- Email: [email protected]
Mapping sea water intrusion in coastal area using time-domain electromagnetic method with different loop dimensions
Journal Pre-proof Mapping sea water intrusion in coastal area using time-domain electromagnetic method with different loop dimensions
Hesham El-Kaliouby PII:
S0926-9851(19)30636-6
DOI:
https://doi.org/10.1016/j.jappgeo.2020.103963
Reference:
APPGEO 103963
To appear in:
Journal of Applied Geophysics
Received date:
17 July 2019
Revised date:
19 January 2020
Accepted date:
28 January 2020
Please cite this article as: H. El-Kaliouby, Mapping sea water intrusion in coastal area using time-domain electromagnetic method with different loop dimensions, Journal of Applied Geophysics(2019), https://doi.org/10.1016/j.jappgeo.2020.103963
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
Journal Pre-proof Mapping sea water intrusion in coastal area using time -domain electromagnetic method with different loop dimensions
Hesham El-Kaliouby (1) (1) National Research Center, Geophysics Dept., Egypt Email: [email protected] Abstract Time Domain Electromagnetic (TDEM) method has been widely used in mapping and assessment of groundwater resources and sea water intrusion along coastal alluvial plains. TDEM soundings were used to define aquifer zones in the alluvial aquifer in Al Khoud coastal
of
area in North Oman. The main aquifer zones, composed of clean gravels, clayey gravels, and cemented gravels, could be identified, along with the basal contact of the aquifer. The TDEM
ro
data were also used to map the depth to the interface between freshwater and saline-water along
-p
the coast. The inland extents and thickness of the saline water intrusion zone were mapped along two N-S TDEM profiles. The depth to the intrusion zone was investigated using different
re
transmitter loop areas. The resistivity distribution derived from TDEM data agrees closely with
lP
the electrical conductivity and resistivity logs from available well data in the study area. TDEM proved to be a successful tool for mapping saline zones and fresh water in arid environment with
na
high depth of penetration and speed of operation.
1. Introduction
ur
Keywords: sea water intrusion, groundwater, loop size, Time-domain Electromagnetic, TDEM.
The coastal plain in northern Oman, that extends between Oman mountains and the Gulf of
Jo
Oman, represents the main source of fresh water. The alluvial coastal aquifer provides an important freshwater resource for both domestic and agricultural use. Almost 50% of the agriculture production is coming from the northern coastal plain, which depends on groundwater as the primary source of agriculture water. However, over pumping from the coastal aquifer resulted
in
large
water-level declines
and
associated
environmental impacts,
including
groundwater quality deterioration and sea water intrusion, which represents a growing potential threat to water supplies. The salinity increased to levels that affected the crops and human consumption. The yield of the crops, including the date palms which represent one of the main economic agricultures in Oman, has been affected. Many wells and farms have been abandoned as soil salinity has increased within 6 km of the coastline. This highlights the need for urgent conservation measures to sustain this precious economic wealth (Young, et al. 1998, 2004).
Journal Pre-proof The study area (figure 1) is located at Al Khoud fan, northern coastal area of Oman; represent the lower reaches of Samail Catchment, which extends from the mountains region into the coastal plain. Groundwater recharge in the Al Khoud fan occurs from infiltration of flood runoff originating as direct rainfall in the upper Samail catchment. As the volume of abstracted water has increased in the downstream coastal plain, the water level has declined and as a consequence, sea water has moved landwards. This hazard is highlighted by the shallow depth of the water table, which is less than 1m above mean sea level and often below sea level. For these reasons, there is a great concern and a pressing requirement to assess the available groundwater
of
reserve in the northern coastal plain aquifer (Young, et al. 1998).
ro
It is important to have a fast and reliable tool for regular monitoring of salinity changes along the coastal aquifer in order to get better future prediction and to improve management
-p
plans to limit or prevent saltwater intrusion. This is conventionally achieved through drilling a network of monitoring wells along the coastal plain to monitor changes in water salinity over
re
time. However, installation and operation of these wells is expensive and provides limited spatial
lP
and vertical coverage. An alternative approach to determine the existence or potential of groundwater resources and saline water intrusion is to use noninvasive geophysical methods
na
(Nenna et.al., 2013; El-Kaliouby and Abdallah, 2015). TDEM is a relatively low cost, rapid, and noninvasive reconnaissance tool to locate
ur
potential freshwater resources. TDEM method is well suited to near-surface hydrological studies as it can be used to differentiate between fresh, brackish, and saline water. TDEM method the
electrical
conductivity
Jo
investigates
of
the
subsurface
through
measurements
of
electromagnetic field over time. The characteristics of the TDEM responses of the earth due to a primary signal from a large wire square-loop laid on the ground surface yield information about the variations of the electrical conductivity with depth. The conductivity of soils and rocks are controlled by mineralogy, clay content, water content, salinity, and porosity. Changes in the conductivity of soils and rocks produce variations in the electromagnetic signature. The exact position of the saline water/ fresh-water interface in most coastal areas is poorly constrained on a local level. A combination of existing monitoring well water quality measurements borehole data, and surface TDEM can be used to accurately map the position of the interface and intrusion zone (e.g. Al-Garni and El-Kaliouby, 2011; El-Kaliouby and Abdallah, 2015).
