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Pedersen, Laust B.
Alternative names
Publications (10 of 54) Show all publications
García Juanatey, M. d., Hübert, J., Tryggvason, A., Juhlin, C., Pedersen, L. B., Bauer, T. E. & Dehghannejad, M. (2019). 2D and 3D MT in the central Skellefte Ore District, northern Sweden. Tectonophysics, 764, 124-138
Open this publication in new window or tab >>2D and 3D MT in the central Skellefte Ore District, northern Sweden
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2019 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 764, p. 124-138Article in journal (Refereed) Published
Abstract [en]

New broadband magnetotelluric (MT) data have been acquired along two parallel profiles in the central part of the metallogenic Skellefte district in northern Sweden. The data were recorded as part of the Swedish 4D modelling of mineral belts project and cover an area with several economical and sub-economical deposits. The dimensionality and quality of the data were carefully analyzed and new error floors were systematically determined prior to inverse modelling in 2D and 3D. The algorithms used were EMILIA and WSINV3DMT. For the 2D inversion, only the determinant of the impedance tensor was used, while for the 3D inversion all elements were considered. The obtained models fit the inverted data, and image the main regional features. A detailed comparison reveals the superiority of the 3D model, both in model structures and data fit. After assessing the main features in the model, an interpretation is proposed and refined with the support of previous geophysical studies. The most interesting features are large and medium-sized conductors associated with crustal-scale shear zones and faults within the Skellefte Group rocks. These may be depicting a network of fossil pathways for hydrothermal fluid transport and as such, provide new insight into past processes in the area.

Keywords
Magnetotellurics, 3D inversion, Deep exploration, Brownfield, Shear zones, Mineral systems
National Category
Geophysics
Research subject
Geophysics with specialization in Solid Earth Physics
Identifiers
urn:nbn:se:uu:diva-178057 (URN)10.1016/j.tecto.2019.04.003 (DOI)000472697900008 ()
Projects
VINNOVA 4D modeling of mineral belts
Funder
Vinnova
Available from: 2012-07-26 Created: 2012-07-26 Last updated: 2019-08-16Bibliographically approved
Wang, S., Kalscheuer, T., Bastani, M., Malehmir, A., Pedersen, L. B., Dahlin, T. & Naser, M. (2018). Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden. Geophysical Journal International, 213(1), 511-533
Open this publication in new window or tab >>Joint inversion of lake-floor electrical resistivity tomography and boat-towed radio-magnetotelluric data illustrated on synthetic data and an application from the Äspö Hard Rock Laboratory site, Sweden
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2018 (English)In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 1, p. 511-533Article in journal (Refereed) Published
Abstract [en]

The electrical resistivity tomography (ERT) method provides moderately good constraints for both conductive and resistive structures, while the radio-magnetotelluric (RMT) method is well suited to constrain conductive structures. Additionally, RMT and ERT data may have different target coverage and are differently affected by various types of noise. Hence, joint inversion of RMT and ERT data sets may provide a better constrained model as compared to individual inversions. In this study, joint inversion of boat-towed RMT and lake-floor ERT data has for the first time been formulated and implemented. The implementation was tested on both synthetic and field data sets incorporating RMT transverse electrical mode and ERT data. Results from synthetic data demonstrate that the joint inversion yields models with better resolution compared with individual inversions. A case study from an area adjacent to the Aspo Hard Rock Laboratory (HRL) in southeastern Sweden was used to demonstrate the implementation of the method. A 790-m-long profile comprising lake-floor ERT and boat-towed RMT data combined with partial land data was used for this purpose. Joint inversions with and without weighting (applied to different data sets, vertical and horizontal model smoothness) as well as constrained joint inversions incorporating bathymetry data and water resistivity measurements were performed. The resulting models delineate subsurface structures such as a major northeasterly directed fracture system, which is observed in the HRL facility underground and confirmed by boreholes. A previously uncertain weakness zone, likely a fracture system in the northern part of the profile, is inferred in this study. The fractures are highly saturated with saline water, which make them good targets of resistivity-based geophysical methods. Nevertheless, conductive sediments overlain by the lake water add further difficulty to resolve these deep fracture zones. Therefore, the joint inversion of RMT and ERT data particularly helps to improve the resolution of the resistivity models in areas where the profile traverses shallow water and land sections. Our modification of the joint inversion of RMT and ERT data improves the study of geological units underneath shallow water bodies where underground infrastructures are planned. Thus, it allows better planning and mitigating the risks and costs associated with conductive weakness zones.

