Microtremor Data Research Articles

The Effect of Weathered Layer Thickness and Slope on Potential Areas of Landslides in Gerbosari Village, Samigaluh District, Kulonprogo Regency, Indonesia

Gerbosari Village has a history of landslides with intensity and risk of 56 occurrences over 5 years. Gerbosari Village, Samigaluh District, Kulonprogo Regency is located at geographic coordinates 7◦ 38 '45.33 "- 7◦ 41' 35.24" LS and 100◦ 9 '20.80 "- 110◦ 11' 16.52" BT with topographic conditions at an altitude around 500 - 1000 mdpl. This study aims to determine the subsurface structure of landslide-prone areas in the form of weathered layer thickness and the effect of slope in landslide-prone areas so that it can be used in making micro zonation maps of landslide-prone areas. This study uses 43 microtremor data with a distance between points of 650 m. The microtremor signal was analysed using the horizontal to vertical spectrum ratio ( HVSR) method. From the measurement results, it is obtained that the value of the dominant frequency ranges from 1 - 22 Hz, the value of the amplification factor is obtained in the range of 1 - 10.5, the value of the peak ground acceleration ranges from 60 - 300 cm/s2, the thickness of the weathered layer is obtained in the range of 12 - 22 meters. Based on the results of the slope analysis, the study area is on a slope classified as a bit steep - very steep.

Site characterization study with microtremor data: A case study at the West Borneo NPP potential site

ABSTRACTA site characterization using a microtremor measurement study was conducted at the West Borneo nuclear power plant (NPP) potential site as a preliminary feasibility study. This study measured the natural resonant frequency of soil and estimated the subsurface VS as prior information for the design planning of an NPP building foundation. The single station microtremor data were processed using the horizontal‐to‐vertical spectral ratio (HVSR) method to obtain the f0 and A0 values, and the array data were processed using the spatial autocorrelation (SPAC) method. The study area was classified based on the f0 and A0 values. Due to the unavailability of borehole data and limited array data, the HVSR curve was used to estimate the VS profile. The HVSR curve VS estimation was calculated using Rayleigh wave ellipticity inversion. The inversion results were constrained by VS profiles from SPAC and regional geology data to ensure that the inverted model was rational. It was demonstrated that the constrained inversion process could generate a rational subsurface model for Pantai Gosong's coastline. Based on the result, the feasible bedrock for the NPP building foundation was found at a depth of 40 m in the coastline area.

Site effect estimation for shallow engineering bedrock using microtremor HVSR method and geotechnical borehole data, Riyadh city, Saudi Arabia

The estimation of local site response and geotechnical properties of near-surface deposits becomes essential for improving the building codes especially in big cities as Riyadh. This study aims to estimate the site response parameters in the northern expansion zone of Riyadh city. Microtremor measurements were recorded at 45 sites at the study area, north of Riyadh, and the resonance frequency and amplification factor were calculated using the H/V spectral ratio approach. The microtremor data has been processed using geopsy software to get the H/V curve and get the value of the resonance frequency (fo) and amplitude (Ao). The values of the resonance frequency span from 0.79 to 45 Hz, while the amplification factor ranges from 1.22 to 8.5, indicating the presence of foundation bedrocks at various levels with varying soil thickness. Moreover, the N60 parameter was calculated to correct the SPT-N uncorrected field readings in order to estimate the shear wave velocity parameter down to 30 m depth (Vs30). The computed shear wave velocity for a depth of 30 m (Vs30) ranges from 210 to 630 m/s. Accordingly, the soil at the studied area can be divided into two site soil classes C and D according to the National Earthquake Hazard Reduction Program (NEHRP). The findings of this study should be considered as part of the development of the Saudi Building Code for Riyadh City.

Deep-Neural-Network-Based Estimation of Site Amplification Factor from Microtremor H/V Spectral Ratio

ABSTRACT Site amplification factors (SAFs) of S wave at ground surface are crucial for evaluating and predicting seismic ground motions. This study proposed a novel methodology for directly estimating S-wave SAF from microtremor horizontal-to-vertical spectral ratio (MHVR) based on deep neural network (DNN) model. We analyzed site amplifications obtained from generalized spectral inversion technique and microtremor data observed at Kyoshin net and Kiban–Kyoshin network sites in Chugoku district, western Japan. The DNN model was developed using peak frequency and the frequency-dependent relationship between MHVRs and SAFs. The sites were divided into training set, validation set, and test set. The training set and validation set were used in k-fold cross-validation technique to evaluate and select optimal model. Once the optimal model had been determined, the model was employed on the test set that was completely independent of the training and validation set for evaluating the generalization performance. Residuals and root mean square errors between the estimated and observed SAFs were evaluated to discuss the applicability of the proposed model. We also confirmed that the DNN model shows better performance in estimating SAFs compared with the existing double empirical correction method.