Journal Pre-proof 2. Field Site Characteristics Major groundwater flows in Northern Oman are from the mountains towards the sea. The coastal alluvial deposits in wadi beds form excellent aquifers and contain renewable resources of good quality groundwater by regular recharge from rainfall, surface water flow and locally the inflow of groundwater from adjacent hard rock aquifers. Recharge from local rainfall appears to constitute an important component in the areas, where coarse gravel deposits provide favorable infiltration conditions. Many streams originating in the mountains may flow for an extended period in downstream to recharge coastal aquifer (Stanger, 1986; Young, et al. 1998, Macumber,
of
2003).
ro
The study area is covered by Quaternary unconsolidated to semi-consolidated sands and gravels with interbedded clay lenses or layers, which rest on clayey conglomerates or directly
-p
upon fissured Ophiolites. The Quaternary cover has generally a wedge shape, thickening from a few meters close to the mountains to more than 100 m along the coast. A thick sequence of marls
re
and clays underlies the coarser Quaternary sediments in some areas near the coast; the thickness
lP
of this sequence may reaches up to 600 m (Gibb, 1976; Stanger, 1986). Three major units dominate the lithological succession of the alluvium: upper, middle and lower gravel units. The upper gravel unit is composed predominantly of large-size gravels and sand including boulders
na
(figure 2). This unit is loose and very poorly sorted of permeable alluvium with good water quality that makes it the main aquifer zone. The middle unit is discontinuous clayey gravel,
ur
which is found in the form of lenses and inter-bedded claystone bands between the upper and
Jo
lower gravel units. The lower unit is cemented gravely marl and clays, which is more compacted and conglomeratic, due to the cementation caused by diagenesis associated with a fluctuating water table. (Gibb, 1976; Stanger, 1986; Young, et al. 1998).
3. TDEM Method TDEM made by transmitting input current into square loops of insulated wire deployed on the land surface. The transmitter loop current consists of equal periods of time-on and time-off, which produce an electromagnetic field near the loop of wire. Termination of the current flow is not instantaneous but occurs over a few to hundreds of microseconds, known as the ramp time (or turn-off time), during which the magnetic field is time-variant. The time-variant nature of the induced electromagnetic field creates a primary electromagnetic field in the ground underneath
Journal Pre-proof the loop in accordance with Faraday’s Law. This resultant field instantly begins to decay, generating additional eddy currents that spread downward and outward into the subsurface like a succession of smoke rings. These eddy currents then produce a secondary magnetic field that is recorded as voltage measurements throughout the time-off period by a receiver positioned in the center or at an off-set distance from the transmitter loop. The voltage magnitude received from the eddy currents at specific times and depths is determined by the overall conductivity of subsurface rock units and fluids. From these voltage measurements, apparent resistivity values can be calculated. Depth of exploration depends on the transmitter's magnetic moment (loop area
of
times the injected current intensity). Large loop dimensions allow current to penetrate deeper and
ro
covers a larger volume of the subsurface materials compared with smaller loops. Late time voltage, is measuring eddy currents from increasingly greater depths, while early time voltage
re
4. Data Acquisition And Processing
-p
corresponds to shallow near surface layers (McNeill, 1990, 1994).
lP
A Geonics Protem-47 system using a square transmitter (Tx) loop was used to collect the TDEM soundings. A 1-meter-diameter multi-turn coil receiver (Rx) with a 31.4-square-meter area measured the secondary magnetic fields with central loop and an off-set loop of 25m from
na
the center of the transmitter loop. The Protem-47 transmitter used an injection current of 3.5A with preset base frequencies of 25Hz for shallow soundings and 6.5Hz for intermediate sounding
ur
and 2.5Hz for deeper soundings. The Protem TDEM unit samples signal amplitudes in 20
Jo
subintervals (time gates) during each measurement and collect three voltage data sets. Data collected with TDEM method, provided significant contributions to image the extent of saltwater intrusion at many sites in relatively short time. The depths and inland extent of the sea water intrusion was mapped through two main profiles perpendicular to the coastline using different loop sizes. The two profiles extend from Al Khoud dam northward towards the sea, (figure 1). The large loop data was measured by the Ministry of Water Resources of Oman and was provided as a raw data to process it and compare it with the measured small loop data. The depths of investigations range from tens to hundreds of meters below the ground surface, depending on Tx loop dimensions. The apparent resistivity data were plotted as a function of time on a log scale. Outliers that deviated severely (a judgment decision) more than 10 percent above or below other data points
Journal Pre-proof were removed or masked before inverse modeling. Inversion is the process of creating an estimate of the true distribution of subsurface resistivity from the measured apparent resistivity obtained from TDEM soundings. Smooth and layered-earth models were generated for each sounding. A smooth model is a vertical delineation of calculated apparent resistivity that represents a non-unique estimate of the true resistivity (Constable, et al., 1987). Ridge regression and Occam’s inversion were used by the inversion software (IX1D, Interpex) in a series of iterations to create a smooth model consisting of 20–30 layers for each sounding site. Layeredearth models were then generated based on results of smooth modeling. The layered-earth
of
models are simplified to fewer layers to represent geologic units with depth. Throughout the
ro
area, the layered-earth models range from 3 to 4 layers, depending on observed changes in the raw data and smooth model inversions.
