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Geophysical Prospecting, 2019, 67, 1582–1594

doi: 10.1111/1365-2478.12771

Vertical–vertical controlled-source electromagnetic instrumentation and acquisition Stefan L. Helwig1,2∗ , William Wood3 and Bernard Gloux3 1 Consens,

Houston, TX 77019, USA, 2 Formerly PetroMarker, Houston, TX 77055, USA, and 3 PetroMarker, Stavanger, 4029, Norway

Received March 2018, revision accepted January 2019

ABSTRACT Vertical–vertical controlled-source electromagnetic is an alternative to other techniques for providing three-dimensional resistivity images of the subsurface. It utilizes a large and powerful vertical dipole transmitter and arrays of E-field receivers with vertical and horizontal dipole sensors. The necessary instrumentation and acquisition procedures which differ strongly from other controlled-source electromagnetic methods are described in the paper. Key words: Controlled-source electromagnetic, Electromagnetics, Acquisition, Instrumentation.

1 INTRODUCTION When marine controlled-source electromagnetic (CSEM) was commercially introduced to the hydrocarbon industry in the early 2000s (Eidesmo et al. 2002; Ellingsrud et al. 2002), the methodology was based on previous developments in academia for studying ocean basins and active spreading centres. The roots go back to the 1980s and before when experiments with active source units were carried out by SCRIPPS in the Pacific (Cox, Deaton and Pistek 1981; Young and Cox 1981). Potential usefulness for hydrocarbon exploration was also realized very early on (Srnka 1986) but was commercially not available until the early 2000s when several companies started to provide services that in essence all relied on the same data-acquisition principles. A grid of autonomous offshore magnetotelluric (MT) receivers that continuously acquire horizontal E and H-field data are deployed on the seafloor. To allow for the detection of thin resistive layers, electrical current flow in the vertical plane is necessary, which is not provided by the natural MT-fields, thus an active source is needed and provided in the form of a deep-towed horizontal dipole, transmitting a low-frequency source signal. While many important improvements in acquisition, processing and interpretation have been made to the

∗ E-mail:

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slh@consens.technology

technology over the last one and a half decades, the fundamental acquisition scheme of horizontal source CSEM has remained the same. An alternative marine CSEM acquisition approach was introduced by Barsukov, Fainberg and Singer (2007) and Holten et al. (2009). It is based on a vertical dipole transmitter and the measurement of the vertical electrical field component on the seafloor. Contrary to the horizontal dipole which emits a transverse electric mode with current flow in the horizontal plane and a transverse magnetic (TM) mode with current flow in the vertical plane, a vertical dipole transmitter only creates a TM mode. This leads to differences in the interaction of the EM fields with the environment and to differences in the sensitivity towards thin hydrocarbon layers. The detrimental effect of the air wave in shallower water acquisition, for example, can be mitigated by acquisition and interpretation strategies that generate and extract only the TM-mode component of the field (Andréis and MacGregor 2008). The differences in the sensitivity distribution are important for the detection of smaller targets. In horizontal–horizontal acquisition, the maximum sensitivity is at large offsets and low frequencies. Vertical–vertical CSEM is typically measured in time domain, and the maximum sensitivity occurs at small offsets and late times. This smaller source–receiver offset also makes the vertical–vertical approach less prone to disturbing influences from three-dimensional (3D) bodies outside of the

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Figure 1 Source system: The white containers house two separate source generators each capable of transmitting 3000 A max. The blue A-frames are used to position the source electrodes. The vertical orange cables connect the source generators to the electrodes.

area of interest. In a two-dimensional (2D) study, Alumbaugh et al. (2010) came to the conclusion that while the horizontal transmitter, horizontal receiver methodology provides better sensitivity to targets where the target width is much greater than the targets depth, the vertical–vertical approach provides better sensitivity as the target’s lateral extent becomes comparable or smaller than its depth. In addition, they further conclude the vertical–vertical mode appears to provide better lateral resolution. Frafjord et al. (2016) compare different CSEM methods and show larger depth penetration for vertical–vertical CSEM. To utilize the benefits of the vertical–vertical approach, a different set of instrumentation as well as a different acquisition approach is needed. The vertical source, the vertical and horizontal E-field receiver and the acquisition are described in the following sections.

