Near-Surface Electromagnetic Surveying

Near-Surface Electromagnetic Surveying
The E&P industry typically focuses on deep formations, but frequently the nearsurface layers also need to be evaluated. Land-based electromagnetic surveys
provide insights into this often complex zone. The interpreted resistivities of these
layers help map and define features for applications as diverse as seismic studies
and aquifer delineation.
Mohamed Dawoud
Environment Agency–Abu Dhabi
Abu Dhabi, UAE
Stephen Hallinan
Milan, Italy
Rolf Herrmann
Abu Dhabi
Frank van Kleef
Dubai Petroleum Establishment
Dubai, UAE
Oilfield Review Spring 2009: 21, no. 1.
Copyright © 2009 Schlumberger.
For help in preparation of this article, thanks to Marcus
Ganz, Houston.
1. The principle describing the changing magnetic field
is Faraday’s law of induction. Including the sign of the
induced current is Lenz’s law. The converse principle
involving a changing current or electric field is Ampère’s
law. These laws are included in Maxwell’s equations.
The near surface of the Earth is a complex place,
the result of dynamic action by wind, water and
other forces of nature. In those first tens of
meters below the surface, the jumbled detritus of
weathering is gradually buried. The near-surface
layers, like those below, vary in resistivity according to their mineral and fluid compositions. This
property allows their investigation using electromagnetic (EM) surveys.
Often, surveys are performed using an artificial source of EM radiation, rather than the
magnetotelluric (MT) radiation resulting from
the interaction of the solar wind with the Earth’s
magnetosphere. On land, there are two general
methods of controlled-source electromagnetic
(CSEM) measurement for generating the signal
and detecting the response. The grounded-source
method requires burial of source and receiver
electrodes in electrical contact with the earth.
The inductive-source method uses a current loop
on the surface to induce a variable magnetic
field, and the same or another loop to detect the
response signal.
The grounded-source method is efficient and
sensitive to horizontal resistive targets because
the electric field has a vertical component.
The grounded receivers measure the response
electric field; response magnetic fields are also
measured to provide control during modeling.
On land, however, the surface conditions must
be suitable for creating and maintaining electrical contact. This prerequisite excludes the
practical application of this method in large, arid
dunes—sand grains are nonconducting. But in
some areas, contact can be improved by drilling
patterns of shallow holes for source and receiver
electrodes and wetting the soil as the hole is
refilled. Deeper investigation into the Earth
requires stronger current sources, among other
factors, and the high contact resistances on land
mean this requires high-voltage systems to drive
that current.
The inductive-source method does not require
electrical contact, since the current loop generates a magnetic field through a time-varying
signal. This field generates a response electric
field, but because the electric field is largely
horizontal, the process is not as efficient for
imaging horizontal layers of resistive hydrocarbons as the direct injection of current using the
grounded-source method. Again, both the electric
and magnetic response fields can be measured
using the inductive-source technique. Coils
for the current loops are square and for nearsurface investigation range from about 10 to 300 m
[30 to 1,000 ft] on a side. Far larger loops have been
used for deeper, but low-resolution, investigation.
A companion article (see “Electromagnetic
Sounding for Hydrocarbons,” page 4) describes
the basic physics of the EM interaction with the
Earth and discusses marine EM studies. It also
covers MT in detail, because the objectives of
those studies are similar for land and marine environments. This article focuses on investigations
Oilfield Review
using the inductive-loop method for nearsurface imaging, illustrated by two WesternGeco
cases from the United Arab Emirates. One study
mapped an aquifer in Abu Dhabi for a waterstorage project. The second determined nearsurface resistivity variations in Dubai sand dunes,
providing valuable input for making static corrections in a seismic survey of the area.
