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Non-Seismic Methods
Multi-transient EM technology in practice
Chris Anderson,1 Andrew Long,2 Anton Ziolkowski,3 Bruce Hobbs,3 and David Wright3 explain
the principles of multi-transient EM technology and provide some recent survey results from the
North Sea. The authors claim significant advantages for their method over the marine controlled source EM operations which currently dominate this emerging market.
E
lectromagnetic (EM) radiation is a self-propagating
wave in space with electric and magnetic components.
These components oscillate at right angles to each
other and to the direction of propagation, and are 90°
out of phase with each other. Electromagnetic radiation is
classified into types according to the frequency of the wave:
these types include, in order of increasing frequency, radio
waves, microwaves, terahertz radiation, infrared radiation,
visible light, ultraviolet radiation, X-rays, and gamma rays.
EM as used in geophysical exploration operates at very low
frequencies from 0.1 Hz up to several tens of kHz, corresponding to the very far left of Figure 1.
In its simplest description, the geophysical study of EM
behaviour in the shallow earth (upper few km) can identify the spatial location of resistive zones. In a sedimentary basin, because hydrocarbon-bearing rocks are known to
show increased resistivity relative to water-bearing rocks, the
zones that appear highly resistive may indicate the presence of
hydrocarbons.
EM methods are members of a family of methods commonly known as the geoelectrical methods. These include
direct current (DC) resistivity, induced polarization (IP), and
magnetotelluric (MT) methods, all of which now represent
precursors to the multi-transient EM methodology operated by
PGS. In the DC method the electrical resistivity of subsurface
materials in the earth is measured by causing an electric current
to flow in the earth between one pair of electrodes (a ‘bipole’)
while the steady-state voltage across a second pair of electrodes
is measured. The measured voltage is converted to an apparent resistivity which is a value representing the weighted average resistivity over the volume of the earth between the source
and receiver. The IP method extends the DC resistivity method by making an additional measurement of the ability of the
ground to store electrical charge. IP instruments measure both
the conductive and capacitive properties of the subsurface, and
operate in the frequency range of 0.01-10 Hz, while the frequency range of active EM measurements on land is approximately 10 Hz-30 kHz. MT is a passive EM technique that uses
naturally occurring currents in the ionosphere as the source
field to probe the earth. The MT method is by nature confined
to much lower frequencies than controlled source EM, and is
thus traditionally a low-resolution tool.
Role of resistivity data for hydrocarbon exploration
As schematically illustrated in Figure 2, high-quality seismic
data may discriminate lithology or in the best case, fluids
(typically gas rather than oil). Wireline electric logging on
a seismic survey scale is impossible, but multi-transient EM
surveying can be viewed as ‘logging from the surface’, providing spatial representations of the apparent resistivity in
the earth. With appropriate constraints, these data can be
spatially correlated with the structural information provided
by seismic data, thus allowing a direct pre-drilling discrimination of whether a prospect contains hydrocarbons or not.
Figure 1 EM spectrum. Wavelengths in metres (m).
1
Petroleum Geo-Services, London, England, E-mail: [email protected].
Petroleum Geo-Services, Perth, Australia.
3
Petroleum Geo-Services, Edinburgh, Scotland.
2
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Figure 2 Schematic illustration of multi-transient EM-derived resistivity profiles as a complement to the structural information provided by seismic data.
Of all geophysical attributes, resistivity is the most sensitive
indicator of hydrocarbons in a rock – resistivity can cover three
orders of magnitude whereas acoustic impedance may change
±30% due to fluids. In that context, resistivity profiling has the
potential to discriminate between commercial and non-commercial hydrocarbon saturation within a reservoir. When combined with seismic AVO data, high resistivity and low acoustic
impedance may identify hydrocarbons within porous carbonates. Non-porous non-reservoir carbonates should correspond
to high resistivity and high impedance. PGS multi-transient
EM experience demonstrates an excellent repeatability of ±1%
in extracted resistivity profiles when survey geometry is repeated, and thus the method is applicable to 4D reservoir monitoring of production-related changes. Local reservoir conditions
(low porosity and/or heavy oil) can often render a producing
reservoir unsuitable for seismic 4D (weak 4D signal), but multi-transient EM is only sensitive to resistivity variations.