or in parallel. At peak load, 1.4 MW of power is needed to operate the system. Access heat is dissipated by water cooling. As the source dipole spans almost the whole water column, a subsea conversion from high voltage, low current to low voltage, high current is not beneficial in a vertical dipole system. Instead, the current is directly fed to the source electrodes using very low impedance cables. Hydraulically controlled Aframes, blue structures in Fig. 1, are connected to winches and sheaves and allow the vertical launch and recovery of the source system. At a transmitter location, the lower electrodes are placed on the seafloor first. Their position is measured about 100 times with the acoustic ultra short baseline telemetry system and the resulting position record is fixed in the dynamic positioning (DP) system of the vessel. During transmission, the source dipole is stationary and the source signal is repeated to lower the signal-to-noise ratio by stacking. The position of the upper electrodes is continuously controlled with the acoustic telemetry, and the DP system of the vessel is set to position the upper electrodes vertically above the lower ones. Figure 2 shows the deviations of the upper electrodes relative to the lower ones. Transmitter dipole verticality is specified to be within one degree. If deviations over one degree occur during a source sequence, the sequence is marked as ‘out of specification’ and the acquisition is extended by one source sequence. Data shown in Fig. 2 consists of 5384 single position measurements during a 90-min transmission. The source dipole length was 250 m, which leads to an accepted maximum deviation of 4.5 m. Nearly all measurement points shown on the left scatter plot are within specification. Only single data points deviate more than 4.5 m, and in this case can all be traced to distortions in the acoustic telemetry system. The histogram on the right shows the distribution of deviations between upper and lower electrode in the East component. It is a normal distribution and centred around zero without significant bias.

2.1 Source signal 2 SOURCE GENERATION AND SOURCE HANDLING The source system consists of containerized units that convert the three phase 440 V power to a coded ternary waveform and deliver it to the source electrodes. The main components of the source system are shown in Fig. 1. The two white containers house the power conversion units. They receive input power from the ship’s generators. Each unit is designed for a maximum output current of 3000 A and can be run individually

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Typically the transmitter signal is coded according to a Thue– Morse sequence with eight pulses (so-called P8 sequence). Two periods of a current measurement are shown in the left panel of Fig. 3. As the interpretation is done based on transients in the time domain, each pulse is followed by a listening time where the transmitter is switched off. During transmission the P8 sequence is periodically repeated. The bandwidth of the signal is determined by the P8 waveform and the decay at switch-off. A detail of a current

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Figure 2 Left panel: Measured position of the upper electrode relative to the lower one. Shown are 5384 measurements over the course of 90 min. Single deviations from the average behaviour are typically due to distortions in the acoustic telemetry. Right panel: Histogram of the East deviation.

recording at switch-off time is shown in the right panel of Fig. 3. The recording shows no undershoot, and 90% of the current decays in less than 10 ms resulting in a wide bandwidth signal.

3 RECEIVER For vertical time domain controlled-source electromagnetic (CSEM) measurements, various signal aspects should be taken into account in the receiver design. The highest sensitivity

towards the target is at short offsets. The receiver must be capable of measuring at these offsets without saturating the data logger. Based on time after switch-off and offset, the transients decay strongly. Consequently, a large dynamic range and low noise floor is needed. Transients also decay fast, requiring a relatively high bandwidth and sampling frequency of the recording unit. In time domain, the interpretation relies on the time of decay and the shape of the recorded signal. The synchronization of timing between the transmitter and receiver, the relative phase, must be known with high

Figure 3 Measurements of the transmitter current. Left panel: Two periods of a P8 sequence. Right panel: Current decay after switch-off.

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Figure 4 1D vertical electric dipole voltage responses to a 5000-A source signal. The model is a 300-m thick water layer with 0.3 m over a 1 m half space. At 1000 m depth below seafloor is a 100-m thick 100 m resistive layer. Left panel: Background responses for different transmitter receiver offsets. Colors and labels refer to the offsets in metres. Right panel: Response with and without resistive layer at 200 m offset.