Stacking Time Sequences
Maxwell’s equations describe the basic physics
behind the interplay of electric and magnetic
fields in a time-varying current loop. A current
loop generates a magnetic field. If the current
changes, the induced field also changes. The
opposite is also true: Changing the flux of a magnetic field within a conducting loop induces a
changing current.1 A simple way to generate such
a current is to move a magnet toward or away
from a wire loop. The movement changes the flux
through the loop, inducing a current. This current induces a response magnetic field oriented
to oppose the change in flux through the loop
caused by the moving magnet.
No actual magnet is required for this effect
to take place. One coil with an imposed timevarying current sets up a time-varying magnetic
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> Basic induction loops. An alternating current
passing through a set of coils (blue) induces a
cyclic magnetic field. When this field passes
through a second set of coils (red), it induces
a cyclic current in that circuit. Thus, energy
passes from one circuit to another without a
direct electrical contact. This is the basis for a
transformer, which is a device that converts from
an input voltage to a different output voltage by
having different numbers of loops in the two coils.
Ramp time
Batteries and
signal electronics
Transmitter current and
primary magnetic field
Ramp time
Time on Time off Time on
field in response. Current is induced in a second
coil positioned close enough to experience the
changing flux. This is the configuration of a transformer (left). Energy passes from one circuit to
the other through the changing magnetic field.
One method that uses inductive measurement for evaluating the near surface is a
time-domain electromagnetic (TDEM) survey.
A conducting loop of wire set out in a square
on the surface of the Earth acts as the first
coil, and the second loop forms in conducting formations of the Earth itself. The primary
magnetic field from the transmitter loop generates horizontal currents, called eddy currents,
immediately beneath the loop. These currents
induce a response field that can be detected at
a surface receiver loop, but this field also travels
farther into the subsurface, generating progressively weaker eddy current loops with larger
radii and smaller response fields (below).2
Source current loop
Induced electromotive force
in nearby conducting layers
Eddy currents
induced by
field change
Response magnetic field
induced by eddy currents
Receiver-coil voltage from
response magnetic field
magnetic field
Eddy currents
at later times
> TDEM inductive method. A large square loop is placed on the surface at the sounding site (top right).
Passing a current pulse through this loop generates the primary magnetic field. The field induces
a secondary, eddy current loop in the ground, as described by Faraday’s law (middle right ). This
secondary current induces a response magnetic field, which can be recorded by a receiver loop on
the surface. In this case, the same loop is used as source and receiver. The primary field decays with
depth into the ground, generating response fields from each subsequent depth (bottom right ). The
timing of the signals and response fields is also shown (left).
The transmitter and receiver loops can
be the same wire coil when an appropriate
time sequence of current steps is applied, or
coaxial but separate coils in a typical setup.
In all inductive-loop TDEM surveys, the time
sequence begins by turning the current on to a
constant DC value. Sufficient time elapses for
transient responses in the subsurface to decay.
Then, electronics shut the current off in a rapid,
controlled ramp, inducing a known electromotive force in the immediate subsurface. The
transient electromotive force generates eddy
currents, producing a secondary magnetic field
that decays with time. The secondary field is
detected by the receiver coil. After enough time
has elapsed, the sequence repeats with opposite polarity. Stacking many repeated responses
improves the signal/noise ratio.
As the eddy currents move progressively
deeper into the subsurface, the response field
contains resistivity information about deeper
layers. Fundamentally, the resistivity variation
with depth determines the rate of decay of the
transient; higher conductivity results in slower
decay. Inversion of the stacked data from a
TDEM sounding reveals the distribution of nearsurface resistivity.
The measured signals are very small, so land
surveys must consider and avoid, if possible, any
sources of noise. Electric trains, power lines,
electric fences, buried utility cables, pipelines
and water pumps distort the local measurement;
large temperature variations and wind impact
stability; and variations in soil moisture and permeability affect uniformity.3
TDEM is not commonly used directly in oil
and gas exploration, although it has utility in
evaluating surface statics for seismic studies.
The method is applied widely for exploration
within the mining industry, employing both terrestrial and airborne sources. It is also a tool for
environmental and water-resource management,
as shown in the first of the following case studies
from the Middle East.
