Controlled-source Electromagnetic Sounding Research Articles

Study on the Anisotropy of Formation Resistivity by Dual Controlled-source Electromagnetic Sounding

We proposed a method to obtain the anisotropy of formation resistivity by dual controlled-source electromagnetic sounding. The two field sources are arranged in two orthogonal directions outside the measuring area: the two grounded dipole sources excite two uncorrelated signals current step by step alternately, and record the current into time series; At sounding station of survey area, the orthogonal horizontal electric field response signals or the vertical magnetic field response signals were recorded into time series; according to the recorded current time series, As well as the time series of electromagnetic response signal, and the parameters of working layouts such as transceiver distance and so on, the whole zone apparent resistivity of the sounding station were calculated by circular cross-correlation system identification method, and then the anisotropy coefficient of the formation resistivity was calculated according to the whole zone apparent resistivity. This method has the advantages of fewer observation components, simple processing, strong anti-interference ability, and high working efficiency.

The range of induction-coil magnetic field sensors for geophysical explorations

This paper reports the long-term teamwork results obtained by an expert group from the Radiophysical Research Institute (NIRFI) and VEGA Geophysics Ltd. (VEGA, St. Petersburg) in the course of designing modern induction-coil magnetic field sensors. A comparative analysis of the world’s most famous sensor models is carried out, and basic design and testing methods and features are considered. The range of low-noise induction-coil magnetometers (IMS-007, IMS-008, and IMS-009) designed by the authors for scientific and geophysical ground-based explorations is described. Research results are obtained from using the sensors for geothermal water exploration by broadband magnetotelluric sounding, investigation of the deep structure of the lithosphere by controlled source electromagnetic sounding, and monitoring of the Earth’s natural electromagnetic field.

Modelling the low-frequency electrical properties of pyrite-bearing reservoir sandstones

Marine controlled source electromagnetic sounding has developed rapidly in recent years to complement conventional seismic method for better discrimination of pore fluid. To obtain quantitative and reliable interpretation of the CSEM data, a robust rock physics model is vital to link the bulk rock electrical conductivity to the electrical properties and microstructure of the rock constituents, especially when some of the constituents exhibit extreme electrical behaviours (e.g., pyrite, a common mineral associated with reservoir rocks). Based on the multi-phase incremental model validated on published experimentally measured electrical conductivity of pyrite-bearing sandstones, the effects of key parameters, i.e., pyrite content, porosity, pyrite conductivity, grain aspect ratio and water saturation on the electrical conductivity of pyrite-bearing sandstones and their corresponding CSEM responses were comprehensively studied. The results are expected to assist in the CSEM data interpretation when a pyrite-bearing sandstone reservoir is encountered in the future.

A seafloor electromagnetic receiver for marine magnetotellurics and marine controlled-source electromagnetic sounding

In planning and executing marine controlled-source electromagnetic methods, seafloor electromagnetic receivers must overcome the problems of noise, clock drift, and power consumption. To design a receiver that performs well and overcomes the abovementioned problems, we performed forward modeling of the E-field abnormal response and established the receiver’s characteristics. We describe the design optimization and the properties of each component, that is, low-noise induction coil sensor, low-noise Ag/AgCl electrode, low-noise chopper amplifier, digital temperature-compensated crystal oscillator module, acoustic telemetry modem, and burn wire system. Finally, we discuss the results of onshore and offshore field tests to show the effectiveness of the developed seafloor electromagnetic receiver and its performance: typical E-field noise of 0.12 nV/m/rt(Hz) at 0.5 Hz, dynamic range higher than 120 dB, clock drift lower than 1 ms/day, and continuous operation of at least 21 days.

Review paper: Instrumentation for marine magnetotelluric and controlled source electromagnetic sounding

ABSTRACTWe review and describe the electromagnetic transmitters and receivers used to carry out magnetotelluric and controlled source soundings in the marine environment. Academic studies using marine electromagnetic methods started in the 1970s but during the last decade these methods have been used extensively by the offshore hydrocarbon exploration industry. The principal sensors (magnetometers and non‐polarizing electrodes) are similar to those used on land but magnetotelluric field strengths are not only much smaller on the deep sea‐floor but also fall off more rapidly with increasing frequency. As a result, magnetotelluric signals approach the noise floor of electric field and induction coil sensors (0.1 nV/m and 0.1 pT) at around 1 Hz in typical continental shelf environments. Fluxgate magnetometers have higher noise than induction coils at periods shorter than 500 s but can still be used to collect sea‐floor magnetotelluric data down to 40–100 s. Controlled source transmitters using electric dipoles can be towed continuously through the seawater or on the sea‐bed, achieving output currents of 1000 A or more, limited by the conductivity of seawater and the power that can be transmitted down the cables used to tow the devices behind a ship. The maximum source‐receiver separation achieved in controlled source soundings depends on both the transmitter dipole moment and on the receiver noise floor and is typically around 10 km in continental shelf exploration environments. The position of both receivers and transmitters needs to be navigated using either long baseline or short baseline acoustic ranging, while sea‐floor receivers need additional measurements of orientations from compasses and tiltmeters. All equipment has to be packaged to accommodate the high pressure (up to 40 MPa) and corrosive properties of seawater. Usually receiver instruments are self‐contained, battery powered and have highly accurate clocks for timekeeping, even when towed on the sea‐floor or in the water column behind a transmitter.