precision, and the signal chain should not include elements with frequency-dependent phase relations, which alter the signal shape. An illustration of the above points is given in Fig. 4. Calculations shown are for a 1-m half space below a 300-m thick water layer of 0.3 m. In the right panel, a 100m thick 100 m resistive layer has been put 1000 m below the seafloor. Transmitter and receiver dipole lengths were set to 250 and 3.65 m, respectively, and the transmitter current is 5000 A. Especially at close offsets, the dynamic range of the signal is very large. The 200-m offset curve has a maximum value of approximately 2 mV, which decays six orders of magnitude to 1 nV over the course of 20 s. The differences between the model with resistor and the one without occur at late times and reach their maximum 3 s after switch-off. By this time, the signal level for the background model has fallen to 0.18 μV while the signal for the model with resistor has fallen to 12 nV. Low noise design is a must. 3.1 Measurement electronics Design limits are bounded by the Johnson–Nyquist noise of the sensing electrodes and their matched amplifiers. The noise density is impedance dependent and given by   4kB T RV/ (Hz) with kB denoting the Boltzman constant, T denoting temperature in Kelvins and R the resistance of the electrode pair or electrode pair amplifier combination. A high quality Ag/AgCl electrode pair has an impedance √ of 4.5  resulting in a noise density of 0.26 nV/ Hz at seafloor temperatures. To benefit from the low sensor impedance, the electrodes need to be combined with a low

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impedance amplifier. In addition, the onset of the pink noise spectrum typical in semiconductors must be pushed to as low of frequencies as possible to get a low noise floor over a large frequency range. Both aspects are achieved with chopper amplifiers based on a design developed in the 1980s (Webb et al. 1985). A comprehensive overview on CSEM acquisition technology can be found in Constable (2013). For the vertical E-field transient electromagnetics receiver, the gain is chosen lower than in other marine applications to allow for a large dynamic range without gain switching. Figure 5 shows a histogram of spectral density values for a shorted amplifier in the frequency range from 0.1 to 10 Hz. The power spectral density was estimated using Welch’s method. The resulting values in the frequency range from 0.1 to 10 Hz have been binned, and a log-normal distribution was fitted to the histogram. The resulting noise value √ of the shorted amplifier is 0.24 nV/ Hz. The amplifier noise density is the same as the noise density of the sensor. The √ √ combined noise density is 0.262 + 0.242 = 0.35 nV/ Hz, equivalent to the Johnson noise density of an 8  resistor. After amplification, the signal is digitized using a DCcoupled 32 bit analog digital converter (ADC) developed for modern seismic data acquisition systems. Linear phase response filters are used in the ADC settings to ensure the signal integrity of the transient signals. The vertical E-field is measured four times with independent sensor pairs and electronics to further improve the signal to noise ratio, reduce the risk of data loss and detect sensitivity and response deviations. Data is stored on flash memory and is downloaded to on-board servers after the recovery of the receiver. An eight-channel receiver produces around 1.5 GB of data every day.

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Figure 5 Histogram of amplitude spectrum density values for a shorted amplifier in the frequency range from 0.1 to 10 Hz.

In vertical–vertical transient CSEM, the E-field signals are typically sampled with 500 Hz resulting in a phase drift of 1/125 of the sample period in a week. During operations, the phase of the receiver clock is synchronized with the main time reference on the vessel prior to each receiver launch. When the receiver is recovered, the phase drift is recorded. With previous generations of clocks, timing corrections had to be applied. Since the timing was changed to CSAC units, no time correction had to be applied to any data.

3.2 Receiver structure

Figure 6 Phase measurement of an early production CSAC compared to GPS-disciplined rubidium standard.

Under water, no global positioning system (GPS) time reference is available. In previous equipment generations, timing was provided by quartz clocks. As oven-heated clocks would consume too much power for a battery-powered unit, clock drift was a typical issue. In modern CSEM instrumentation, timing of the data logger is provided by miniature atomic clocks (CSAC). Figure 6 shows the phase difference of an early model CSAC against a GPS-disciplined rubidium reference in nanoseconds. Over the course of a week, the phase drift is less than 16 μs and can be lowered even more by careful disciplining of the CSAC unit to the reference timing unit.

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The structure used to acquire data needs to be designed to best support the goal of making high-quality vertical E-field measurements while fulfilling the requirements of 3D offshore operations at the same time. Deviations from verticality result in signal distortions as parts of the much stronger horizontal E-field are mixed into the vertical response (Goldman et al. 2015). Receiver designs utilizing a vertical dipole have been introduced by Constable (2006) and MacGregor et al. (2008). While the prior uses an orthogonal system of three electrical antennae, the latter uses a tetrahedral antennae arrangement. In both cases, the vertical component needs to be derived by a linear combination of at least three measured electrical fields as the seafloor can’t be expected to be perfectly flat. Frafjord et al. (2014) showed that the differences in noise between a true vertical recording and a mathematically rotated one can be significant as the horizontal E-field components are