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-327094 (URN)10.1093/gji/ggx414 (DOI)000448715000035 ()
Funder
Swedish Research Council Formas, 25220121907
Available from: 2017-08-02 Created: 2017-08-02 Last updated: 2019-01-18Bibliographically approved
Mehta, S., Bastani, M., Malehmir, A. & Pedersen, L. B. (2017). CSRMT Survey on Frozen Lake - A New Technique with an Example from the Stockholm Bypass Tunnel. In: : . Paper presented at 23rd European Meeting of Environmental and Engineering Geophysics, 3-7 September 2017, Malmö, Sweden.
Open this publication in new window or tab >>CSRMT Survey on Frozen Lake - A New Technique with an Example from the Stockholm Bypass Tunnel
2017 (English)Conference paper, Oral presentation only (Refereed)
Abstract [en]

More than 7% of the Scandinavian landmass is covered with fresh-water bodies in the form of lakes and rivers. This poses a unique challenge to carry out electromagnetic survey on shallow-water bodies for various purposes for example geotechnical investigations. Recently boat-towed RMT (radio-magnetotelluric) technique was introduced and used for measurements over the Lake Mälaren in Stockholm, Sweden. The RMT covers a wide range of frequencies (10-250 kHz) and provides good resolution for shallow subsurface studies although it lacks resolution at greater depths. Using controlled-source frequencies in the range of 1-10 kHz sufficient penetration depths can be achieved for most of the near surface targets. In this study, we present the results from the combined use of controlled-source and RMT (CSRMT) data that were obtained over frozen Lake Mälaren. The objective of this study was to map bedrock surface and fractures in the middle of the profile where using only RMT data these were not adequate. We demonstrate a new technique where CSRMT surveys were carried out over frozen-shallow-water bodies and we expect the idea to be used in the near future for other applications where moderately-resistive water bodies are present. 

National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-328045 (URN)10.3997/2214-4609.201702056 (DOI)
Conference
23rd European Meeting of Environmental and Engineering Geophysics, 3-7 September 2017, Malmö, Sweden
Funder
Swedish Research Council Formas, 25220121907
Available from: 2017-08-16 Created: 2017-08-16 Last updated: 2017-08-22Bibliographically approved
Shan, C., Kalscheuer, T., Pedersen, L. B., Erlström, M. & Persson, L. (2017). Portable audio magnetotellurics - experimental measurements and joint inversion with radiomagnetotelluric data from Gotland, Sweden. Journal of Applied Geophysics, 143, 9-22
Open this publication in new window or tab >>Portable audio magnetotellurics - experimental measurements and joint inversion with radiomagnetotelluric data from Gotland, Sweden
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2017 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 143, p. 9-22Article in journal (Refereed) Published
Abstract [en]

Field setup of an audio magnetotelluric (AMT) station is a very time consuming and heavy work load. In contrast, radio magnetotelluric (RMT) equipment is more portable and faster to deploy but has shallower investigation depth owing to its higher signal frequencies. To increase the efficiency in the acquisition of AMT data from 10 to 300 Hz, we introduce a modification of the AMT method, called portable audio magnetotellurics (PAMT), that uses a lighter AMT field system and (owing to the disregard of signals at frequencies of less than 10 Hz) shortened data acquisition time. PAMT uses three magnetometers pre-mounted on a rigid frame to measure magnetic fields and steel electrodes to measure electric fields. Field tests proved that the system is stable enough to measure AMT fields in the given frequency range. A PAMT test measurement was carried out on Gotland, Sweden along a 3.5 km profile to study the ground conductivity and to map shallow Silurian marlstone and limestone formations, deeper Silurian, Ordovician and Cambrian sedimentary structures and crystalline basement. RMT data collected along a coincident profile and regional airborne very low frequency (VLF) data support the interpretation of our PAMT data. While only the RMT and VLF data constrain a shallow (similar to 20-50 m deep) transition between Silurian conductive (<30 Omega m resistivity) marlstone and resistive (>1000 Omega m resistivity) limestone, the single-method inversion models of both the PAMT and the RMT data show a transition into a conductive layer of 3 to 30 Omega m resistivity at similar to 80 m depth suggesting the compatibility of the two data sets. This conductive layer is interpreted as saltwater saturated succession of Silurian, Ordovician and Cambrian sedimentary units. Towards the lower boundary of this succession (at 600 m depth according to boreholes), only the PAMT data constrain the structure. As supported by modelling tests and sensitivity analysis, the PAMT data only contain a vague indication of the underlying crystalline basement. A PAMT and RMT joint inversion model reveals all the aforementioned units including the less than 80 m deep limestone and marlstone formations and the conductive sedimentary succession of Silurian, Ordovician and Cambrian units. Our test measurements have proven the PAMT modification to be time saving and easy to set up. However, PAMT data suffer from the same noise disturbances as regular AMT data. Since man-made EM noise can propagate over great distances through resistive underground, PAMT measurements are recommended to be carried out in areas with low resistivity. The PAMT method is proven to be applicable in shallow depth studies, especially in areas where normal AMT measurements are inconvenient and/or too expensive to carry out.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-329162 (URN)10.1016/j.jappgeo.2017.05.002 (DOI)000407524800002 ()
Funder
The Geological Survey of Sweden (SGU), 611404/2010
Available from: 2017-09-08 Created: 2017-09-08 Last updated: 2017-11-13Bibliographically approved
Mehta, S., Bastani, M., Malehmir, A. & Pedersen, L. B. (2017). Resolution and sensitivity of boat-towed RMT data to delineate fracture zones - Example of the Stockholm bypass multi-lane tunnel. Journal of Applied Geophysics, 139, 131-143
Open this publication in new window or tab >>Resolution and sensitivity of boat-towed RMT data to delineate fracture zones - Example of the Stockholm bypass multi-lane tunnel
2017 (English)In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 139, p. 131-143Article in journal (Refereed) Published
Abstract [en]