Seismic Site Characterization Using Ambient Noise and Earthquake HVSR in the Easternmost Part of Shillong Plateau, India

This study provides site characterization results using passive seismic techniques for the easternmost part of Shillong plateau of Northeastern India. The prime objective of this research is to generate the Horizontal-to-Vertical Spectral Ratios (HVSR) from earthquakes and ambient microtremor data for generation of 1-D shear wave velocity profiles to reveal the basement depths and fundamental resonance frequencies underneath three stations of Shillong plateau. The earthquake waveforms are recorded for a period of eighteen months (i.e. December 2018 to June 2020) using broadband seismometer; while the ambient microtremor data is collected from a three component highly sensitive velocity meter. The earthquake HVSRs and noise HVSRs are consistent with each other and range from ∼3.6 Hz to ∼14.5 Hz. The Rayleigh wave ellipticity are generated from the corresponding HVSR curves for inversion to determine compressional and shear wave velocity structure. We report that the compressional and shear wave velocity profiles match fairly well for both earthquakes and ambient microtremor data. Overall, Vs30 lies in the range of 480 m/s to 2600 m/s at all locations in the easternmost part of Shillong plateau; while the sedimentary layer velocity lies between 450 m/s and 1100 m/s. The thickness of the sedimentary layer is assessed from the velocity profiles and using an empirical formula makes a good match and varies from 11.3 m to 31.2 m.

Evaluation of site effects using horizontal-to-vertical spectral ratio in Kuala Pilah, Negeri Sembilan

The destruction experienced by an area due to tremors is commonly related to the size of magnitude and the distance of the source to the site where it is well known that larger magnitude earthquakes cause greater damage. Similarly, the closest area to the source will experience much devastating damage compared to the area further away. In addition to that, the local site effect also plays a significant role in the impact of earthquake hazards for specific sites despite the site being hundreds of kilometers away from the epicenter. This had been proven by the Michoacán earthquake in 1985 where the situation is similar to Peninsular Malaysia located approximately 400 km away from one of the most active plate tectonics in the world, the Sumatra Subduction Zone. This paper aims to evaluate the local site effects to improve the result of seismic hazard assessment by applying the Horizontal-to-Vertical Spectral Ratio (HVSR) method. The recorded microtremors data are processed to get two parameters that are closely related to the local site condition which is the resonant frequency and the amplification factor. The overall values extracted from the H/V curves are intermediate for both parameters. Overall, Kuala Pilah is divided into two zones; the western region associated with a thin, well-compacted soil layer and the eastern region associated with loose and unconsolidated deposits.

Soil Vulnerability Levels based on Microtremor Data using the HVSR method in the Old City Area of Semarang

<p class="AbstractEnglish">Kota Lama Semarang with the Dutch colonial era relics is currently used as a cultural heritage tourist spot. As a cultural heritage tourism place, it is necessarily aware of the potential for earthquake disasters due to active faults and volcanic eruptions, as well as the phenomenon of land subsidence caused by geological conditions of young alluvium. One of the geophysical techniques to determine subsurface structure is microtremor measurement. This study aims to determine vulnerability level based on dominant frequency, amplification, and seismic vulnerability index using HVSR analysis in Kota Lama area. Microtremor data measurement contained 11 points with a data retrieval duration of 30 minutes. The results of microtremor data analysis obtained dominant frequency values of 7.03 - 19.36 Hz, amplification of 0.72 – 3.7, and seismic vulnerability index of 0.074 – 0.95. Based on the parameters, it can be concluded that the Kota Lama Semarang area has a low level of vulnerability.</p>