The following fact, however, must be observed: ‘While
all hydrocarbons are resistive, not all resistors are hydrocarbons’. Notable resistors include salt, impermeable carbonates, coals, igneous intrusions, and volcanic layers. Interpretation of apparent resistivity profiles should be constrained
by all other available data.
It can be shown from Archie’s law (see later) that in a clayfree formation the resistivity of a rock increases with decreasing
water saturation/increasing hydrocarbon saturation, increasing
water resistivity, and decreasing porosity. It can be shown that
the resistivity associated with hydrocarbon-bearing reservoirs
increases approximately by an order of magnitude, but can be
as high as three orders of magnitude compared with the resistivity associated with water-filled reservoirs.
The resolving power of low-frequency EM methods is
intrinsically limited by the spatially smooth nature of the diffusing EM fields. EM methods are generally sensitive to the
transverse resistance of a layer; the resistivity-thickness product. Therefore, a thin highly resistive layer may be almost
indistinguishable from a thicker less resistive layer with the
same transverse resistance, and seismic constraints may thus
be invaluable. By making measurements at many sourcereceiver offsets and employing a broadband signal this equivalence problem can be greatly reduced and the measurement of
other EM field components can help reduce it even further.
Multi-transient EM data benefits
Figure 3 The typical range of resistivity found in earth materials.
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This section is not supposed to be exhaustive, although the
intention is to demonstrate that the accurate knowledge of
the spatial resistivity cross-section of the earth opens many
exciting opportunities.
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Multi-transient EM technology supplements high-resolution seismic data to support decision-making and risk reduction. Operationally, real-time appraisal is an attractive bonus of
the multi-transient EM method; ensuring optimum acquisition
results are recorded for every survey, and every source-receiver configuration. Initial resistivity profile results can be generated in the field within 24 hours, potentially providing an early
indication of the presence of hydrocarbons and their approximate depths.
Because multi-transient EM is sensitive to fluid types contained within the reservoir rocks, it is the ideal tool for regional reconnaissance and initial exploration efforts in new plays
and frontier areas. Besides its application to reduce or eliminate
the risk of drilling dry holes, the technology can also be used to
high-grade (or low-grade!) exploration prospects. This expedites
planning and maximizes the economics of a drilling program:
n Direct resistivity indicator,
n Imaging through complex overburden, provided that it is
conductive
n Imaging resistors beneath seismic fault shadows
n Capable of identifying stacked resistors
n Confirmation where seismic AVO results are inconclusive
n Confirmation where other direct hydrocarbon indicators
(DHIs) are inconclusive
n Imaging top resistor surfaces in carbonates
n Discriminate hydrocarbon-saturated carbonates
n Shallow gas/drilling hazard identification
n Mapping tar sand deposits
n Locating fresh water aquifers
n Gas hydrates identification
n Lower-risk drilling decisions.
Multi-transient EM has the capability to maximize recoverable reserves and extend the economic life of a field. The
technology can be used for 4D (time-lapse) applications to
improve reservoir characterization and management. 4D
surveys are also becoming increasingly commonplace to
monitor movement of water, gas, chemicals, etc., injected
into reservoirs during enhanced recovery programs:
n Improve upon earlier reservoir imaging and characterization
n Distinguish fizz water from commercial gas and accurately
image resistors below low-saturation gas clouds
n Identify sweet spots and bypassed pay in geological settings where lack of seismic resolution inhibits imaging
n Identify and estimate reserves
n Enhance reservoir parameter prediction because of high
sensitivity to water saturation
n Easily integrated into reservoir characterization workflows by direct correlation to log data, structural and
stratigraphic interpretations and seismic data
n 4D monitoring including permanent monitoring: seismic
4D does not always work
n CO2 sequestration
Multi-transient EM vs. CSEM
Strictly speaking, multi-transient EM can be considered
as a class of controlled source EM (CSEM), as the source
function used is man-made and therefore ‘controlled’. The
implementation, however, is strikingly different.