The resolution and sensitivity of water-borne boat-towed multi-frequency radio-magnetotelluric (RMT) data for delineating zones of weaknesses in bedrock are examined in this study. 2D modeling of RMT data along 40 profiles in joint transverse electric (TE) and transverse magnetic (TM) as well as determinant mode was used for this purpose. The RMT data were acquired over two water passages from the Lake Malaren near the city of Stockholm where one of the largest underground infrastructure projects, a multi-lane tunnel, in Europe is currently being developed. Comparison with available borehole coring, refraction seismic and bathymetric data was used to scrutinize the RMT resistivity models. A low-resistivity zone observed in the middle of all the profiles is suggested to be from fracture/fault zones striking in the same direction as the water passages. Drilling observations confirm the presence of brittle structures in the bedrock, which manifest themselves as zones of low-resistivity and low-velocity in the RMT and refraction seismic data, respectively. Nevertheless, RMT is an inductive electromagnetic method hence the presence of conductive lake sediments may shield detecting the underlying fractured bedrock. The loss of resolution at depth implies that the structures within the bedrock under the lake sediments cannot reliably be delineated. To support this, a synthetic data analysis was carried out providing useful information on how to improve and plan the lake measurements for future studies. Synthetic modeling results for example suggested that frequencies as low as 3 kHz would be required to reliably resolve the bedrock and fracture zone within it in the study area. The modeling further illustrated the advantage of a fresh water layer that acts as a near-surface homogeneous medium eliminating the static shift effects. While boat-towed RMT data provided substantial information about the subsurface geology, the acquisition system should be upgraded to enable controlled-source data acquisition to increase the penetration depth and to overcome the shortcomings of using only radio-frequencies.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2017
Keywords
Boat-towed radio magnetotelluric, Fracture zone, Urban geophysics, Resolution, Synthetic modeling, Controlled source
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-322221 (URN)10.1016/j.jappgeo.2017.02.012 (DOI)000399269400013 ()
Funder
Swedish Research Council Formas, 252-2012-1907The Geological Survey of Sweden (SGU), 363-26512013
Available from: 2017-05-17 Created: 2017-05-17 Last updated: 2017-08-22Bibliographically approved
Yan, P., Andersson, M., Kalscheuer, T., Garcı́a Juanatey, M. A., Malehmir, A., Shan, C., . . . Almqvist, B. S. G. (2016). 3D magnetotelluric modelling of the Alnö alkaline and carbonatite ring complex, central Sweden. Tectonophysics, 679, 218-234
Open this publication in new window or tab >>3D magnetotelluric modelling of the Alnö alkaline and carbonatite ring complex, central Sweden
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2016 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 679, p. 218-234Article in journal (Refereed) Published
Abstract [en]

Thirty-four broadband magnetotelluric stations were deployed across the Alno alkaline and carbonatite ring intrusion in central Sweden. The measurements were designed such that both 2D models along existing seismic profiles and a 3D model can be constructed. Alno Island and surrounding areas are densely populated and industrialized and in order to reduce the effect of noise, the remote reference technique was utilized in time series processing. Strike and dimensionality analyses together with the induction arrows show that there is no homogeneous regional strike direction in this area. Therefore, only the determinant of the impedance tensor was used for 2D inversion whereas all elements of the impedance tensor were used for 3D inversion. Representative rock samples were collected from existing outcrops and their resistivities were measured in the laboratory to facilitate interpretation of the inversion models. The results from these measurements show that coarse grained (sovite, white color) and fine-grained (dark color) carbonatites are the most conductive and resistive rock types, respectively. In accordance with the interpretation of the reflection seismic images, the 2D and 3D resistivity models depict the caldera-related ring-type fault system and updoming faulted and fractured systems as major 10-500 Omega m conductors, extending down to about 3 km depth. A central similar to 4000 Omega m resistive unit at about 3 km depth appears to correspond to a solidified fossil magma chamber as speculated from the reflection seismic data and earlier field geological studies.