-p
5. Interpretation and Discussion
re
The results shown in figures (3-7) are for two main NE-SW profiles starting behind Al Khoud dam and extend northward towards the sea. These two profiles (figure 1) were measured
lP
by different TDEM loop sizes to be able to penetrate at different depths of investigation. Generally, there is a sharp transition from unsaturated to saturated conditions highlighted as
na
geoelectric boundary that is marked by a decrease from relatively highly resistive dry sediment to less resistive layer. TDEM models show an upper dry and fresh-water saturated gravels
ur
followed by an intermediate resistivity that may attributed to clayey gravel and a bottom low resistivity layer that is attributed to brackish and saline water intrusion. The thickness of the
Jo
intruded zone increases towards the sea.
The first profile to the West of the dam was measured using three different TDEM loop sizes (20m, 50m and 300m-loop side). Small loop (20m by 20m) survey (figure 3) covered a depth of 140m showing upper high resistivity layer (100-200 Ohm.m) of dry sand and gravel, that extends to about 15-20m followed by fresh-water saturated gravels of resistivity (40-90 Ohm.m) and extends to 30m depth . A deeper layer of resistivity (10-40 Ohm.m) that corresponds to clayey gravels extend to 80-100m depth. A very low resistivity bottom layer (<2 Ohm.m), extend towards the sea at a depth of more than 100m, is detected and could be attributed to the saline interface, since the distance from the last NE station to the sea is about 4km. The small-loop transect showed only the top of the saline zone but the base of the saline
Journal Pre-proof zone is not well defined by this loop dimensions (figure 3). The geoelectric section shows a high resolution of the upper shallow alluvial deposits including the water table. Figure 4 shows the results of co-located TDEM profile with the 20m-loop profile (figure 3) but with larger loop dimensions (50m by 50m) in order to investigate deeper depth. The smooth inversion results revealed the saline intrusion zone more clearly but the base of the saline zone is still not well defined. The section shows lower resolution of the shallow layers compared with the smaller 20m-loop data as the operating base frequency is lower (6.5 Hz) and the transmitter
of
turn-off (ramp) time is longer than the 20m-loop survey that lead to recording the earliest time channel of the secondary magnetic field at deeper depth and misses the voltage response of
ro
shallower layers.
-p
Larger loop dimensions (300m by 300m) provided a good definition of the base of the saline water at a depth of 260m below ground surface. However, the near surface layers are
re
poorly resolved due to: averaging of resistivities over the area of the loop, the low base
lP
frequency used (2.5 Hz) and the long turn-off time (300 s) after which the first time channel is recorded. This prevents accurate estimation of the resistivity of the overlying aquifer or the other
na
resistivity boundaries of the upper layers that only provide qualitative estimate of the variation of water quality compared with small loop data, which distinguish TDEM response from shallower
ur
depths. Below the intrusion zone, an intermediate resistivity layer (20-50 Ohm.m) that could be attributed to cemented gravel composed of low porosity limestone and marl with clay or due to
Jo
water of lower salinity confined by impermeable clay layer (figure 5 and 7). The results of this survey correlates well with induction resistivity logs measured at a well close to the mid of this profile (highlighted by arrow).
The second profile to the East of the dam was measured using two different TDEM loop sizes (20m and 300m-loop side). The small loop (20m by20m) survey (figure 6) show a similar behavior as the corresponding one measured at the Western profile (figure 3) but we can notice that the top of salt water intrusion is detected at shallower depth (60m) at the northern station as the distance from the last station to the shore is only 2.4 km. This also indicates that there is a better underground recharge at the western part of the dam compared with the eastern part. The results from this profile correlate well with water Electrical Conductivity (EC) log measured at a
Journal Pre-proof well close from the northern station (highlighted by arrow). The EC which reflects the total dissolved salt (TDS) within the well shows a very high value (50,000 S/cm starting from 60m depth) that is related to sea water salinity. The large loop (300m by 300m) Eastern survey (figure 7) show a similar behavior as the corresponding one measured at the western profile (figure 5) but we can notice that the depth of the intrusion zone extends to a depth of 240m with a thickness of about 150m at the northern stations. Below the intrusion zone, there is a clear intermediate resistivity layer (30-50 Ohm.m)
of
due to low porosity cemented gravel or brackish water of lower salinity.
ro
Conclusion
-p
TDEM has proven to be a cost-effective method tool in mapping the saline water intrusion and for locating the depth of fresh water aquifer within depths from a few tens to several hundred
re
meters. TDEM offers important advantages over the conventional resistivity depth sounding for mapping the electrically conductive saline zones in the arid environment with improved depth of
lP
penetration, depth resolution and speed of acquisition. The TDEM data collected were considered as good results on the basis of root mean square errors calculated after inversion,
na
comparisons with borehole logging data, and repeatability. The depth and thickness of the saline intrusion zone interpreted from TDEM data and inferred from borehole EC and induction
ur
resistivity logs are well correlated. Models estimated from TDEM data using different
Jo
transmitter loop sizes suggest that there is a definite geoelectrical boundary between saltwatersaturated and freshwater-saturated sediments, but with different resolutions. Large loop cannot resolve details in a freshwater-saturated zone, while small loop could resolve it clearly. On the other hand, Large loop could resolve the limits of saltwater-saturated zone at depth, where smaller loop could not penetrate to that depth. Because the different transmitter loops provide complimentary information about the hydrogeological setup at different depths, further analysis using a joint inversion approach of both data sets may improve the estimated geoelectric models of the subsurface. Monitoring or time-lapse TDEM measurements should be carried out at the same time of the year along the same profiles to monitor the movement of the fresh/saline water interface over time.