EM exploration for hydrocarbons is usually considered in
the context of the electric field component with a source current injected across bipole electrodes because that approach
is established as being better for detecting hydrocarbons. It
is important to note the concepts of a primary and secondary EM field more commonly associated with the magnetic
field component of EM. Normally, if a magnetic source (e.g.,
a conducting loop) is switched on for a while, the primary
Figure 4 Comparison of the time and frequency domain representations of a multi-transient EM PRBS source function and a CSEM continuous square wave
source function.
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Figure 5 Illustration of the PRBS source function current (left), the measured potential across a receiver bipole (middle), and a deconvolved impulse function for
the earth (right). The time scale on the right is 0 – 30 ms.
field generated by that source will induce currents in the subsurface of the earth, and these will create secondary magnetic
fields. The measured field at a receiver is the total field (primary + secondary). When the primary field is switched off,
only the secondary field will be present, from which deductions will be made about the conductivity of the subsurface.
In a CSEM frequency domain EM system a receiver measures
the secondary field of frequency f generated by a source field
also of frequency f. However, in all frequency domain EM
systems the secondary field generated in the earth is always
recorded in the presence of the primary field, which limits
the detection of the secondary signal. Time domain EM systems are usually recorded in the absence of the primary signal, which makes the signal more detectable.
Figure 6 Schematic layout of a PGS multi-transient EM onshore survey.
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A patented deconvolution procedure to separate the primary and secondary fields recorded during a multi-transient
EM survey (see below) has been developed by PGS.
CSEM for hydrocarbon detection is only considered in
the marine environment. Multi-transient EM is equally applicable to both the land and marine environments, although
there are operational differences as discussed later. Geophysically speaking, the first-order difference between CSEM and
multi-transient EM is that CSEM is a frequency domain pursuit and multi-transient EM is a time domain pursuit. In theory the two techniques will produce equivalent results, but
CSEM in practice fails to record sufficient frequency range
data to match the advantages of multi-transient EM.
Another key difference between multi-transient EM and
CSEM is the frequency content of the source waveform. The
CSEM method uses a continuous harmonic signal that has
traditionally consisted of a square wave with a fundamental frequency of typically 0.01-10 Hz and odd harmonics of
the fundamental frequency. Individual frequencies are transmitted one at a time, a process that is inefficient and timeconsuming. As shown in the RHS of Figure 4, in addition
to a less-than-ideal frequency spectrum, the CSEM frequency domain amplitudes are proportional to the inverse of frequency. The source waveform used most commonly in multi-transient EM is a pseudo-random binary sequence (PRBS)
that has a frequency range from close to zero frequency up
to the Nyquist frequency of the receiver unit or the source
bit rate of the PRBS.
If a step function or PRBS function is used, the current
input to the ground does not exactly follow the applied voltage, because the circuit impedance is complex (see Figure 5).
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Thus, the input current for each source event is measured. It is
then possible to deconvolve the measured output voltage for the
measured input current to yield the earth’s impulse response.
The receiver voltage response is related to the input
source current via the convolutional equation:
v(t) = i(t)*g(t) + n(t), where g(t)is the impulse response of the
earth, and is uncorrelated noise.
This equation applies provided the input signal is constant
or zero for a period exceeding the duration of the impulse
response g(t). The impulse response can then be recovered by
signature deconvolution. The PRBS source type enables the
frequency content of the input sequence to be tuned to the
depth of interest: a shallow target would employ a high frequency sequence, while deeper targets would use lower frequencies.
The layout of an onshore multi-transient EM survey
resembles that of a 2D seismic reflection survey (refer to Figure 6). In the onshore EM case, the response includes an airwave that travels at about the speed of light, arriving at the
receivers almost instantaneously. After deconvolution the
airwave appears as a sharp impulse that precedes the earth’s
impulse response. The airwave and impulse response are
therefore separable in time, and onshore multi-transient EM
can be considered to have no airwave problem.
For offshore surveying, PGS multi-transient EM utilizes mature, reliable ocean bottom cable (OBC) technology in water depths ranging from 10-500+ m. The OBC
is deployed in a linear configuration with 200 m receiver
bipole interval on the seabed where it remains stationary.