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The electrodes for the vertical field measurements are mounted on a centre pole, which houses a verticality sensor connected to a control unit that aligns the centre pole to vertical orientation after the receiver has settled on the seafloor (Gloux, Helwig and Wood 2016). By this method, verticality can be achieved with a high degree of precision. A statistical overview from 221 receiver positions from a survey offshore Norway is shown in Fig. 8. In the significant majority of cases, the deviation from verticality towards gravity is less than 0.1â—Ś . Records of the verticality measurements that are automatically performed once every hour after aligning the centre pole show that in some cases a receiver has sunken further into the mud after the verticality alignment was carried out and stayed in that position for the rest of the measurement.

3.3 Release mechanism

Figure 7 MK5 receiver during launch. The main components of the receiver are pointed out.

typically much noisier than the vertical component. Designs with gravitational verticality alignment, based on a hinged dipole with a large weight on the bottom (Kjerstad 2010), are impractical for larger operations due to their size and weight (Helwig et al. 2016). For operational efficiency, the receiver needs to sink down to the seafloor and get back to the surface without the use of a remotely operated vehicle (ROV) or crane. This is achieved with a buoyant frame that is sunk with added ballast and retrieved by releasing that ballast on command of an acoustic signal (Constable 2013). Figure 7 shows the PetroMarker MK5 receiver during launch. The frame structure consists of a nonmetallic tripod with a hinged centre pole mounted on a bottom frame. On the corners of the bottom frame are ballast buckets that are filled with sand prior to launch of the receiver. Borosilicate glass spheres house the electronics and batteries and provide part of the buoyancy. An acoustical transponder facilitates communication with the unit and release commands. Horizontal arms support the electrodes for the horizontal E-field measurements.

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The tripod shape of the MK5 receiver was chosen for optimal support of the centre pole. Its ballast is distributed in three buckets at corners of the lower frame. A synchronized release of all three buckets is needed for reliable operation. If, for example, only two buckets release the ballast while one is still retained, the receiver will over-rotate and the ballast in the last bucket will not be properly released. Release mechanisms with poorly defined reaction time, like the wellproven electrolytic wire erosion ones, cannot be used. Actuators add much weight, and mechanisms based on explosives are an offshore safety risk. For the MK5 receiver, a different release system was designed. The weights needed to sink the receiver are attached to the main structure with ultra-highmolecular-weight polyethylene ropes. This material combines an extremely high tensile strength with a low melting point. The release is based on heating the ropes up over their melting point. Based on an acoustic telemetry signal, all three ropes melt and release the ballast at the same time. To enhance reliability, each bucket is equipped with two independent release mechanisms. Figure 9 shows a receiver filmed with an ROV immediately after release. The ballast of all three buckets was released, and the receiver is on its way to the surface.

4 OPERATIONS AND SURVEY CONFIGURATION Figure 10 shows the arrangement of the equipment on-board a multi-purpose vessel. The equipment is modularized and can be installed on a large variety of platform support,

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Figure 8 Deviation of the centre pole from verticality at 221 receiver locations.

Figure 9 Vertical E-field receiver rising from the seafloor shortly after releasing.

multi-purpose or offshore construction vessels. The containers in the stern are storage and workplace units utilized in routine maintenance work. Next to them, on the port side, spare transmitter electrodes are stored. Between the stern containers and the first white transmitter, container receivers are stored. The transmitter configuration is like the one shown in Fig. 1, with two power conversion containers, one each side, and the A-frames for launch and recovery of the transmitter dipoles in the middle. Further towards the bow is the

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Figure 10 Equipment on-board a multi-purpose vessel during a survey in the Norwegian Sea.

designated launch and recovery area for the receivers as well as the operator’s container. The large crane on the port side of the vessel is not used in the operation. The system was mobilized onto the vessel in two days and demobilized in half a day. The survey configuration of a

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Figure 11 Survey layout for a 2017 Norwegian sea 3D survey. Red triangles indicate receiver locations, and blue dots indicate transmitter locations.