Journal Pre-proof Acknowledgements The author would like to thank Sultan Qaboos University for providing the resources to conduct the field survey. The author would like to thank the Ministry of Regional Municipalities and Water Resources for providing the well data. The author also would like to thank the students who helped him during the field work: Raja AlRahbi and Ibrahim AlSawafi References
of
Al-Garni, M. El-Kaliouby, H. 2011. Delineation of saline groundwater and sea water intrusion zones using transient electromagnetic (TEM) method, Wadi Thuwal area, Saudi Arabia. Arabian Journal of Geosciences 4 (3-4), 655-668
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Constable, S.C., Parker, R.L., and Constable, C.G., 1987. Occam’s inversion—A practical algorithm for generating smooth models from EM sounding data: Geophysics, 52, p. 289–300.
re
-p
El-Kaliouby, H. and Abdallah, O. 2015. Application of time-domain electromagnetic method in mapping saltwater intrusion of a coastal alluvial aquifer, North Oman. Journal of applied Geophysics v.115, p 59-64.
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Gibb, A. 1976. Water resources survey of northern Oman. Ministry of Water Resources report. Vols. 1 to 7.
na
Macumber P.G. 2003 Lenses, plumes and wedges in the Sultanate of Oman: a challenge for groundwater management. In: Alsharhan, A.S, Wood, W.W. (eds). Water Resources Perspectives: Evaluation, Management and Policy. Elsevier Science, Amsterdam, the Netherlands: 349–370
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McNeill, J.D. 1990. Use of electromagnetic methods for groundwater studies. In: Ward S.H. (Ed.), Geotechnical and Environmental Geophysics, vol. 1., SEG Tulsa, OK, p. 191–218.
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McNeill, J.D., 1994. Principles and application of time domain electromagnetic techniques for resistivity sounding: Geonics technical note TN-27, Mississauga, Ontario, p.1-15. Nenna, V., Herckenrath, D., Knight, R., Odlum, N., and McPhee, D., 2013. Application and evaluation of electromagnetic methods for imaging saltwater intrusion in coastal aquifers: Seaside Groundwater Basin, California. Geophysics, 78, p. B77–B88. Stanger G., 1986. The hydrogeology of the Oman Mountains. PhD Dissertation, Open University, Milton Keynes, England, 355 pp. Young, M, Macumber, P , Watts, M and Al Touqy, N. 2004. Electromagnetic detection of freshwater lenses in a hyper-arid terrain. Journal of Applied Geophysics, 57, p. 43-61. Young, M, de Beuijn, R, and Al-Ismaily, A. 1998. Exploration of an alluvial aquifer in Oman by time-domain electromagnetic sounding. Hydrogeology Journal, v. 6, p. 383-393.
Journal Pre-proof
Iran Arabian Gulf
Gulf of Oman
U.A.E
Muscat
.
Study Area Sultanate of Oman
Saudi Arabia
Arabian Sea
N
AlKhoud Dam
0
200 km
Salalah
Jo
ur
na
lP
re
-p
ro
of
Figure 1. Location of the study area showing AlKhoud dam and the TDEM profiles.
Figure 2. Field photo showing wadi gravel and silt from a road cut and the lithologic log from a well in the study area. The cross section is (After Abdalla and Al-Rawahi, 2013).
Journal Pre-proof
SW
Alluvial deposits
Water table
NE
Clayey gravels Salt water intrusion
Figure 3. TDEM profile (1.6 km length) using 20mx20m loop showing the water table and the top of the salt-water intrusion zone.
NE
Salt water intrusion
ro
of
SW
lP
re
-p
Figure 4. TDEM profile (1.5 km length) using 50mx50m loop showing part of the salt-water intrusion zone.
Jo
ur
na
Figure 5. TDEM profile (3.35 km length) using 300mx300m loop showing the full salt-water intrusion zone and its correlation with induction resistivity logs.
Figure 6. TDEM profile (2.9 km length) using 20mx20m loop showing the water table and the top of the salt-water intrusion zone and its correlation with EC log.
SW
NE Salt water intrusion Clayey gravels Cemented gravels
Figure 7. TDEM profile (4.15 km length) using 300mx300m loop showing the full salt-water intrusion zone.
Journal Pre-proof
• Increase of salinity in groundwater at coastal areas due to over pumping • Salinity causes potential health problems, and destroy agricultural crops
Jo
ur
na
lP
ro -p
re
• TDEM showed great ability in locating the depth and extent of the intrusion • The depth to the intrusion zone was investigated using different transmitter loop areas • TDEM identified salinity variation with the subsurface in the study area • TDEM delineated the depth of the coastal fresh water alluvial aquifer • Results showed the effect of different transmitter dimensions for the depth of investigation • TDEM revealed the role of recharge dams to counter advancing sea water intrusion
of
• Transient Electromagnetic method is employed to map the saline water intrusion
Journal Pre-proof
Jo
ur
na
lP
re
-p
ro
of
The author (Hesham El-Kaliouby) proposed the idea, implemented the research work and wrote the manuscript.