Thus, the PGS multi-transient EM approach automatically measures the inline component of the electric field, has
much denser (inline) receiver spatial sampling than sparsely deployed CSEM nodes, and is operationally much faster to deploy, relocate, and retrieve than CSEM nodes (refer
also to Figure 7). The transient source comprises two large
electrodes connected to the source vessel by heavy cables,
and is deployed via a second vessel. Multi-transient EM
uses a stationary source. The data are recorded until sufficient signal-to-noise ratio is achieved, and as is the case on
land, real-time quality control is used to monitor the quality of the data as they are acquired. This makes it possible
to determine if the signal-to-noise ratio is adequate prior
to moving the source to the next point-another advantage
over CSEM.
As is the case for land EM data, at the water surface there
is a refracted wave coupled to the earth; the inappropriatelynamed airwave that travels at almost the speed of light. The
receiver receives energy from below that has diffused through
the earth, and energy from above that has diffused through
the water and refracted at the water-air interface. In shallow
water (less than 500 m), the amplitude of the refracted air-
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Figure 7 Schematic layout of a PGS multi-transient EM offshore survey.
wave at the receiver may be much larger than the wave from
below, and can often swamp the measured response, thus
masking the response from deeper resistors/hydrocarbon reservoirs. Its amplitude is attenuated by increasing water depth
and becomes negligible if the depth is great enough. CSEM
with an AC source is therefore normally restricted to water
depths exceeding 300 m. In contrast, because multi-transient
EM use a non-continuous source function it is possible to
separate the airwave response from the earth response in the
time domain for water depths less than a few hundred metres
using patented technology.
Resolution
In terms of resolution, marine EM measurements are inherently lower in resolution than land measurements. Seawater strongly attenuates the high frequency end of the signal, the result being that only low-frequency signals penetrate to the depths required for hydrocarbon exploration.
As discussed earlier, PGS multi-transient EM typically use
a PRBS source function to efficiently inject a broad frequency spectrum into the ground (refer also to Figure 8).
The frequency range of the source function is modified for
each source-receiver offset, so unnecessarily high frequencies are not wasted. Horizontal resolution is half the receiver interval (like reflection seismic), and existing resistivity
inversion methods can resolve the top of a resistor to about
10% of the depth. However, multi-transient EM generally
resolves the transverse resistance of a reservoir, the product
of thickness and resistivity. If seismic data can constrain the
target thickness, or well logs can constrain the resistivity,
then the other quantity is much better determined by multi-transient EM.
Analysis and interpretation of data
CSEM surveys derive a resistivity profile of the earth by
frequency domain inversion or forward modelling of the
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Figure 8 Time (left) and frequency (right) domain representations of mono-frequency CSEM vs. DC resistivity vs. multi-transient EM. CSEM typically only has one
or a few frequencies, spatially sampled at a coarse interval over a finite source-receiver offset range with a moving source dipole and stationary receiver dipoles.
DC resistivity can be considered as the low frequency limit of CSEM, and uses stationary source and receiver bipoles. Multi-transient EM uses a broadband signal sampled at discrete offsets over a finite source-receiver offset range with stationary source and receiver bipoles. Sampling of the offset range is typically
denser with multi-transient EM than CSEM. A transient input generates a broadband input signal, which means that a continuous frequency spectrum dataset
is recorded. As the signal passes through more of the subsurface the energy in higher frequencies is absorbed, and the data remaining after this absorption is
shown as the pink shaded area on the graph on the right. Therefore at any given offset the multi-transient EM system records all the available frequencies, the
orange line on the graph.
amplitudes and phase derived at each receiver location.
In contrast to multi-transient EM surveys, CSEM surveys
cannot recover the earth’s impulse response because of the
limited source frequencies available. At least 32 frequencies
would be required to recover the impulse response from a
CSEM survey, which operationally would require repeatedly
surveying the same line with a different switching frequency
and exceptional source-receiver geometry repeatability.
The remaining discussion below only focuses on multitransient EM methodology.
With reference to Figure 9, when a single transient
(step) current is applied to the earth, the step response in
voltage at a multi-transient EM receiver (land example)
starts with an instantaneous increase at time t = 0, whose
strength depends on the surface resistivity. This is the airwave, which precedes the earth impulse response. The airwave is followed by a period of little change after which
the transient responds to the subsurface resistivity. The
response varies with offset. The late time value can be used
to determine the DC resistivity value, and is the value that
would be achieved if the current remained on long enough.