人工信号源電磁探査法の海底熱水鉱床への適用可能性

人工信号源電磁探査法の海底熱水鉱床への適用可能性

Controlled-source electromagnetic sounding in shallow water: Principles and applications

The marine controlled-source electromagnetic (CSEM) method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings. Until recently, its use was confined to deepwater (water depths greater than approximately [Formula: see text]) because of the interaction of signals with the atmosphere in shallower water depths. The purpose of this study was to investigate, using a simple 1D analytical analysis, the physics of CSEM in shallow water. This approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency-domain or time-domain methods. Stepping away from wavelike approaches, which if applied without care can be misleading for the diffusive fields of CSEM, we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the EM signal into modes and using only the mode least affected by interaction with the atmosphere. Such decomposition is straightforward in a 1D earth, and we demonstrate that the approach remains valid in higher dimensional structures. We also show that the coupling between signals diffusing through the earth and those that have interacted with air can be used to our advantage in the interpretation of marine CSEM data.

Electrical resistivity structure of the Valu Fa Ridge, Lau Basin, from marine controlled-source electromagnetic sounding

SUMMARY In December 1995 we carried out a comprehensive controlled-source electromagnetic survey of the Valu Fa Ridge at 22u25k Si n the Lau Basin. The Valu Fa Ridge is ab ack-arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity. Seismic studies have imaged a melt lens at an average depth of 3.2 km below the seafloor, surrounded by a zone of lowered seismic velocity, interpreted as a region of partial melt in the crust. The electromagnetic experiment was part of a multidisciplinary study which included wide-angle and reflection seismics, bathymetry and potential field measurements. Electromagnetic signals at frequencies between 0.25 and 40 Hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea-bottom receivers at ranges of up to 20 km from the source. Over 80 hr of data, consisting of the magnitude of the horizontal electric field at the seafloor, were collected. These data have extremely low scatter compared to similar data from previous surveys. The data were interpreted using a combination of 1- and 2-D forward modelling and inversion. The vertical resistivity gradient in the upper crust at the Valu Fa Ridge is abnormally low, with resistivities of less than 10 V m observed throughout layer 2 of the crust to a depth of 3 km. This is significantly more conductive at depth than the axis of the slow-spreading Reykjanes Ridge at 57u45kN, and the fastspreading East Pacific Rise at 13uN, where similar data sets have been collected in the past. Although the structure of layer 2 is well constrained by the electromagnetic data, its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it, and hence the data are not sensitive to the structure in layer 3, in particular the structure of the melt lens or surrounding low-velocity zone. The seismic velocity structure of the Valu Fa Ridge, determined from the coincident wide-angle seismic study, is similar to that observed at other mid-ocean ridges, with a steep seismic velocity gradient through layer 2 (although overall velocities are slightly lower). The seismic velocity anomaly calculated relative to an average off-axis structure is also small. This suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity. However, hot and/or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity. Since the conductive region extends unbroken from 3 km depth to the seafloor, it is probable that these fluids circulate to (or close to) the magma chamber itself.

Magmatic processes at slow spreading ridges: implications of the RAMESSES experiment at 57° 45′N on the Mid-Atlantic Ridge

SUMMARY This paper is the first in a series of three (this issue) which present the results of the RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics). RAMESSES was an integrated geophysical study which was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57°45aeN. It consisted of three major components: wide-angle seismic profiles along and across the AVR, using ocean-bottom seismometers, together with coincident seismic reflection profiles; controlled-source electromagnetic sounding (CSEM); and magnetotelluric sounding (MT). Supplementary data sets included swath bathymetry, gravity and magnetics. Analyses of the major components of the experiment show clearly that the sub-axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crustal magma body, at a depth of 2.5 km below the sea surface. The magma body is characterized by low seismic velocities constrained by the wide-angle seismic data; a seismic reflection from its upper surface; and a region of anomalously low electrical resistivity constrained by the CSEM data. It includes a thin, ribbon-like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone, which flanks and underlies the melt lens. RAMESSES is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge. The result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion, but are short-lived rather than steady-state features. The magma body can exist for only a small proportion of a tectono-magmatic cycle, which controls crustal accretion, and has a period of at least 20 000 years. These findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle, and of its delivery into the crust, beneath slow-spreading mid-ocean ridges.