Sounding for Water Storage
A recent land-based EM application helped
to locate potential water-storage sites in the
UAE.4 The Environment Agency–Abu Dhabi
(EAD) is managing a study for the government
to evaluate storage plans for 30 billion British
imperial gallons [136 million m3, 36 billion
galUS] of fresh water in the northeast area of
the Emirate.5 The country wants a freshwater
reserve for emer­gency periods and to meet peak
Oilfield Review
summer demands. The water for this aquifer
storage and recovery (ASR) project will be transported via pipeline from a water-desalination
plant in the Emirate of Fujairah.
EAD retained Schlumberger to identify and
test a potential ASR site. This involved defining
the subsurface storage zone and surrounding formations, aquifer thickness and related hydraulic
parameters. Schlumberger selected a preferred
site and constructed three pilot wells, which
were tested to determine the aquifer’s potential.
Geologic studies of the area found that
deep-seated faults had been reactivated in the
Late Tertiary Period by the northeastward displacement of the Arabian Peninsula, resulting
in a series of tightly folded faults. The overlying
layer of Quaternary Period sediments, consisting of eolian sands and alluvium, were primarily
deposited along the reactivated faults and in the
synclines between them, giving the sediment
thickness a directional bias. The directionality
can influence groundwater flow, creating a preference for flow parallel to the structure.
Schlumberger evaluated this geologic structure in 2006 during the drilling, logging and
testing periods. Logs from the pilot wells indicated a resistivity contrast between the targeted
sand-and-gravel aquifer and the underlying clayrich layer. Because TDEM data are useful for
aquifer characterization, the evaluation over the
ASR site included a survey to define the lateral
extent of the aquifer—necessary to compute the
potential water-storage volume. In a time-domain
survey, the apparent resistivity of the underlying
formations is determined from the time variation of the electric and magnetic response fields.
For the ASR study, the same set of coils placed
on the surface was used for both current and
receiver loops. The survey covered a 6- by 7-km
[3.7- by 4.4-mi] area.
A 1D Occam inversion at each receiver yielded
resistivity input for constructing a 3D model.6 The
maximum depth obtained by the inversion was
about 250 m [820 ft]. Resistivity logs from the
three wells were compared with the inversion
results at adjacent sounding locations (right).
The top of the underlying clay unit is clear in the
eastern part of the survey area but less obvious in
the western part.
2. Nabighian MS: “Quasi-Static Transient Response of a
Conducting Half-Space—An Approximate Representation,”
Geophysics 44, no. 10 (October 1979): 1700–1705.
3. Constable SC, Orange AS, Hoversten GM and Morrison HF:
“Marine Magnetotellurics for Petroleum Exploration, Part I:
A Sea-Floor Equipment System,” Geophysics 63, no. 3
(May–June 1998): 816–825.
Spring 2009
Sounding S049
Depth, m
TDEM Resistivity
ohm.m 100
Well SWS17
Log Resistivity
ohm.m 100
Sounding S071
Well SWS15
Log Resistivity
TDEM Resistivity
ohm.m 100
ohm.m 100 1
Sounding S013
Well SWS16
TDEM Resistivity
Log Resistivity
ohm.m 100 1
ohm.m 100 Depth, m
Water table
Water table
Water table
Bottom of aquifer
Bottom of aquifer
Bottom of aquifer
> Comparison of TDEM soundings with resistivity profiles from logged wells. The TDEM resistivity
measurements from soundings correlate closely with the resistivity logs from adjacent wells. The
soundings at S049 and S013 show reasonable correlation for the contact between the aquifer and
the clay-rich unit below (violet), which is true for most other soundings in the eastern part of the
investigated area. The contrast is not as clear at S071, where the more resistive lower unit does not
provide a sufficient contrast for the TDEM measurement. This trend follows for most of the soundings
in the western part of the survey. The top of the water table (blue dashed line) was determined from a
map of the water depth by interpolating between wells in the area.