Keywords
Alnö; Carbonatite; Alkaline; 3D; Magnetotelluric; Resistivity
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-292604 (URN)10.1016/j.tecto.2016.05.002 (DOI)000378177700017 ()
Funder
Swedish Research Council, 621-2009-4439
Available from: 2016-05-04 Created: 2016-05-04 Last updated: 2017-11-30Bibliographically approved
Abtahi, S. M., Pedersen, L., Kamm, J. & Kalscheuer, T. (2016). Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden. Geophysics, 81(3), B65-B76
Open this publication in new window or tab >>Consistency investigation, vertical gravity estimation and inversion of airborne gravity gradient data – A case study from northern Sweden
2016 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 3, p. B65-B76Article in journal (Refereed) Published
Abstract [en]

For airborne gravity gradient data, it is a challenge to distinguish between high-frequency intrinsic and dynamically produced noise caused by the aircraft and small-scale effects from shallow density variations. To facilitate consistent interpretation, techniques that include all of the measured gravity gradient components are particularly promising. We represented the measurements by a common potential function accounting for lateral and height variations. Thus, it was possible to evaluate the internal consistency between the measured components and to identify components with bias or particularly strong noise. As an extra benefit for data sets that contain terrain-corrected and nonterrain-corrected gravity gradient measurements at flight altitude, we estimated terrain-corrected anomalies on the topographic relief using downward continuation and retrieved nonterrain-corrected gravity gradient data suitable for inversion using upward continuation. For a field data set from northern Sweden, the largest differences (up to 50 eotvos) between the measured and estimated components of the gravity gradient data were found in areas of high topographical relief. But the average residual standard deviations of the individual components were between 3.6 and 7.4 eotvos, indicating that the components were consistent in an average sense. We have determined the successful conversion of terrain-corrected airborne gravity gradient data to Bouguer gravity data on the topographic relief using ground-based vertical gravity data as a reference. A 3D inverse model computed from the nonterrain-corrected data clearly showed the depth extent of the geologic structures observed at the surface, but it only produced a weak representation of the shallow structure. In contrast, a 2D surface density model in which only lateral variations of density in the topographic relief was allowed exhibited more realistic density distributions in fair correlation with geology.

Keywords
gravity, modeling, noise, processing
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-300024 (URN)10.1190/geo2014-0428.1 (DOI)000384984900008 ()
Available from: 2016-08-02 Created: 2016-08-02 Last updated: 2017-11-28Bibliographically approved
Abtahi, S. M., Pedersen, L., Kamm, J. & Kalscheuer, T. (2016). Extracting geoelectrical maps from vintage very-low-frequency airborne data, tipper inversion, and interpretation: A case study from northern Sweden. Geophysics, 81(5), B135-B147
Open this publication in new window or tab >>Extracting geoelectrical maps from vintage very-low-frequency airborne data, tipper inversion, and interpretation: A case study from northern Sweden
2016 (English)In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 81, no 5, p. B135-B147Article in journal (Refereed) Published
Abstract [en]

In 1985, the mining company Luossavaara-Kiirunavaara Aktiebolag collected airborne very-low-frequency (VLF) data in northern Sweden. The operators stored only the vertical component and the total magnetic field, which at that time were believed to be sufficient for qualitative interpretation. Therefore, the data could not be directly used for quantitative tensor VLF processing and inversion. To avoid the costs of resurveying, we have developed a novel technique to estimate the tippers from the measured VLF data by computing anomalous and normal parts of the horizontal components of the magnetic field from two transmitters separately. Retrieval of the normal horizontal components was possible because one component of the horizontal magnetic field was used as the phase reference during the measurements. Additionally, we have determined how the approximate apparent resistivity suitable for data visualization can be computed from the components of the magnetic field assuming an average normal resistivity of the subsurface. Maps of apparent resistivity combined with topography show a clear correlation between high topography and high resistivity, whereas conductive zones are found in valleys in between. More importantly, the 3D model inverted from the calculated tippers shows excellent agreement with a map of the surface geology. Based on this comparison, some less resistive zones can be related to fluids in fractures and others can be related to mineralized contact zones. We suggest to focus further exploration on conductive zones surrounding areas with basaltic composition.