Journal Pre-proof Declaration of interests
Jo
ur
na
lP
re
-p
ro
of
I have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Hesham El-Kaliouby PII:
S0926-9851(19)30636-6
DOI:
https://doi.org/10.1016/j.jappgeo.2020.103963
Reference:
APPGEO 103963
To appear in:
Journal of Applied Geophysics
Received date:
17 July 2019
Revised date:
19 January 2020
Accepted date:
28 January 2020
Please cite this article as: H. El-Kaliouby, Mapping sea water intrusion in coastal area using time-domain electromagnetic method with different loop dimensions, Journal of Applied Geophysics(2019), https://doi.org/10.1016/j.jappgeo.2020.103963
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
Journal Pre-proof Mapping sea water intrusion in coastal area using time -domain electromagnetic method with different loop dimensions
Hesham El-Kaliouby (1) (1) National Research Center, Geophysics Dept., Egypt Email: [email protected] Abstract Time Domain Electromagnetic (TDEM) method has been widely used in mapping and assessment of groundwater resources and sea water intrusion along coastal alluvial plains. TDEM soundings were used to define aquifer zones in the alluvial aquifer in Al Khoud coastal
of
area in North Oman. The main aquifer zones, composed of clean gravels, clayey gravels, and cemented gravels, could be identified, along with the basal contact of the aquifer. The TDEM
ro
data were also used to map the depth to the interface between freshwater and saline-water along
-p
the coast. The inland extents and thickness of the saline water intrusion zone were mapped along two N-S TDEM profiles. The depth to the intrusion zone was investigated using different
re
transmitter loop areas. The resistivity distribution derived from TDEM data agrees closely with
lP
the electrical conductivity and resistivity logs from available well data in the study area. TDEM proved to be a successful tool for mapping saline zones and fresh water in arid environment with
na
high depth of penetration and speed of operation.
1. Introduction
ur
Keywords: sea water intrusion, groundwater, loop size, Time-domain Electromagnetic, TDEM.
The coastal plain in northern Oman, that extends between Oman mountains and the Gulf of
Jo
Oman, represents the main source of fresh water. The alluvial coastal aquifer provides an important freshwater resource for both domestic and agricultural use. Almost 50% of the agriculture production is coming from the northern coastal plain, which depends on groundwater as the primary source of agriculture water. However, over pumping from the coastal aquifer resulted
in
large
water-level declines
and
associated
environmental impacts,
including
groundwater quality deterioration and sea water intrusion, which represents a growing potential threat to water supplies. The salinity increased to levels that affected the crops and human consumption. The yield of the crops, including the date palms which represent one of the main economic agricultures in Oman, has been affected. Many wells and farms have been abandoned as soil salinity has increased within 6 km of the coastline. This highlights the need for urgent conservation measures to sustain this precious economic wealth (Young, et al. 1998, 2004).
Journal Pre-proof The study area (figure 1) is located at Al Khoud fan, northern coastal area of Oman; represent the lower reaches of Samail Catchment, which extends from the mountains region into the coastal plain. Groundwater recharge in the Al Khoud fan occurs from infiltration of flood runoff originating as direct rainfall in the upper Samail catchment. As the volume of abstracted water has increased in the downstream coastal plain, the water level has declined and as a consequence, sea water has moved landwards. This hazard is highlighted by the shallow depth of the water table, which is less than 1m above mean sea level and often below sea level. For these reasons, there is a great concern and a pressing requirement to assess the available groundwater
of
reserve in the northern coastal plain aquifer (Young, et al. 1998).
ro
It is important to have a fast and reliable tool for regular monitoring of salinity changes along the coastal aquifer in order to get better future prediction and to improve management
-p
plans to limit or prevent saltwater intrusion. This is conventionally achieved through drilling a network of monitoring wells along the coastal plain to monitor changes in water salinity over
re
time. However, installation and operation of these wells is expensive and provides limited spatial
lP
and vertical coverage. An alternative approach to determine the existence or potential of groundwater resources and saline water intrusion is to use noninvasive geophysical methods
na
(Nenna et.al., 2013; El-Kaliouby and Abdallah, 2015). TDEM is a relatively low cost, rapid, and noninvasive reconnaissance tool to locate
ur
potential freshwater resources. TDEM method is well suited to near-surface hydrological studies as it can be used to differentiate between fresh, brackish, and saline water. TDEM method the
electrical
conductivity
Jo
investigates
of
the
subsurface
through
measurements
of
electromagnetic field over time. The characteristics of the TDEM responses of the earth due to a primary signal from a large wire square-loop laid on the ground surface yield information about the variations of the electrical conductivity with depth. The conductivity of soils and rocks are controlled by mineralogy, clay content, water content, salinity, and porosity. Changes in the conductivity of soils and rocks produce variations in the electromagnetic signature. The exact position of the saline water/ fresh-water interface in most coastal areas is poorly constrained on a local level. A combination of existing monitoring well water quality measurements borehole data, and surface TDEM can be used to accurately map the position of the interface and intrusion zone (e.g. Al-Garni and El-Kaliouby, 2011; El-Kaliouby and Abdallah, 2015).