Characterization of Site Effect and Natural Frequency of Railway Bridges

Resonance could occur as a wave propagates through the soil resulting from seismic vulnerability activities. The resonance impact resulting from the local site effects may cause civil infrastructures failure such as railway bridge collapse. This study presented possibilities of the resonance in the BH77 railway bridge in Lampung, Indonesia. Characteristics of the site effect around the bridge were evaluated using microtremor data. Measurements were made at nine locations, with two points were on the bridge, and seven other points were scattered around the bridge. Microtremor data were analyzed using the Horizontal to Vertical Spectral Ratio (HVSR) method to find natural frequencies and amplification factors. Furthermore, the site effect around the bridge was analyzed using the natural frequency, the amplification factor, and the seismic vulnerability index (Kg) to have site effect characteristics. Results of the study indicated that the research area showed amplification factors (A0) ranging from 5.15 to 12.19. The site natural frequency obtained from the study was about 4.01-7.27 Hz. The seismic vulnerability index in the study area was 4.2-19.4. The obtained natural frequency (F0) on the bridge had a value between 5.20 and 6.19 Hz, and the natural frequency value at the site was 4.01-7.27 Hz, respectively. This phenomenon indicated that there would be a possibility of resonance on the railway bridge. It is interesting to note that this case study might be beneficial to practicing engineers and researchers.

Investigation of the Effects of Waste Tire, Water Content and Relative Density on Soil Dominant Frequency in Separate Grained Soils by Microtremor Method

This paper presents the effects of the waste tire, water content, and relative density on soil dominant frequency using the experimental application. In this study, metal cylindrical molds with a diameter of 30 cm and a height of 50 cm are prepared to understand the effects of these materials. These molds are filled with sand in different relative densities (60% and 80%). Also; the samples are prepared by adding waste tire particles at different rates (0%, 0.5%, 1%, 2%) to this sand for different relative densities and water contents (0%, 5%, 10%, 15%). The soil dominant frequency and H/V amplitude are determined with the microtremor technique by measuring at 60 minutes of measurement on each cylinder using a broadband seismometer. Microtremor data are analyzed by using the Horizontal/Vertical spectral ratio method. As a result of the experiments, it has been observed that the waste tire particles added to the sand affect the H/V amplitude in a decreasing way. It is detected that the most suitable environment is obtained to be 1% waste tire and 0% water content for 60% relative density. Moreover, the most suitable environment has been achieved in 0% water content and 0.5% waste tire ratio for 80% relative density. It has been concluded that the prepared composite material with waste tires may have positive effects on decreasing the earthquake devastating effects.

Determination of earthquake prone zones at university of tadulako based on dominant periods and peak ground acceleration (PGA)

University of Tadulako is the largest State University in Palu City. When an earthquake with a magnitude of 7.4 Mw on September 28, 2018, occurred, many buildings were damaged and even collapsed at that time, even causing casualties due to the rubble. Research on the Local Site Effect is essential for the assessment of seismic hazard. In this study, the local site effect was analyzed using the HVSR method based on microtremor data. The predominant Period (To) ranges between 1.709 s to 3.816 s, indicates that this area consists of alluvium and has a very thick sediment layer. Another parameter calculated in this paper is the peak ground acceleration (PGA) with values from 0.914 g to 0.924 g. This value is the first indicated soil damage level due to ground motions. The results of this study can be used as a consideration in the development of regional spatial planning and building structures based on earthquake analysis.

Multiple-peak HVSR curves: Management and statistical assessment

The Horizontal-to-Vertical Spectral Ratio (HVSR) obtained from microtremor data recorded at three test sites are analyzed in order to highlight some issues related to the computation of the SESAME criteria that define the statistical robustness of possible peaks. In case of multiple-peak HVSR curves, it is shown that to properly assess the statistical properties of a peak and avoid the problem of multimodal data distribution, it is crucial to isolate each peak by reducing the frequency range around it.It is also shown that, while the standard approach used to obtain a homogeneous data set is represented by the removal of large-amplitude transient events before the computation of the HVSR, the removal of outlier HVSR curves can be a more effective way to obtain such a goal, being outlier HVSR curves not necessarily associated to large-amplitude transient events. It is eventually briefly discussed the problem of the proper management of multi-peak HVSR curves in microzonation studies while defining the so-called soil frequency maps. It is argued that to focus these maps just on the lowest-frequency (f0) value is questionable because higher-frequency peaks can be much more important when the local urban landscape is characterized by low-rise buildings.