Keywords
case history, inversion, interpretation, electromagnetics
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-300086 (URN)10.1190/GEO2015-0296.1 (DOI)000392752200002 ()
Available from: 2016-08-02 Created: 2016-08-02 Last updated: 2018-10-29
Mansoori, I., Oskooi, B., Pedersen, L. B. & Javaheri, R. (2016). Three-dimensional modelling of magnetotelluric data to image Sehqanat hydrocarbon reservoir in southwestern Iran. Geophysical Prospecting, 64(3), 753-766
Open this publication in new window or tab >>Three-dimensional modelling of magnetotelluric data to image Sehqanat hydrocarbon reservoir in southwestern Iran
2016 (English)In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 64, no 3, p. 753-766Article in journal (Refereed) Published
Abstract [en]

A detailed magnetotelluric survey was conducted in 2013 in the Sehqanat oil field, southwestern Iran to map the geoelectrical structures of the sedimentary Zagros zone, particularly the boundary between the Gachsaran Formation acting as cap rock and the Asmari Formation as the reservoir. According to the electrical well logs, a large resistivity contrast exists between the two formations. The Gachsaran Formation is formed by tens to hundreds of metres of evaporites and it is highly conductive (ca. 1 m-10 m), and the Asmari Formation consists of dense carbonates, which are considerably more resistive (more than 100 m). Broadband magnetotelluric data were collected along five southwest-northeast directed parallel lines with more than 600 stations crossing the main geological trend. Although dimensionality and strike analysis of the magnetotelluric transfer functions showed that overall they satisfied local 2D conditions, there were also strong 3D conditions found in some of the sites. Therefore, in order to obtain a more reliable image of the resistivity distribution in the Sehqanat oil field, in addition to standard 2D inversion, we investigated to what extent 3D inversion of the data was feasible and what improvements in the resistivity image could be obtained. The 2D inversion models using the determinant average of the impedance tensor depict the main resistivity structures well, whereas the estimated 3D model shows significantly more details although problems were encountered in fitting the data with the latter. Both approaches resolved the Gachsaran-Asmari transition from high conductivity to moderate conductivity. The well-known Sehqanat anticline could also be delineated throughout the 2D and 3D resistivity models as a resistive dome-shaped body in the middle parts of the magnetotelluric profiles.

Keywords
Magnetotellurics, 2D and 3D inversion, Asmari Formation, Gachsaran Formation, Sehqanat oil field, Zagros zone
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-297808 (URN)10.1111/1365-2478.12328 (DOI)000374690000017 ()
Available from: 2016-06-28 Created: 2016-06-28 Last updated: 2017-11-28Bibliographically approved
Oskooi, B., Mansoori, I., Pedersen, L. B. & Koyi, H. A. (2015). A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran. Pure and Applied Geophysics, 172(2), 491-502
Open this publication in new window or tab >>A Magnetotelluric Survey of Ophiolites in the Neyriz area of southwestern Iran
2015 (English)In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 172, no 2, p. 491-502Article in journal (Refereed) Published
Abstract [en]

A wide band magnetotelluric study of the ophiolitic zone of the Zagros orogenic belt was conducted in the Neyriz area of southwestern Iran. The purpose of the study was to image subsurface structures electrically and relocate the main Zagros thrust fault in the region. The thrust fault has a complex structure with obscure behavior and is believed to be located within a zone of ongoing continental plate convergence. The fault zone with a NW-SE geological trend is parallel to the Zagros orogenic belt and separates the Neyriz ophiolite assemblage from the adjacent Sanandaj-Sirjan metamorphic zone. Magnetotelluric data were collected along a SW-NE profile across the geologic strike; the study included 18 stations and modeling was performed using a 2-D inversion scheme. Analysis of both modes of magnetotelluric data (TE and TM) clarifies the signatures of large resistivity variation in the study area. Due to the presence of a high contrast in resistivity between the ophiolites and neighboring rocks, we are able to discern two sharp boundaries as faulting planes and borders of the ophiolite-radiolarite zone in the north-eastern and southwestern parts of the 2-D resistivity models, respectively.

Keywords
Magnetotelluric, Neyriz ophiolite assemblage, Zagros orogenic belt, main Zagros thrust fault, resistivity, Sanandaj-Sirjan metamorphic zone
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-247142 (URN)10.1007/s00024-014-0925-5 (DOI)000348787100018 ()
Available from: 2015-03-17 Created: 2015-03-13 Last updated: 2017-12-04Bibliographically approved
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