2017 multi-client acquisition in the Norwegian Sea is shown in Fig. 11. A total of 218 transmitter and 221 receiver locations, shown as blue dots respective red triangles have been used in the acquisition. At each transmitter location, the lower electrodes have been put to the seafloor while the upper ones were suspended 50 m below the vessel, resulting in transmitter dipole lengths between 250 and 280 m. On most shot locations, 29 periods of the ternary sequence shown in Fig. 3, each 144 s long and consisting of eight individual pulses, were transmitted. The survey was conducted in a roll-along mode. As shown in Fig. 4, the vertical E-field dies off quickly with offset. Receivers too far from the transmitter do not contribute to the analysis of the subsurface resistivity distribution. Therefore, the three-dimensional survey area is subdivided into smaller subsections. First, receivers were launched and a specified number of source points were shot. Following that, a subset of the receivers were recovered, the data downloaded, the receivers re-launched in a new position and the next points were shot. The vertical transmitter can be launched and recovered fast, and as no turns have to be made outside of the area of interest there is no time penalty for switching between receiver and transmitter operations.

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At each receiver launch, the same operational steps are applied. The ballast is loaded, the functionality of the receiver and the release mechanism is checked and the internal CSAC is synchronized with the main time reference. After the receiver is in its position on the seafloor, it automatically adjusts the centre pole. The adjustment and other parameters can be checked with the acoustic telemetry. To transmit, the vessel is brought into position and held there with the vessel dynamic positioning system. The lower transmitter electrodes are lowered to the sea floor using the hydraulic A-frames and winches. The position of the lower electrodes is measured, and the upper electrodes are positioned above them in the water column 50 m below the sea level. Operation of the current transmission as well as the position control are automated and continuously monitored.

4.1 Data example One period of an Ez recording is shown in the left panel of Fig. 12. The dataset was acquired with 647 m offset between the transmitter and the receiver. It is normalized by receiver dipole length and current. The view on the right of Fig. 12 shows the result of processing the time series. All 29 signal periods have been used in processing. Each receiver measures

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Figure 12 Example of an Ez-transient with 647 m transmitter–receiver offset. Left panel: Section of the time series showing a single transmission period. Right panel: Stacked result in log–log domain.

four vertical E-field records. If all pass the quality check, as in this case, they are averaged. The maximum signal level is 20 μV. At the latest time point, 12 s after switching off the transmitter, the voltages have decayed to 12 nV.

Section 3.1. Uncertainties in the amplitude of the transmitter current are also very small and therefore not considered in the discussion.

5.1 Source-related uncertainties 5 MEASUREMENT UNCERTAINTIES The impact of data uncertainties needs to be taken into account during the interpretation of results from any measurement. Controlled source electromagnetic (CSEM) is no exception, and an in-depth discussion on the vertical–vertical CSEM measurement uncertainties will be presented here. Currently available CSEM systems show a high degree of similarity and since Maaø and Nguyen (2010) and Mittet and Morten (2012) wrote a comprehensive overview and discussion on uncertainties, we will focus here on those aspects and features that are complementary and characteristic for the implementation of a stationary CSEM system using a vertical dipole source and a vertical E-field receiver system. During the measurement, both transmitter and receiver are stationary and the signal-to-noise ratio can be improved by stacking. The uncertainties will therefore be grouped into those that will improve due to stacking and those that will not. In particular, the following sources of measurement uncertainty will be included in the discussion: transmitter dipole deviation from verticality, transmitter dipole variation in length due to rolling movement of the vessel or elevation change due to tides during transmission, receiver dipole deviation from verticality, uncertainty in offset, receiver calibration uncertainty and uncertainty of the sea water resistivity. Uncertainties in the relative phase are negligible as explained in

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During transmission, the lower transmitter electrode rests on the seafloor and the upper electrode hangs 50 m below the vessel. Its position vertically above the lower electrode is measured with an acoustic system and controlled with small vessel moves. As shown in Fig. 1, the A-frames for the transmitter are installed on the side of the vessel. A rolling movement of the vessel results in an up and down movement of the sheave and an up and down movement of the upper electrode. Based on safety reasons, transmitter operations are permitted in weather conditions up to 4 m significant wave height. The exact change of dipole length under such conditions depends on the individual equipment setup on the vessel, the vessel orientation the angle of the A-frame and on other parameters. To estimate the length influence, we take a worst case approach and calculate responses for a ±3 m length deviation. In addition to the change in dipole length there also occurs a horizontal dislocation of the upper electrode. A deviation from verticality up to 1◦ is normally accepted during operation. This creates a small additional horizontal transmitter component. The geometry of the setup is shown in Fig. 13. Depending on the vertical dipole length, the maximum allowed deviation on the top can fall on a circle around position 0. Four example positions (1–4) are indicated in the top view on the left. From the geometry of the problem, it is obvious that