4. For more on water storage: Black B, Dawoud M,
Herrmann R, Largeau D, Maliva R and Will B: “Managing
a Precious Resource,” Oilfield Review 20, no. 2
(Summer 2008): 18–33.
5. A British imperial gallon is equivalent to 1 galUK.
6. An Occam inversion is a smooth inversion that does not
predefine the number of layers.
2 km
80 m
2 km
> Resistivity discontinuity in the bottom of an aquifer. The discontinuity is a band with 15- to 20-ohm.m
resistivity (yellow and green), which contrasts with the 1- to 10-ohm.m resistivity (blue and violet)
elsewhere in the resistivity cube. The resistivity of the clay at the bottom of the aquifer (violet) to the
east of the discontinuity is lower than that to the west. This discontinuity roughly aligns with a thrust
fault (tan) that was identified at about 3,000 m [9,800 ft] by a seismic interpretation. Outcrop studies
performed to the south of the survey area (not shown) support the surface expression of the fault
being slightly west of the deeper seismic interpretation, and those observations are consistent with
the location of the near-surface resistivity discontinuity. The syncline axis (blue) from the seismic
interpretation, another thrust fault (purple) and some wells are also shown. (Seismic lines adapted
from Woodward and Al-Jeelani, reference 7.)
The TDEM data clearly show a discontinuity in the resistivity distribution in the clay unit
(left). There is also a difference in resistivity
between the eastern and western compartments;
the western part exhibits significantly greater
resistivity at a given depth. The anomaly aligns
with a seismically mapped thrust fault.7 The
seismic interpretation was based on a survey
acquired in the early 1980s and reprocessed in
1992 to highlight the shallow structures.
The shallow units above the anomaly are
expected to show some structural complexity; they
exhibit rapid variations in saturated thickness and
possibly no saturated thickness in some areas. On
the eastern side of the discontinuity, the horizontal extent of saturated thickness is suitable for an
ASR unit. The western side of the discontinuity
shows some potential, but has a larger risk: The
interpretation in that area has a greater uncertainty because of the poor resistivity contrast
between the saturated sands and the underlying clay. The discontinuity in the clay formation
should not be seen as a complete hydraulic barrier
in the shallower aquifer layer. Paleochannels or
tear faults—those striking perpendicular to the
overthrust fault—are expected to provide preferential flow paths from east to west across the line
of the discontinuity.
This TDEM study suggests that around 4 billion imperial gallons [18 million m3, 4.8 billion
galUS] of water can be stored at this site, giving
it the capacity for daily production of more than
20 million imperial gallons [91,000 m3, 24 million
galUS] for 200 continuous days.
Mapping the Dunes
Within the same regional setting as the waterstorage site, a 2D seismic survey was conducted
for Dubai Petroleum Establishment (DPE). The
same clay layer forming the base of the storagesite aquifer is present in this area; it provides
a marker for the base of the weathered surface
layer. The depth of the clay varies across the survey area, and lines of dunes add local variation
7. Woodward DG and Al-Jeelani AH: “Application of
Reprocessed Seismic Sections from Petroleum Exploration
Surveys for Groundwater Studies, Eastern Abu Dhabi,
UAE,” paper SPE 25538, presented at the SPE Middle East
Oil Show, Bahrain, April 3–6, 1993.
8. An uphole is a shallow well used to determine nearsurface velocities for a seismic survey.
9. Colombo D, Cogan M, Hallinan S, Mantovani M,
Vergilio M and Soyer W: “Near-Surface P-Velocity
Modelling by Integrated Seismic, EM, and Gravity Data:
Examples from the Middle East,” First Break 26
(October 2008): 91–102.
> Desert terrain at the ASR site. The blue cable is part of a TDEM sounding
loop. The building houses a submersible pump and water tank for the nearby
wells (blue caps) of the ASR project.
