussian filter. Then, we generated derivative maps, Euler deconvolution solutions map and a 3D gravity model to delineate different anomalies and to estimate the depth-to-basement parameter and the subsurface density contrasts. Positive and negative anomalies mapping and 3D gravity modelling showed that the El Houdh basin is associated with an asymmetric ‘perched syncline’ with a segmentation into NE–SW sub-basins (local negative residual anomalies) separated by NW–SE lineaments. The TDEM survey was calibrated using boreholes to image the karst saturated and unsaturated zone and to deduce the epikarst and endokarst relationship. Finally, the proposed method provides a fast and valuable approach for better management of hydrogeological exploitation. Thematic collection: This article is part of the Climate change and resilience in Engineering Geology and Hydrogeology collection available at: https://www.lyellcollection.org/cc/climate-change-and-resilience-in-engineering-geology-and-hydrogeology

Identification of Geological Structure Based on Gravity Method in Tangkuban Parahu Volcano, Bandung, Indonesia

Tangkuban Parahu Volcano, located in Cikahuripan, Lembang, West Bandung Regency, West Java, Indonesia, has a geothermal potential. However, this potential is accompanied by geohazard risks, one of which is a volcanic eruption that can interfere with the development of geothermal potential. On February 12, 2022, there was a gust of volcanic gas from Ratu Crater, indicating an increase in volcanic activity after an eruption in 2019. Based on the geological map of Tangkuban Parahu/Sunda Volcano Complex, it is estimated that there is a fault structure in a northwest-southeast direction, where if a volcanic eruption occurs, it will cause greater deformation because it cuts through Ratu Crater. Further studies with geophysical methods are needed to estimate the fault structure. The study of the subsurface geological structure of Tangkuban Parahu Volcano was carried out using the gravity method by studying variations in the gravitational field on the earth’s surface to determine differences in rock density. The Complete Bouguer Anomaly (CBA) value from the gravity method at Tangkuban Parahu Volcano shows 68 – 73 mGal, and residual anomaly values ranged from -0.95 to 0.67 mGal. The negative residual anomaly value between -0.95 to 0 mGal shows the weak zone due to volcanism in the crater. On the other hand, high residual anomaly values ranging from 0 to 0.67 mGal are interpreted as the influence of Lava 1 Tangkuban Parahu (Tl1). Analysis using First Horizontal Derivative (FHD) and Second Vertical Derivative (SVD) shows a fault structure trending Northwest – Southeast, which can be a way out of the accumulation of high-pressure water vapor due to volcanic activity.

Delineate the Main Structural Trend in Anah and Surrounding Area, Western Iraq Using Pie-Slice Technique on Gravity Data

The gravity data were analyzed and interpreted to delineate the main structural trend in western Iraq. The Pie-Slice technique was applied to the residual anomalies of gravity data with an interval angle of 20o, which results from eighteen trending maps. The major trends are defined as N25E, N15E, E-W, N50W and N30W and the minor trend N-S. This trend is generally related to deep structural and to detect their effects on the surface used lineaments extracted from Landsat Satellite Imagery. The prominent direction of lineaments N25E-N35E and N55W, and less prominent direction N45W, Generally the final results show lineaments trend coinciding perfectly with the trend of subsurface structures in NE direction and in NW trends of lineaments show nearly coinciding with gravity trends.

UTILIZATION OF SATELLITE GRAVITY ANOMALY DATA FOR TWO-DIMENSIONAL MODELING OF SUBSURFACE STRUCTURE OF SLAMET VOLCANO, CENTRAL JAVA, INDONESIA

Study using the satellite gravity method has been carried out in the Slamet Volcano area, Central Java, Indonesia. The gravimetric satellite has produced gravity anomalies data which have been corrected up to free air correction. This study purposes to determine the subsurface structure of Slamet Volcano by modeling residual gravity anomalies data. Data processing which has been conducted includes bouguer correction, terrain correction, data reduction to a horizontal surface, separation of regional and residual anomalies data, modeling, and interpretation. The residual gravity anomalies data obtained from the processing range from -40.62 – 66.22 mGal. The anomalies data are modeled in two dimensions (2D) to obtain the subsurface structure model of Slamet Volcano. The 2D-subsurface models on the AB and CD trajectories show the presence of magma chamber and several lava rocks which form the body of the volcano. The rocks are interpreted as andesitic lava with a density value of 2.72 g/cm3, andesitic-basaltic lava with a density value of 2.89 g/cm3, basaltic lava with a density value of 2.97 g/cm3, and magma chamber with a density value of 1.32 g/cm3. Such low value of magma chamber density relative to the surrounding rocks, indicates that Slamet Volcano is still active.