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Figure 13 Geometry for calculations on the influence of a tilted transmitter. Left: View from top. The ideal upper electrode position is marked with 0, positions 1–4 are example positions of electrode variations with the distance r from position 0. Right: View from the side. If the upper electrode moves from position 0 to position 2, a horizontal transmitter component is added to the source field.

deviations in the Y-direction will cancel out as long as they are randomly distributed around point 0. Deviations in the Xdirection will result in a length difference of the offset vector a as shown in the right panel of Fig. 13. For a maximum uncertainty calculation, we therefore focus on positions 2 and 4. The influence of a change in transmitter dipole length is more prominent for shallow water situations; therefore, only a shallow water estimation is presented here. In deeper water, the relative change of dipole length as well as the relative horizontal component are both small. In Fig. 14, the effect of a change in transmitter length as well as the additional field created by a tilted dipole is compared to the response of a model similar to the one used by Mittet and Morten (2012). It is an isotropic one-dimensional (1D) model consisting of a 0.3125 m water layer, a 1.5 m formation layer, a 50-m resistive layer and a 2-m lower halfspace. The water layer is 500 m thick, the formation layer is 1250 m thick and the reservoir layer is 50 m thick. The transmitter dipole is 450 m long and reaches from the sea-floor to 50 m below the water surface. In the background model, the 50 m resistive layer was replaced with a 1.5-m layer. The vertical E-field response of the background model at 500 m transmitter receiver offset and 2500 m transmitter receiver offset is shown on the left respective right panel as a blue continuous line. The influence of the resistive layer as well as the influences of uncertainties is shown as anomalous fields in the sense that the anomalous field is the total field of a model including the resistive layer or uncertainty minus the background field. The anomalous or scattered field of the

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resistive layer is shown as red continuous line. It can be seen that at short offset and early times, the configuration is not sensitive to the resistive layer. Here the anomalous field is more than a factor of 100 smaller than the primary field of the background. At late times, the anomalous field is sensitive to the resistive layer and reaches 48% of the background field at 3.4 s after switch off. The anomalous field due to a 1◦ transmitter tilt, the maximum allowed in operations, is shown as red dashed line. As shown in Fig. 2, the upper position can be controlled within ±2.5 m and is randomly distributed. On a 450-m long transmitter, a deviation of 1◦ equals a displacement of ±7.8 m. Therefore, the assumption used here is a worst case scenario that is unlikely to happen in a real measurement. At early times and small offsets, the anomalous field introduced by the tilted transmitter is 3.4% of the background field. At late times this value drops to 1.5%. Due to the stationary nature of the acquisition, the average influence of the transmitter tilt is a much better uncertainty indicator than the maximum tilt influence. The cyan dashed line shows a the stacked anomalous field of 50 simulations with random tilt values between the positive maximum tilt and the negative maximum tilt. One can see that the error is greatly reduced. It now reaches 0.3% at early times and three orders of magnitude at late times. It is common to acquire about 200 stacks in vertical–vertical CSEM; this results is a further reduction to below three orders of magnitude at early times and 0.02% at late times. The yellow/greenish dashed line shows the maximum anomalous field due to the change in length of the transmitter dipole. The effect is orders of magnitude smaller

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Figure 14 Vertical electrical field responses for the 1D model described in the text. The left panel shows fields at 500 m transmitter receiver offset, the right panel at 2500 m offset. Blue continuous lines (BG) show the field response of the background field, red continuous lines (REL) show the anomalous field of the resistive layer and red dashed lines (TXT) show the maximum anomalous field due to the horizontal field component of a single transmitter pulse with 1◦ tilt. Cyan dashed lines (TXS) show the anomalous field of 50 stacked responses with varying receiver tilt, and the dashed yellow line (ROL) shows the anomalous field caused by a transmitter length change due to a rolling vessel.