Journal Pre-proof 2. Field Site Characteristics Major groundwater flows in Northern Oman are from the mountains towards the sea. The coastal alluvial deposits in wadi beds form excellent aquifers and contain renewable resources of good quality groundwater by regular recharge from rainfall, surface water flow and locally the inflow of groundwater from adjacent hard rock aquifers. Recharge from local rainfall appears to constitute an important component in the areas, where coarse gravel deposits provide favorable infiltration conditions. Many streams originating in the mountains may flow for an extended period in downstream to recharge coastal aquifer (Stanger, 1986; Young, et al. 1998, Macumber,
of
2003).
ro
The study area is covered by Quaternary unconsolidated to semi-consolidated sands and gravels with interbedded clay lenses or layers, which rest on clayey conglomerates or directly
-p
upon fissured Ophiolites. The Quaternary cover has generally a wedge shape, thickening from a few meters close to the mountains to more than 100 m along the coast. A thick sequence of marls
re
and clays underlies the coarser Quaternary sediments in some areas near the coast; the thickness
lP
of this sequence may reaches up to 600 m (Gibb, 1976; Stanger, 1986). Three major units dominate the lithological succession of the alluvium: upper, middle and lower gravel units. The upper gravel unit is composed predominantly of large-size gravels and sand including boulders
na
(figure 2). This unit is loose and very poorly sorted of permeable alluvium with good water quality that makes it the main aquifer zone. The middle unit is discontinuous clayey gravel,
ur
which is found in the form of lenses and inter-bedded claystone bands between the upper and
Jo
lower gravel units. The lower unit is cemented gravely marl and clays, which is more compacted and conglomeratic, due to the cementation caused by diagenesis associated with a fluctuating water table. (Gibb, 1976; Stanger, 1986; Young, et al. 1998).
3. TDEM Method TDEM made by transmitting input current into square loops of insulated wire deployed on the land surface. The transmitter loop current consists of equal periods of time-on and time-off, which produce an electromagnetic field near the loop of wire. Termination of the current flow is not instantaneous but occurs over a few to hundreds of microseconds, known as the ramp time (or turn-off time), during which the magnetic field is time-variant. The time-variant nature of the induced electromagnetic field creates a primary electromagnetic field in the ground underneath
Journal Pre-proof the loop in accordance with Faraday’s Law. This resultant field instantly begins to decay, generating additional eddy currents that spread downward and outward into the subsurface like a succession of smoke rings. These eddy currents then produce a secondary magnetic field that is recorded as voltage measurements throughout the time-off period by a receiver positioned in the center or at an off-set distance from the transmitter loop. The voltage magnitude received from the eddy currents at specific times and depths is determined by the overall conductivity of subsurface rock units and fluids. From these voltage measurements, apparent resistivity values can be calculated. Depth of exploration depends on the transmitter's magnetic moment (loop area
of
times the injected current intensity). Large loop dimensions allow current to penetrate deeper and
ro
covers a larger volume of the subsurface materials compared with smaller loops. Late time voltage, is measuring eddy currents from increasingly greater depths, while early time voltage
re
4. Data Acquisition And Processing
-p
corresponds to shallow near surface layers (McNeill, 1990, 1994).
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A Geonics Protem-47 system using a square transmitter (Tx) loop was used to collect the TDEM soundings. A 1-meter-diameter multi-turn coil receiver (Rx) with a 31.4-square-meter area measured the secondary magnetic fields with central loop and an off-set loop of 25m from
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the center of the transmitter loop. The Protem-47 transmitter used an injection current of 3.5A with preset base frequencies of 25Hz for shallow soundings and 6.5Hz for intermediate sounding
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and 2.5Hz for deeper soundings. The Protem TDEM unit samples signal amplitudes in 20
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subintervals (time gates) during each measurement and collect three voltage data sets. Data collected with TDEM method, provided significant contributions to image the extent of saltwater intrusion at many sites in relatively short time. The depths and inland extent of the sea water intrusion was mapped through two main profiles perpendicular to the coastline using different loop sizes. The two profiles extend from Al Khoud dam northward towards the sea, (figure 1). The large loop data was measured by the Ministry of Water Resources of Oman and was provided as a raw data to process it and compare it with the measured small loop data. The depths of investigations range from tens to hundreds of meters below the ground surface, depending on Tx loop dimensions. The apparent resistivity data were plotted as a function of time on a log scale. Outliers that deviated severely (a judgment decision) more than 10 percent above or below other data points
Journal Pre-proof were removed or masked before inverse modeling. Inversion is the process of creating an estimate of the true distribution of subsurface resistivity from the measured apparent resistivity obtained from TDEM soundings. Smooth and layered-earth models were generated for each sounding. A smooth model is a vertical delineation of calculated apparent resistivity that represents a non-unique estimate of the true resistivity (Constable, et al., 1987). Ridge regression and Occam’s inversion were used by the inversion software (IX1D, Interpex) in a series of iterations to create a smooth model consisting of 20–30 layers for each sounding site. Layeredearth models were then generated based on results of smooth modeling. The layered-earth
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models are simplified to fewer layers to represent geologic units with depth. Throughout the
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area, the layered-earth models range from 3 to 4 layers, depending on observed changes in the raw data and smooth model inversions.