Struktur Kecepatan Gelombang Geser (Vs) di Daerah Rawan Gerakan Tanah (Longsor) Jalan Lintas Kabupaten Bengkulu Tengah-Kepahiang

Shear wave velocity <em>(V<sub>s</sub>)</em> structure at along of Central Bengkulu-Kepahiang crossline has been mapped. This research aims to identify the subsurface structure and to estimate the constituent material type of rock in landslide-prone areas (Central Bengkulu-Kepahiang crossline). Shear wave velocity structure on each site is obtained by the HVSR-inversion of 146 microtremor data (ambient noise recording of seismometer). <em>V<sub>s</sub></em> structure at the line mapped from the surface until to 30 meters of the depth. Groups of Vs are identified in class E (<em>V<sub>s</sub></em> <180), Class D (180≤<em> V<sub>s</sub></em> <360), Class C (360≤<em> V<sub>s</sub></em> <760), and Class B (760≤<em> V<sub>s</sub></em> <1500). The subsurface structure at the depth of 0 to 10 meters are dominated by stiff soil, very dense soil, and soft rock which has highly fractured and weathered rock properties. At the depth of 15 meters to 30 meters, the subsurface structure is dominated by hard rock but it is high potential or easy to fracturing and weathering like the properties of the rocks in areas that have landslides in the past. Based on <em>V<sub>s</sub></em> value, rock constituent materials are deposition of sand, clay, gravel and alluvium ranging from soft to relatively hard structures at the depth.

An integrated approach using microtremor data for field development and reservoir monitoring: an example from Betung Field, Jambi

Reservoir monitoring is one of the most important aspects in oil and gas field management, because it provides industries with the required data in hydrocarbon optimization. This is often carried out in Indonesia by the TAC Pertamina-Pusako Betung Muaro Senami Jambi (PBMSJ), an organization responsible for the management and coordination of oil well activities. In 2012, the organization conducted passive seismic data acquisition and series of drilling campaigns, which successfully increased oil production in Betung Block from 43 to 652 bopd. This campaign was successfully carried out through one and two wells in 2013 and 2014, respectively. Furthermore, the pre-2017 interpretation describes the Betung Anticline as a relatively symmetrical and low angle N-S trending field, which defies the South Sumatra Basin Northwest-Southeast regional trend. This anticline, bound by two high angle normal faults, is unable to define the LKO/OWC position, leading to the consideration of a more integral interpretation approach by the PBMSJ team in early 2017. The procedure was conducted through the use of a 2d seismic line bounded with new wells and SRTM imagery, to combine subsurface structural data with the surface fundamental analogue. In 2017, a passive seismic study was repeatedly conducted in the Betung field, with only 30 measuring points. The urgency in identifying the reservoir is essential within the petroleum industry, in order to produce sufficient hydrocarbons. It is also essential in confirming the potential uses of microseismic data in storage decision-making, through a dataset acquired in Betung, based on reservoir management.

Improving Quality of Microtremor Data with Application of Empirical Mode Decomposition Method, Case Study: East Tanjung Karang, Bandar Lampung

Microtremor can be used for determining ground dynamical characteristic. We have measured microtremor data in East Tanjung Karang, Bandar Lampung. Collecting proper data in some urban areas like East Tanjung Karang is a difficult work because it produced data with a lot of transient signal caused by many human activities and industrial work. One of method to removed transient signal is Empirical Decomposite Method (EMD). EMD decompose an input signal (in time series) into several intrinsic mode functions (IMF) and a residual function. We made the first synthetic signal contained frequency with 1, 10, 20, and 30 Hz. EMD method was applied to the signal and obtained some Instrinsic Mode Function (IMF). IMF 1 has some frequency peaks that are 10, 20, and 30 Hz with power spectral density (PSD) amplitude 0.2, 0.9, and 1 W/Hz respectively. The second synthetic signal contained same frequency with the first one butbut the amplitude was 1, 1, 10, and 50 m. The results in IMF 1 from the second signal still have some peaks but spectral density in 10 Hz increases from 1 W/Hz (before applied EMD) to ±5 W/Hz (after applied EMD, in IMF 1). We applied EMD method also to Y1, Y2, and Y8 microtremor data. We remove IMF 1 or IMF 2 from the original data. The results are the removed IMF 1 data has less spike or transient signal in microtremor time series, but the removed IMF 2 data did not have significant change from the original microtremor. We sugested that higher IMF number, lower the dominant frequency. IMF cannot be related as a single oscillatory source because the result showed that IMF content more than one frequency peak. Removing IMF 1 produced by EMD method can improving quality of micrtremor data because it remove some transient data. This transient can be related to noise in higher frequency that originated locally. it can also decrease maximum standard deviation of HVSR curve. This study revealed revealed that removed IMF 1 data is has less change for both fundamental frequency and amplification factor than removed IMF 2 data in HVSR Curve.