Note that the derivative of the EM field step response is the
earth’s impulse response function (Figure 9). The impulse
response function is the key to multi-transient EM data
interpretation - it contains all the information about the
subsurface resistivity.
Note in Figure 9 that if the reservoir contains hydrocarbons (red curve) the peak is larger at the reference offset of 1 km than if the reservoir contains water (black
curve). Both the amplitude and travel time information in
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the impulse response are used for building resistivity profiles of the earth. Note again that multi-transient EM typically deconvolve the earth’s impulse response from a PRBS
source function, rather than the step function used here to
explain the fundamental geophysical principles involved.
It can be mathematically proven that the ‘peak’ time of the
impulse response to a modelled half-space resistivity model
is an explicit function of source-receiver offset and apparent
resistivity (ρH). Furthermore, by measuring the gradient of the
peak time vs. increasing source-receiver offset an apparent
interval resistivity (ρI) can also be estimated. As the modelled
half-space impulse response closely matches the real impulse
response measured in the field, forward modelling plays a
large part in multi-transient EM data interpretation. The relationship between ρH and ρI may be considered as roughly analogous to the relationship between the stacking velocity and
the interval velocity in reflection seismology. The extraction
of apparent interval resistivity is attractive because these have
higher resolution than the apparent half-space resistivities.
Real-time appraisal products quickly derived in the field
include two pseudo-resistivity analyses that enable real-time
decision-making:
n Common offset amplitude analysis: Amplitude information from different offsets provides an approximate indication of the depth of resistors being imaged, and enables
rapid and accurate delineation of their lateral extent.
n Travel time (tpeak in the impulse response) to resistivity
mapping, either as an average apparent resistivity or as
an apparent interval resistivity estimated from the offsetdependent gradient in tpeak. The pseudo-resistivity value
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Figure 9 Modelled step response (left), impulse response without system response (middle), and impulse response with system response included (right), both for
water and hydrocarbons in the model half-space. The modelled offset in each case is 1 km.
derived is simply the resistivity of a modelled uniform halfspace that has the same arrival time as the peak of the earth
impulse response. Thus, the amplitudes will not match the
true resistivity. Depth scale is only approximate.
Figure 10 shows impulse response data displayed as a series
of common-offset sections. The effect of a resistor is noticeable
at offsets greater than about twice its depth. High amplitudes
are observed between horizontal distances along the profile
of 1800-4000 m, and at offsets of about 1400 m and greater.
This is interpreted as the effect of the resistor corresponding
to the known gas storage reservoir at a depth of 500 m and
greater. Turnaround to delivery of this product is typically less
than 24 hours.
Figure 11 is derived from analysis of the travel time peak
of the impulse responses at each source-receiver offset. Again,
it is clear that there is a deep resistor between horizontal positions 1800-4000 m overlain by a conductor and variable nearsurface resistors. This interpretation is consistent with the
results of the common-offset sections in Figure 10.
Office-based processing and inversion products can also
be delivered within a week of acquisition (refer to Figure 12):
rd
n 2D DC dipole-dipole resistivity inversion using 3 party
software to fit the late time component of the step response
(integrated from the impulse response).
n Full waveform multi-offset inversion: Several offsets in
CMP gathers are inverted simultaneously using full waveforms, resulting in absolute resistivities as a cross-section
function of depth. Generally implemented as Occam inversion; either unconstrained or constrained.
Case example: Saturation discrimination
and 4D feasibility
An empirical scenario based upon Archie’s Law (Figure 13)
demonstrates that increasing from 0-30% hydrocarbon saturation in a clay-free rock only doubles the resistivity factor, a
standard petrophysical attribute used in resistivity logging. As
the trend in Figure 13 is followed to decreasing water saturation (increasing hydrocarbon saturation), however, the impact
© 2008 EAGE www.firstbreak.org
upon the resistivity factor becomes exponentially greater.
Therefore, EM can clearly distinguish commercial from noncommercial in-situ hydrocarbon accumulations. In addition,
EM is most sensitive to early production-induced saturation
changes, making it possible to evaluate flow-continuity shortly
after production is initiated and plan for additional wells necessary to extend plateau production of the field.