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5. Interpretation and Discussion
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The results shown in figures (3-7) are for two main NE-SW profiles starting behind Al Khoud dam and extend northward towards the sea. These two profiles (figure 1) were measured
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by different TDEM loop sizes to be able to penetrate at different depths of investigation. Generally, there is a sharp transition from unsaturated to saturated conditions highlighted as
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geoelectric boundary that is marked by a decrease from relatively highly resistive dry sediment to less resistive layer. TDEM models show an upper dry and fresh-water saturated gravels
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followed by an intermediate resistivity that may attributed to clayey gravel and a bottom low resistivity layer that is attributed to brackish and saline water intrusion. The thickness of the
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intruded zone increases towards the sea.
The first profile to the West of the dam was measured using three different TDEM loop sizes (20m, 50m and 300m-loop side). Small loop (20m by 20m) survey (figure 3) covered a depth of 140m showing upper high resistivity layer (100-200 Ohm.m) of dry sand and gravel, that extends to about 15-20m followed by fresh-water saturated gravels of resistivity (40-90 Ohm.m) and extends to 30m depth . A deeper layer of resistivity (10-40 Ohm.m) that corresponds to clayey gravels extend to 80-100m depth. A very low resistivity bottom layer (<2 Ohm.m), extend towards the sea at a depth of more than 100m, is detected and could be attributed to the saline interface, since the distance from the last NE station to the sea is about 4km. The small-loop transect showed only the top of the saline zone but the base of the saline
Journal Pre-proof zone is not well defined by this loop dimensions (figure 3). The geoelectric section shows a high resolution of the upper shallow alluvial deposits including the water table. Figure 4 shows the results of co-located TDEM profile with the 20m-loop profile (figure 3) but with larger loop dimensions (50m by 50m) in order to investigate deeper depth. The smooth inversion results revealed the saline intrusion zone more clearly but the base of the saline zone is still not well defined. The section shows lower resolution of the shallow layers compared with the smaller 20m-loop data as the operating base frequency is lower (6.5 Hz) and the transmitter
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turn-off (ramp) time is longer than the 20m-loop survey that lead to recording the earliest time channel of the secondary magnetic field at deeper depth and misses the voltage response of
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shallower layers.
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Larger loop dimensions (300m by 300m) provided a good definition of the base of the saline water at a depth of 260m below ground surface. However, the near surface layers are
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poorly resolved due to: averaging of resistivities over the area of the loop, the low base
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frequency used (2.5 Hz) and the long turn-off time (300 s) after which the first time channel is recorded. This prevents accurate estimation of the resistivity of the overlying aquifer or the other
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resistivity boundaries of the upper layers that only provide qualitative estimate of the variation of water quality compared with small loop data, which distinguish TDEM response from shallower
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depths. Below the intrusion zone, an intermediate resistivity layer (20-50 Ohm.m) that could be attributed to cemented gravel composed of low porosity limestone and marl with clay or due to
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water of lower salinity confined by impermeable clay layer (figure 5 and 7). The results of this survey correlates well with induction resistivity logs measured at a well close to the mid of this profile (highlighted by arrow).
The second profile to the East of the dam was measured using two different TDEM loop sizes (20m and 300m-loop side). The small loop (20m by20m) survey (figure 6) show a similar behavior as the corresponding one measured at the Western profile (figure 3) but we can notice that the top of salt water intrusion is detected at shallower depth (60m) at the northern station as the distance from the last station to the shore is only 2.4 km. This also indicates that there is a better underground recharge at the western part of the dam compared with the eastern part. The results from this profile correlate well with water Electrical Conductivity (EC) log measured at a
Journal Pre-proof well close from the northern station (highlighted by arrow). The EC which reflects the total dissolved salt (TDS) within the well shows a very high value (50,000 S/cm starting from 60m depth) that is related to sea water salinity. The large loop (300m by 300m) Eastern survey (figure 7) show a similar behavior as the corresponding one measured at the western profile (figure 5) but we can notice that the depth of the intrusion zone extends to a depth of 240m with a thickness of about 150m at the northern stations. Below the intrusion zone, there is a clear intermediate resistivity layer (30-50 Ohm.m)
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due to low porosity cemented gravel or brackish water of lower salinity.
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Conclusion
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TDEM has proven to be a cost-effective method tool in mapping the saline water intrusion and for locating the depth of fresh water aquifer within depths from a few tens to several hundred
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meters. TDEM offers important advantages over the conventional resistivity depth sounding for mapping the electrically conductive saline zones in the arid environment with improved depth of
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penetration, depth resolution and speed of acquisition. The TDEM data collected were considered as good results on the basis of root mean square errors calculated after inversion,
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comparisons with borehole logging data, and repeatability. The depth and thickness of the saline intrusion zone interpreted from TDEM data and inferred from borehole EC and induction
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resistivity logs are well correlated. Models estimated from TDEM data using different
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transmitter loop sizes suggest that there is a definite geoelectrical boundary between saltwatersaturated and freshwater-saturated sediments, but with different resolutions. Large loop cannot resolve details in a freshwater-saturated zone, while small loop could resolve it clearly. On the other hand, Large loop could resolve the limits of saltwater-saturated zone at depth, where smaller loop could not penetrate to that depth. Because the different transmitter loops provide complimentary information about the hydrogeological setup at different depths, further analysis using a joint inversion approach of both data sets may improve the estimated geoelectric models of the subsurface. Monitoring or time-lapse TDEM measurements should be carried out at the same time of the year along the same profiles to monitor the movement of the fresh/saline water interface over time.