than the effect of the horizontal component of the tilt. Like the horizontal tilt effect, it will further reduce with stacking. At larger offsets the relative effect of the anomalous resistor becomes evident at early times. At 2500 m offset, the early times anomaly for this model is 6%, the effect of a single pulse with a maximum tilted transmitter is about 8% and reduces to 0.7% with 50 stacks. A single transmitter pulse with maximum tilt would result in a significant error at early times, but as in the short offset case the error reduces favourably with stacking. To minimize the position uncertainty of the lower electrode, the transmitter electrodes are outfitted with two acoustic transponders of the latest generation. The resulting accuracy in 500 m water depth is better than 1m, and the anomalous field is slightly smaller than the TXS field shown in Fig. 14. This uncertainty however is static and will not improve with stacking. 5.2 Receiver-related uncertainties A similar approach as with transmitter tilt has been applied to understand the influences of receiver tilt. Based on the data shown in Fig. 8, we assume a receiver tilt of 0.1◦ . The position of transmitter and receivers is measured using a ultra short baseline acoustic telemetry system. The measurements are repeated about 100 times and averaged. Based on tests on gravity markers at known locations, we use a 1-m error for the position of transmitter and receiver at 500 m depth. The combined uncertainty in offset for transmitter and receiver

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√ is therefore 2 m. The uncertainty in the receiver calibration consists of the uncertainty in the data acquisition unit and the uncertainty of the sensor impedance. The data loggers are calibrated with a known source to a high precision. Repeated measurements of the electrode impedance over a time of 2 years show a high stability of the impedance values. Differences in the water salinity and pressure conditions may however result in impedance changes. A impedance change from the nominal 5  per electrode pair to 8  per pair is used as the basis for the calibration error. In combination with the amplifier input impedance, this results in a 0.25% calibration error. In addition to the parameters described above, the influence of a measurement error in the conductivity, temperature and depth (CTD) measurement of the seawater is considered with a 2% error on the seawater resistivity. The estimate is based on measured CTD profiles during a larger campaign. Simulation results are presented in Fig. 15. As on the previous figures, the blue solid line with label BG shows the vertical field of the background and the red solid line with label REL shows the anomalous field of the resistive layer. The dashed cyan line with label SEA shows the anomalous field of a 2% error in sea water resistivity. It can be seen that the uncertainty is dominant with an error 3.6% at short √ offsets and early times and 1.5% at late times. A 2 m offset deviation and a 0.1◦ receiver tilt deviation result in similar uncertainties of 0.7% at short offsets and early times and less than three orders of magnitude at late times. The calibration

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Figure 15 Influences of receiver uncertainties compared to the 1D model described in the text for a 500-m transmitter receiver offset on the left and a 2500-m transmitter receiver offset on the right. Blue continuous lines (BG) show the field response of the background field, and red continuous lines show the anomalous field of the resistive layer (REL). The cyan dashed lines (SEA) show the anomalous field due to a 2% uncertainty in the resistivity of the sea water layer. The blue dashed lines (RXT) show the anomalous field due to a 0.1◦ deviation in receiver √ tilt. The dashed green lines (OFF) show the anomalous field due to a 2 m deviation in offset, and the dashed yellow lines show the anomalous field of a 0.25% calibration error.

error is by definition a fixed percentage of the measured field at 0.25%. At larger offsets, the effects are very similar. The uncertainty due to sea water resistivity is the most significant one followed by the receiver tilt-induced uncertainty and the calibration uncertainty. As the offset increases, the relative error of the offset becomes insignificant. The examples in this section provide a guideline on uncertainty estimation in comparison to a particular model anomaly. Due to the non-linear and non-monotonic relationship between the water depth or transmitter dipole length and the vertical electrical field strength at a given offset, the ratio between the anomalous fields due to a resistive anomaly and the anomalous fields due to a particular uncertainty changes in a highly non-linear way depending on model, water depth and other operational factors and should be evaluated on a case by case basis. Non-stationary effects from transmitter movement can be controlled by selecting an appropriate number of stacks. The uncertainties of stationary effects are well below 1% of the background field except for the water resistivity measurements illustrating the high fidelity of the vertical–vertical CSEM measurement.

6 SUMMARY AND CONCLUSIONS A system for vertical–vertical controlled source electro magnetics (CSEM) data acquisition and its application has been presented here.