The RAMESSES experiment-III. Controlled-source electromagnetic sounding of the Reykjanes Ridge at 57°45′N

SUMMARY A controlled-source electromagnetic sounding survey centred on an axial volcanic ridge (AVR) segment of the Reykjanes Ridge at 57°45aeN was performed as part of the RAMESSES experiment. Low-frequency (0.35‐11 Hz) electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source, which was a 100 m long horizontal electric dipole towed at heights of 50‐80 m from the seafloor. Coincident seismic and magnetotelluric studies were conducted during the rest of the RAMESSES experiment. Data were interpreted using a combination of 1-D forward modelling and inversion, and iterative forward modelling in two dimensions. On the axis of the AVR, the resistivity at the seafloor is 1 V m. There is a steep resistivity gradient in the upper few hundred metres of the crust, with the resistivity reaching approximately 10 V ma t a depth of 500 m. In order to explain the low resistivities, the upper layer of the crust must be heavily fractured and saturated with sea water. The resistivity increases with distance from the axis as the porosity decreases with increasing crustal age. The most intriguing feature in the data is the large diVerence in amplitude between fields transmitted along and across the AVR axis. A significant zone of low-resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diVerence. It is coincident with the low-velocity zone required by the seismic data, and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study. The low-resistivity zone can be explained by the presence of a body of partially molten basalt in the crust. Taken together, these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid-ocean ridge. The data constrain the melt fraction within the body to be at least 20 per cent, with a melt volume suYcient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years. Since this body would freeze in the order of 1500 years, this finding lends support to the hypothesis that, at slow spreading rates, crustal accretion is a cyclic process, accompanying periodic influxes of melt from the mantle to a crustal melt reservoir.

Apparent resistivity curves in controlled-source electromagnetic sounding directly reflecting true resistivities in a layered Earth

Apparent resistivity curves in controlled-source electromagnetic sounding directly reflecting true resistivities in a layered Earth

Electromagnetic sounding in the Columbia Basin, Yakima, Washington

A controlled‐source electromagnetic (CSEM) survey was conducted near Yakima, Washington, to map the thickness and resistivity of a thick volcanic sequence overlying a sedimentary section and to infer structure in the sediments. The survey was conducted with a frequency‐domain system employing loop transmitters 400–500 m on a side and three‐component SQUID magnetometer receivers separated from the loop by 1 to 5 km. Data collected along a 30 km profile orthogonal to regional strike were interpreted initially with 1-D layered models, which were then pieced together to make a geoelectric section. Induction logs in a 5000 m exploration hole at one end of the profile agree very well with the CSEM soundings made around the hole. The geoelectric section reveals a smoothly varying thickness of volcanics with a pronounced anticlinal structure approximately concordant with a surface topographic ridge. To assess the validity of inferring lateral structure from 1-D interpretations, we made scale models of an anticlinal structure and of a surface inhomogeneity and conducted CSEM measurements over the scale models. Layered‐model inversions of these data show that the anticlinal structure and its location are very well determined by 1-D inversion, but its height and width are not accurately determined. CSEM sounding over the surface inhomogeneity model shows that this feature does not significantly degrade the interpretation of a deep target layer. In this setting, a geoelectric section made up from 1-D interpretations provides good qualitative measures of subsurface structure and also provides excellent starting models for detailed 2-D or 3-D modeling.

Controlled-source electromagnetic sounding of the oceanic lithosphere

The attenuation of ionospheric signals in the frequency range 0.06–24 Hz by sea water effectively precludes using the magnetotel-luric method to study the electrical structure of the upper oceanic lithosphere. We have carried out a dipole–dipole electromagnetic sounding in the North Pacific by injecting electromagnetic signals into the ocean and sea bed. The crust at the site is 25 Myr old and has a thin sediment cover. The technique, similar to that used in earlier work1,2, involves dragging a horizontal dipole antenna along the sea floor. The electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10–65 km. As the ambient electric field is very small (varying from 10−18 V2 m−2 Hz−1 at 0.1 Hz to 10−24 V2 m2 Hz−1 above 1 Hz; ref. 3), the controlled-source signals could be easily monitored. Our data are consistent with a simple one-dimensional Earth model consisting of a 3–7-km-thick crustal layer of moderate conductivity (∼0.001 S m−1) underlain by a thicker region of very low conductivity (<2 × 10−5 S m−1). The results suggest an upper mantle water content of at most 0.1% by volume.

A seafloor electric field instrument.

A seafloor instrument has been constructed to measure a horizontal component of the electric field, using a long antenna (up to 1000m) to increase sensitivity. The instrument has a depth capability of 5200m and may be deployed for several months at a time, collecting over 14 Mbytes of data under the control of a microprocessor. Instrumental noise is mainly caused by the Ag-AgCl electrodes used to make contact with the sea water, and is about 10-24V2/m2Hz at frequencies above 1Hz, following an f-2 power law at frequencies (f) below 1Hz. For controlled source signals averaging techniques such as synchronous stacking and block averaging have been used to detect narrow band (5.6×10-4Hz) signals as small as 10-12V/m. The instrument is designed to include the pressure transducer described by COX et al. (1984) and has already been used for the study of ocean bottom seismic signals, magnetotelluric experiments and controlled source electromagnetic soundings.

Geophysical applications of SQUIDS

Present and potential geophysical applications of Superconducting Quantum Interference Devices (SQUIDs) include remote reference magnetotellurics, controlled-source electromagnetic sounding, airborne gradiometry, gravity gradiometers, rock magnetism, paleomagnetism, piezomagnetism, tectonomagnetism, the location of hydrofractures for hot dry rock geothermal energy and enhanced oil and gas recovery, the detection of internal ocean waves, and underwater magnetotellurics.