Oilfield Review
Elevation, m
Distance, km
> Soundings along a seismic line. Interpolation between sounding points yields a detailed 2D model of
resistivity along a seismic line. The sharply layered model at each sounding point (filled black square)
is shown as a narrow column (top). Uphole sites (UH-09, -06, -08) contain domains of constant velocity
in the weathered layer (yellow stippling) of about 1,400 m/s [4,600 ft/s] and in the underlying clay and
limestone (gray stippling) of greater than 2,000 m/s [6,560 ft/s]. The variation in properties both within
the dunes and in the lower layer is evident along the entire seismic line (bottom). The higher resistivity
of the bottom layer (south end of the seismic line) indicates a relatively clay-poor region.
The survey comprised 505 sounding sites
to the depth of the weathered layer. Sand dunes
generally exhibit low seismic velocity, and defin- using square loops of 50 m [164 ft] on a side,
ing the velocity variation and thickness of the except for a few sites that used 75-m [246-ft]
surface layer is crucial for deriving a long-wave- square loops for deeper penetration.9 Spacing
length static correction for the seismic data.
between sounding points was generally about
The seismic crew drilled several upholes to 1,000 m [3,280 ft]; GPS was used to position the
log the velocity of the surface and underlying sites. The effective time for decay ranged from
clay layer.8 The upholes typically were at seismic- 0.01 to 10 ms; the pulse repetition rate was 6.3 Hz.
Given the subhorizontal nature of the zone
line intersections and, for practical reasons,
away from the higher dune crests. However, this of investigation and its shallow depth compared
pattern often does not sample the near-surface with the TDEM station spacing, 1D resistivity
variations found in sand dune areas, so finer sam- inversion modeling was selected for the analyTwo 1D resistivity inversion methods were
pling was desired. DPE elected toLAND
resistivity survey to map the area because it applied. The first one incorporated about 15 laywould be more cost-effective than drilling more ers extending to a depth of about 200 m [650 ft].
upholes and would avoid additional drilling on Layer thickness increased logarithmically with
depth. Resistivity was a free parameter, and
the environmentally sensitive dunes.
this inversion yielded a detailed, smooth variation of resistivity.
Spring 2009
The detailed fit provided a starting point for
the second inversion. Termed a layered fit, it used
the minimum number of layers required to fit the
data to less than 5% root-mean-square misfit. This
was typically two to five layers. Analysts selected
the starting definition of these layers from the
detailed fit. The layered model generated stronger resistivity contrasts than the detailed one.
Interpreters created a 2D model with grid
blocks 200 m wide and 5 m [16 ft] deep along
a seismic line. They used the sounding sites
along the seismic line to evaluate the surface
layering. Model resistivity values were obtained
by interpolating between the 1D smooth inversions at those sounding sites (left). The result
is a detailed description of the location of the
bounding clay layer at the bottom of the lowvelocity zone. The resistivity data were not
calibrated to seismic velocities.
The seismic processing team used these maps
during the estimation of the surface static corrections. The velocities for the surface zone were
interpolated from velocity measurements taken
at the upholes. The TDEM approach gave the
seismic interpreters a geologically consistent
way to remove the effects of the laterally varying sand velocities. The resistivity analysis also
highlighted variations within and below the lowvelocity weathered layer.
Sounding Deeper
Both case studies used TDEM methods to examine near-surface features. However, because
using inductive-source techniques is inefficient
for defining deep targets, the method is not the
exploration tool of choice for examining deeper
structures. The industry is improving techniques
for using the alternative, grounded-source method
to inject current into the earth.
The source for the grounded method must
be able to inject a large current at a voltage
that is sufficient to overcome contact resistance
in areas where the soil is dry. This combination
has been difficult to achieve. The method using
direct injection of current is more sensitive to
resistive targets, making it a more likely method
than the inductive-loop option to provide a direct
hydrocarbon indication. —MAA