Investigation of the Superficial Structures of the Foumban Area (West Cameroon) Based on Spectral Analysis and 3D Modelling

The main objective of this study is to characterise the superficial structures of Foumban area. To achieve this, a sample of data from the gravity map of Cameroon is used to build a new Bouguer map to the studied area. The finite element method is used to separate the Bouguer anomalies into regional and residual anomalies. Given that the signatures of gravity anomalies related to surface structures are perceptible on the residual anomaly map, a qualitative analysis of this map helped to identify three anomaly domains in the north-western, central and north-eastern parts of the studied area. These three anomalies peak at 19 mGal, -24 mGal and 27 mGal, respectively. Spectral analysis is used to obtain the appropriate energy spectrum for different profiles that were selected, to estimate the average depth of the intrusive bodies. Thus, thanks to 3D modelling of the anomalous structures of the crust, a gravimetric interpretation is carried out. This interpretation leads correlatively to the geology of the region to conjecture the presence of sediments (clays, conglomerates, sandstones, sands) in the center of the study area; metamorphic rocks (gneisses, micaschists) and magmatic rocks (granites) responsible for positive anomalies in the rest of the area

Implications on Gravity Anomaly Measurements Associated with Different Lithologies in Turkana South Subcounty

The use of gravity data has demonstrated capability for monitoring lithological changes on a large scale as a consequence of differentiating basement and sedimentary of buried valleys. Gravity anomalies are associated with lateral contrasts in density and therefore deformation by faulting or folding will be manifested if accompanied by lateral density changes, otherwise, the vice versa is true. The study’s objective is to evaluate the effectiveness of gravity method in establishing different lithologies in an area. The study has revealed that regional anomaly gravity map presents high anomalies in the Northern region in the NW-SE trend and low anomalies in the southern trend in NW-SE, while the residual anomaly gravity map shows different trends for the low and high gravity anomalies. The gravity anomalies are well interpreted in line with the lithologies of the study area rather than the deformation of the same lithologies. There are observed high values of gravity anomaly values (ranging from -880.2 to -501.2 g.u.) where there are eolian unconsolidated rocks overlying the basement compared to low gravity anomaly values (ranging from -1338.9 to -1088.7 g.u.) where the andesites, trachytes and phonolites overly the basement. The different regional gravity anomalies relate well with different rock densities in the study area along the line profile for radially averaged power spectrum. The gravity highs are noted in the eastern point and are associated with andesites, trachytes, basalts and igneous rocks, while the gravity lows are associated with sandstone, greywacke, arkose, and eolian unconsolidated rock. The utilization of the information from the Power spectrum analysis demonstrates that the depth to the deepest basement rock is 12.8 km which is in the eastern flank, while the shallowest to the basement of 1.1 km to the western flank.

Physical modeling of magma chamber of slamet volcano by means of satellite gravimetric data

Slamet Volcano (3,432 m) is the highest volcano in Central Java, Indonesia, with a weak explosive type of eruption compared to other active volcanoes. Designing the magma chamber model may help reveal the characteristics of Slamet Volcano. The modelling uses the gravimetric satellite data from GGMplus, which is best in spatial resolution compared to other satellite data, i.e. 220 m. Data processing begins with Bouguer correction and terrain correction and has resulted in complete Bouguer anomalies data, with values ranging from 11.068 – 117.451 mGal. Further, residual Bouguer anomalies data were obtained after data reduction to the horizontal surface and removal of regional anomalies data, to obtain values ranging from -67.569 – 38.808 mGal. The residual anomaly contour map shows the lowest anomalous value is under the volcanic cone at positions of 109.21967° E and 7.24281° S which is estimated to be the location of the magma chamber of Slamet Volcano. However, the inversion modeling resulting from the residual Bouguer anomalies data shows that the magma chamber of Slamet Volcano can be observed clearly at positions of 109.22053° E and 7.24719° S. The location of the magma chamber is not perfectly vertical under the volcanic cone but has a slight slope. The obtained model of the magma chamber has a relatively small volume and shallow depth, i.e. about 1 – 4 km. The obtained physical parameters of the magma chamber impact the characteristics of the eruption of Slamet Volcano which tend to be weak explosive.

Sebaran Batuan Daerah Panas bumi Terak Bangka Tengah berdasarkan Anomali Magnetik

Geothermal potential in Indonesia has various non volcanic geothermal sources, one of which is geothermal in the Terak Village area of ​​Central Bangka. The geothermal geological structure of Terak is in the form of granite rocks with normal faults and horizontal faults or what is called the Pemali fault. The purpose of this research is to identify rock spreading on geothermal Terak based on magnetic anomaly. Magnetic data was processing by applied diurnal correction, IGRF correction, RTE (Reduce to the Equator) correction. Reginal dan residual anomaly was separation using upward continuation 350 m, 400 m, and 450 m. The result shown that rock spreading on geothermal Terak was consist by granite rock from Granite Klabat formation in north Mangkol Hill, sandstone from Tanjung Genting formation on West study area. Schist and phyllite was dominated on northeast and northwest area.