Preliminary Result : Classification of Soil Using Fundamental Frequency and Vs30m

Geologically, Ngamprah – Padalarang is located above Lembang Fault which stretched from east to west of Bandung, with a length of 29 Km. therefore, to understand disaster risk around the area, a seismic microzonation study is needed. Microtremor measurement has been done in 21 locations around Ngamprah, West Bandung. Microtremor measurement is a passive seismic method that utilizes natural vibration so it can provide a fundamental frequency of microtremor data and can be applied to understand the lithology of the local site area. Furthermore, the fundamental frequency can be used to obtain Vs30m and correlate it to Eurocode 8 for classification of soil type. The result has shown, the fundamental frequency at Ngamprah around 3 – 4 Hz, meanwhile, we found an anomaly of high frequency around 9 – 11 Hz at Cimeta River. Low frequencies correlated to deep sedimentation meanwhile high frequencies may correlated to Lembang Fault Segment Cimeta which cut through Cimeta River and also indicated as slow sedimentation. In addition to that, VS30m obtained from fundamental frequency show around Cimeta river has high velocity (550 m/s) meanwhile at Ngamprah area is low velocity < 400 m/s . High velocity is indicated as very dense sand or very stiff clay meanwhile low velocity indicates deep deposits of gravel or stiff clay with thickness several tens to hundreds of meters.

Combining single-station microtremor and gravity surveys for deep stratigraphic mapping

Any stratigraphic reconstruction by means of surface geophysical methods is affected by the nonuniqueness of data inversion and by the resolution-depth trade-off. The combination of different geophysical techniques can reduce the number of degrees of freedom of the problem. We have focused on two low-impact single-station geophysical techniques: microtremor and gravity. These have been used by previous authors for stratigraphic mapping only by comparing the results independently. We suggest a procedure to combine microtremor and gravity data into a unique subsoil model and explore to what extent their combined use can overcome their individual weaknesses and constrain the final result. We apply the procedure to the Bolzano sedimentary basin, Northern Italy, to derive a 3D bedrock model of the basin. We use microtremor data to map the ground resonance frequencies and derive an initial 3D bedrock depth model by assuming a [Formula: see text] profile for the sediment fill. Then, we define a density model for rock and sediments and perform 3D gravity forward modeling. We then perturb the [Formula: see text] and density models and find the parameters that best fit the observed gravity anomalies. Data uncertainties are examined to explore the significance of the results. Joint use of the two techniques successfully helps interpret the stratigraphic model: Ground resonance frequencies guarantee the spatial resolution of the bedrock geometry model, whereas gravity data help constrain the frequency to depth conversion.

Liquefaction in Palu: the cause of massive mudflows

The 7.5 Mw tectonic earthquake that hit Palu City on 28 September 2018 was followed by tsunami and liquefaction, triggered massive mudflows in Balaroa, Petobo, and Jono Oge areas. This study focuses on the generating factors of liquefaction such as the condition of soil lithology, depth of water table, the distance to the focal mechanism, and the thickness of soft sediment. Microtremor data, including the Horizontal Vertical Spectral Ratio (HVSR), geological condition, and borehole data, were examined to conduct the liquefaction analysis. The analysis results based on the microtremor data showed that the distribution of ground shear strain values in Palu City ranged from 0.75 × 10–4 to 2.56 × 10–4. The distribution of the locations of the liquefaction was correlated to the distribution of ground shear strain values. High ground shear strain values and a shallow groundwater level were discovered in Palu City valley, which indicates that liquefaction in Palu City will undoubtedly occur. The semi-empirical method confirmed that Balaroa, Petobo, and Jono Oge had undergone large-scale liquefaction at a maximum depth of 16 m below the ground level. The average peak of water runoff that generated the mudflow was estimated to be at 11.31 cm3/s. Since the soil has loose soil grain with high water content, the soil will turn into a massive amount of mud during the liquefaction.