Journal Pre-proof Acknowledgements The author would like to thank Sultan Qaboos University for providing the resources to conduct the field survey. The author would like to thank the Ministry of Regional Municipalities and Water Resources for providing the well data. The author also would like to thank the students who helped him during the field work: Raja AlRahbi and Ibrahim AlSawafi References
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Al-Garni, M. El-Kaliouby, H. 2011. Delineation of saline groundwater and sea water intrusion zones using transient electromagnetic (TEM) method, Wadi Thuwal area, Saudi Arabia. Arabian Journal of Geosciences 4 (3-4), 655-668
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Constable, S.C., Parker, R.L., and Constable, C.G., 1987. Occam’s inversion—A practical algorithm for generating smooth models from EM sounding data: Geophysics, 52, p. 289–300.
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El-Kaliouby, H. and Abdallah, O. 2015. Application of time-domain electromagnetic method in mapping saltwater intrusion of a coastal alluvial aquifer, North Oman. Journal of applied Geophysics v.115, p 59-64.
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Gibb, A. 1976. Water resources survey of northern Oman. Ministry of Water Resources report. Vols. 1 to 7.
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Macumber P.G. 2003 Lenses, plumes and wedges in the Sultanate of Oman: a challenge for groundwater management. In: Alsharhan, A.S, Wood, W.W. (eds). Water Resources Perspectives: Evaluation, Management and Policy. Elsevier Science, Amsterdam, the Netherlands: 349–370
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McNeill, J.D. 1990. Use of electromagnetic methods for groundwater studies. In: Ward S.H. (Ed.), Geotechnical and Environmental Geophysics, vol. 1., SEG Tulsa, OK, p. 191–218.
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McNeill, J.D., 1994. Principles and application of time domain electromagnetic techniques for resistivity sounding: Geonics technical note TN-27, Mississauga, Ontario, p.1-15. Nenna, V., Herckenrath, D., Knight, R., Odlum, N., and McPhee, D., 2013. Application and evaluation of electromagnetic methods for imaging saltwater intrusion in coastal aquifers: Seaside Groundwater Basin, California. Geophysics, 78, p. B77–B88. Stanger G., 1986. The hydrogeology of the Oman Mountains. PhD Dissertation, Open University, Milton Keynes, England, 355 pp. Young, M, Macumber, P , Watts, M and Al Touqy, N. 2004. Electromagnetic detection of freshwater lenses in a hyper-arid terrain. Journal of Applied Geophysics, 57, p. 43-61. Young, M, de Beuijn, R, and Al-Ismaily, A. 1998. Exploration of an alluvial aquifer in Oman by time-domain electromagnetic sounding. Hydrogeology Journal, v. 6, p. 383-393.
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Iran Arabian Gulf
Gulf of Oman
U.A.E
Muscat
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Study Area Sultanate of Oman
Saudi Arabia
Arabian Sea
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AlKhoud Dam
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200 km
Salalah
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Figure 1. Location of the study area showing AlKhoud dam and the TDEM profiles.
Figure 2. Field photo showing wadi gravel and silt from a road cut and the lithologic log from a well in the study area. The cross section is (After Abdalla and Al-Rawahi, 2013).
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Alluvial deposits
Water table
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Clayey gravels Salt water intrusion
Figure 3. TDEM profile (1.6 km length) using 20mx20m loop showing the water table and the top of the salt-water intrusion zone.
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Salt water intrusion
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Figure 4. TDEM profile (1.5 km length) using 50mx50m loop showing part of the salt-water intrusion zone.
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Figure 5. TDEM profile (3.35 km length) using 300mx300m loop showing the full salt-water intrusion zone and its correlation with induction resistivity logs.
Figure 6. TDEM profile (2.9 km length) using 20mx20m loop showing the water table and the top of the salt-water intrusion zone and its correlation with EC log.
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NE Salt water intrusion Clayey gravels Cemented gravels
Figure 7. TDEM profile (4.15 km length) using 300mx300m loop showing the full salt-water intrusion zone.
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• Increase of salinity in groundwater at coastal areas due to over pumping • Salinity causes potential health problems, and destroy agricultural crops
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• TDEM showed great ability in locating the depth and extent of the intrusion • The depth to the intrusion zone was investigated using different transmitter loop areas • TDEM identified salinity variation with the subsurface in the study area • TDEM delineated the depth of the coastal fresh water alluvial aquifer • Results showed the effect of different transmitter dimensions for the depth of investigation • TDEM revealed the role of recharge dams to counter advancing sea water intrusion
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• Transient Electromagnetic method is employed to map the saline water intrusion
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The author (Hesham El-Kaliouby) proposed the idea, implemented the research work and wrote the manuscript.
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I have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.