In the context of 4D (time-lapse) reservoir monitoring,
Figure 13 also demonstrates that if the initial in-situ hydrocarbon saturation is 90% and 2/3 of all reserves are subsequently
produced, the resistivity factor will decrease from 100 to 2, a
significant change that should be easily observable using resistivity cross-sections derived from a multi-transient EM survey. Increasing clay content (e.g. shales) will decrease the comparative resistivity factor from the nominal clean sand trend
shown, and carbonates will increase the resistivity factor.
Case example: Conflicting seismic amplitudes
and multi-transient EM anomaly
A multi-transient EM survey of 21.6 km was acquired in
water depth of 135 m. Survey duration was less than five
days, with the delivery of data products about a week later.
Observe in Figure 14 that seismic amplitudes at the target
level of 1435 m increase to the left of the figure, away from
an existing dry well. In contrast, a strong resistivity anomaly
increases in amplitude in the opposite direction, towards the
well location. Reservoir interval thickness is 10-50 m.
Case example: Confirmation of hydrocarbon
pay zone
14.8 km of multi-transient EM data was acquired in water
depth of 100 m. Survey duration was less than three days,
with the delivery of data products about a week later. A
known producing interval occurs at 1700-1900 m depth.
Observe in Figure 15 that the multi-transient EM amplitude
anomaly correlates with the producing well location, and is
absent below the dry well location. Additional 2D surveying
and 3D processing and inversion is recommended to properly
delineate the multi-transient EM anomaly.
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Figure 10 Common-offset sections of the earth impulse response function for offsets 700–2000 m. The horizontal coordinate is the midpoint between source and
receiver, and the scale is shown at the top of each figure; the vertical coordinate is time, with the complete axis being 0–100 ms. The grey bar at the top of each
section shows where the airwave has been muted out. The relative amplitude scale is denoted by the colour bar (again the airwave has been muted out). The
presence of a deep (> 500 m) resistor can be seen at offsets of about 1400 m and greater. From Ziolkowski et al. (2007).
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Figure 11 Travel time-to-resistivity mapping in CMP-offset coordinates (refer also to Figure 10). From Ziolkowski et al. (2007).
Figure 12 Schematic flow from survey to the delivery of resistivity cross-sections.
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Figure 13 The nominal trend shown is the ‘resistivity factor’ for a clay-free rock
matrix: The resistivity at particular water saturation (Rt) divided by the resistivity at 100% water saturation (R0).
Figure 15 The multi-transient EM anomaly correlates to the producing well,
and is absent below the dry well location.
correlated with the structural information provided by seismic
data. Hydrocarbon fluids and gas can thus be discriminated
in terms of location and saturation - prior to any drilling decisions. The method is equally applicable to land, transition
zone, and marine, providing greater penetration depth and
higher resolution that other EM methods. Multi-transient EM
data have much broader frequency content than CSEM data,
and are unaffected by the airwave problem. The methodology is intuitively analogous to the seismic method, recording
offset, and depth-variant resistivity information from the
earth. An extensive suite of applications includes exploration,
exploitation and production, and 4D production monitoring.
Figure 14 Correlation between a PGS multi-transient EM survey and seismic
data in the North Sea. The multi-transient EM data conflict with the seismic
amplitude trend.
Both North Sea case examples above (acquired in late
2007) verify the resolution and quality of multi-transient EM
data in very shallow water - impossible with CSEM data.
Acknowledgements
PGS Multi-Transient EM is grateful for the cooperation of
Apache Corporation and BP in acquiring the data examples
shown.
References
Wright, D., Ziolkowski, A., and Hobbs, B. [2002] Hydrocarbon detection and monitoring with a multicomponent transient electromag-
Summary
Multi-transient EM is a proven remote sensing method for
direct hydrocarbon detection and delineation. Profiles are
rapidly derived to describe the resistivity of the earth, spatially
netic (MTEM) survey. The Leading Edge, 21(9), 852-864.
Ziolkowski, A., Hobbs, B.A., and Wright, D. [2007] Multi-transient
electromagnetic demonstration survey in France. Geophysics, 72(4),
F197–F209.
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