ESPAC-based 2D mini-array microtremor method and its application in urban rail transit construction planning

Conventional geophysical methods face many challenges in urban areas from limited site accessibility to severe industrial interferences. With the fast-paced development in urban area and urban light rail transit system, it is highly desirable to have a high-resolution geophysical method, which is suitable for the complex urban condition with strong background noises and, at the same time, is environment-friendly.In this paper, we introduce a 2D mini-array microtremor survey method, based on the extended spatial auto-correlation (ESPAC) method. It first uses the ESPAC to extract the dispersion curve of the Rayleigh wave from the microtremor data recorded using the mini array, then reconstructs the apparent S-wave velocity (Vx) structure of the stratigraphic rock formations by the Vr–Vx transformation, and finally, the 2D Vx profile is obtained by using Vx velocity imaging. This method not only makes full use of the multi-mode Rayleigh waves of dispersion curve, but also reduces the uncertainty during inversion caused by the potential pattern misjudgment when inverting the dispersion curve directly. Since the Vx is generally sensitive to the change of rock mass density, its profile is a natural and reliable reflection of the changes in rock properties. This technique provides an accurate and cost-effective tool for mapping complex near-surface geological anomalies such as karst cavities, fractured zone, and the soil/bedrock interface, which are frequently encountered in many urban engineering projects, such as urban rail transit development. The successful application of the microtremor survey in Guangzhou Metro Line 10 project demonstrates that this method can be very effective in identifying the soil/bedrock interface, mapping the severely weathered rock formations, and detecting the abandoned underground structures.

The Implementation of Strain Liquefaction Learning Package to Cultivate the Critical Thinking of High School Students

This study aims to explain the potential of liquefaction in Padang Lekat Village and Pasar Ujung Village Kepahiang District, explaining the feasibility of developed learning packages, and explaining students' critical thinking skills after learning the wave material through the learning package "Strain Liquidity Learning". The type of study used was research and development which aims to develop digital-based physics learning packages that are integrated with disaster mitigation education on the basic competencies of waves in high school education units. This research was conducted using ADDIE development research procedures. The results of research on the analysis of liquefaction potential based on microtremor data using the Ground Shear Strain method, zoning indication of liquefaction potential due to geological conditions and seismic potential, generally Padang Lekat Village and Pasar Ujung Kepahiang Village are areas that have no potential to experience liquefaction. However, there are a number of locations indicated by potential liquefaction, although not significantly. Learning Package Book "Learning Liquid Strain" compiled based on the analysis of liquefaction potential based on expert validation is appropriate to be used as a learning medium for seismic and liquefaction waves. In its application with distance learning, the compiled learning package book has been able to improve critical thinking skills in students even though the N-gain improvement obtained is still in the low and medium categories.

Guidelines and pitfalls of refraction microtremor surveys

The geotechnical industry has widely adopted the refraction microtremor shear-wave velocity measurement technique, which is accepted by building authorities for evaluation of seismic site class around the world. Clark County and the City of Henderson, Nevada, populated their Earthquake Parcel Map with over 10,000 site measurements for building code enforcement, made over a 3-year period. 2D refraction microtremor analysis now allows engineers to image lateral shear-wave velocity variations and do passive subsurface imaging. Along with experience at a basic level, the ability to identify the “no energy area” and the “minimum-velocity envelope” on the slowness-frequency (p-f) image helps practitioners to assess the quality of their ReMi data and analysis. Guides for grading (p-f) image quality, and for estimating depth sensitivity, velocity-depth tradeoffs, and depth and velocity resolution also assist practitioners in deciding whether their refraction microtremor data will meet their investigation objectives. Commercial refraction microtremor surveys use linear arrays, and a new criterion of 2.2% minimum microtremor energy in the array direction allows users to assess the likelihood of correct results. Unfortunately, any useful and popular measurement technique can be abused. Practitioners must follow correct data collection, analysis, interpretation, and measurement procedures, or the results cannot be labeled “refraction microtremor” or “ReMi” results. We present some of the common mistakes and provide solutions with the objective of establishing a “best practices” template for getting consistent, reliable models from refraction microtremor measurements.