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The unique feature of the system, everything being stationary, offers a tremendous signal-processing advantage over systems where the source and/or the receivers are moving in that any variation related to equipment is readily identified as noise and effectively suppressed by averaging, provided that data is recorded over a sufficiently long period of time. Earlier implementations of vertical–vertical CSEM already acquired high-fidelity data, but only with relatively sparse spatial sampling. The present receivers were designed to aim at a significantly improved operational efficiency of the vertical–vertical CSEM method such that three-dimensional (3D) acquisition would become the standard. To achieve this, a host of innovations were incorporated in the present system, like reducing the receiver dimensions, changing the deployment and recovery to that of free-fall systems, designing a novel release mechanism that ensures the near-simultaneous release of ballast from each of the three pods that form the receiver base, and leaving nothing on the seafloor other than a bit of sand. In vertical–vertical CSEM, the highest sensitivity towards a resistive target occurs at short transmitter receiver offsets where the dynamic range of the signal is very large. Also, verticality is of key importance. Consequently, receiver electronics and control systems feature active control of the verticality of the receiver antenna within very tight tolerances, the use of an atomic clock in the data loggers to eliminate the need for post-acquisition phase corrections, the use of stateof-the-art 32-bit analog-to-digital-converter and the use of

2019 European Association of Geoscientists & Engineers, Geophysical Prospecting, 67, 1582–1594


1594 S.L. Helwig, W. Wood and B. Gloux

ultra-low-noise electrodes with matching amplifiers to measure the electric field. The largest gain, however, is obtained by having access to many receivers that are much simpler and faster to operate. This is the enabler to efficiently acquire 3D-CSEM surveys. The source is based on a modular design and is typically used with a 5000-A output. If necessary, higher currents can be obtained by simply adding more modules provided that the overall load is within the capabilities of the vessel. The discussion on measurement uncertainty illustrates the fundamental fidelity of the system with equipment-related uncertainties typically well below 1% of the observed signal which provides an excellent opportunity to detect targets at depth.

ACKNOWLEDGEMENTS The authors thank PetroMarker for the permission to publish this paper, Peter van der Sman for the fruitful discussions and the editor as well as the anonymous reviewers for their constructive remarks.

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Eidesmo T., Ellingsrud S., MacGregor L.M., Constable S., Sinha M.C., Johansen S., et al. 2002. Sea bed logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers in deepwater areas. First Break 20, 144–152. Ellingsrud S., Eidesmo T., Johansen S., Sinha M., Macgregor L. and Constable S. 2002. Remote sensing of hydrocarbon layers by seabed logging (SBL): results from a cruise offshore Angola. The Leading Edge 21, 972–982. Frafjord O., Eide K., El Kaffas A., Helwig S. and Holten T. 2016. A 2.5D comparison between two CSEM methods: 78th EAGE Conference and Exhibition, Expanded Abstracts. Frafjord O., Holten T., El Kaffas A., Borven J. and Helwig S. 2014. Minimizing the noise contribution in vertical electric field measurements: 76th EAGE Conference and Exhibition, Expanded Abstracts. Gloux B., Helwig S. and Wood W. 2016. Apparatus for orienting an electromagnetic field sensor, and related receiver unit and method. Norwegian patent 342689. Goldman M., Mogilatov V., Levi E., Tezkan B. and Haroon A. 2015. Signal detectability of marine electromagnetic methods in the exploration of resistive targets. Geophysical Prospecting 63, 192–210. Helwig S., Wood W., Gloux B. and Holten T. 2016. A new generation of vertical CSEM receiver. 78th EAGE Conference and Exhibition, Expanded Abstracts. Holten T., Flekkoy E., Singer B., Blixt E., Hanssen A. and Maloy K. 2009. Vertical source, vertical receiver, electromagnetic technique for offshore hydrocarbon exploration. First Break 27, 89–93. Kjerstad J. 2010. Device for a vertical electromagnetic field component receiver. World patent WO2010041959A1. Maaø F. and Nguyen A. 2010. Enhanced subsurface response for marine CSEM surveying. Geophysics 75, A7–A10. MacGregor L., Nicholls J., Newall Pratt D. and Sinha M. 2008. Electromagnetic detector for marine surveying. World patent WO 2008152361 A1. Mittet R. and Morten J.P. 2012. Detection and imaging sensitivity of the marine CSEM method. Geophysics 77, E411–E425. Srnka L.J. 1986. Method and apparatus for offshore electromagnetic sounding utilizing wavelength effects to determine optimum source and detector positions. US patent 4,617,518. Webb S., Constable S., Cox C. and Deaton T. 1985. A seafloor electric field instrument. Journal of Geomagnetism and Geoelectricity 37, 1115–1129. Young P. and Cox C. 1981. Electromagnetic active source sounding near the East Pacific Rise. Geophysical Research Letters 8, 1043–1046.

2019 European Association of Geoscientists & Engineers, Geophysical Prospecting, 67, 1582–1594

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