Electromagnetic Sounding System Research Articles

Pulsed Electromagnetic Sounding of the Bazhenov Formation: High-Performance Computing to Justify a New Geophysical Technology

Abstract —The work is devoted to the theoretical substantiation of a new geophysical technology for studying a unique geological object with unconventional hard-to-recover hydrocarbon reserves. The technology is based on transient electromagnetic sounding from a spatially distributed system of highly deviated wells drilled in target objects near the Bazhenov Formation. The results of computer modeling predetermine a new direction for geological exploration of unconventional hydrocarbon deposits. We consider a numerical solution to the 3D direct problem of pulsed electromagnetic sounding and, on its basis, develop a computational scheme and computer program. A mathematical model is built that describes the sensing process through a pulsed source for electromagnetic field excitation. The Fourier transform is used for time discretization, and the vector finite element method for spatial discretization. This approach makes it possible to obtain many independent 3D problems and effectively solve them in parallel by applying the modern multiprocessor technology. Using the KNL and Broadwell computing nodes of the Siberian Supercomputer Center SB RAS, we performed calculations of electromagnetic signals, which showed a high efficiency of the devised computing scheme and a high performance of the implemented algorithm. Despite the fact that the total peak performance of the KNL nodes is 2.5 times higher than that of the Broadwell nodes, their practical application for performing large-scale 3D modeling on the cluster shows a high efficiency of the latter. When choosing the most suitable computing architecture for the implementation of mass calculations, one should not rely on their formal characteristics only; significant performance is achieved taking into account the peculiarities of the computational methods employed for solving a specific problem. The implemented more efficient ways of performing parallel matrix-vector operations did not significantly increase the performance for this computational scheme. The created computational tools form the basis for further design of the configuration of a pulsed electromagnetic sounding system, and for identifying the capabilities of the new geophysical technology for examining complex geological media.

On the Voltage Response of Homogeneous Earth Models in Central Loop Electromagnetic Sounding

An accurate series-form explicit expression is derived for the voltage induced in the small receiving loop of a central loop electromagnetic sounding system positioned above a homogeneous earth model. The solution is obtained by converting the semi-infinite integral representation for the vertical magnetic field induced at the center of the two loops into a series of simpler integrals, amenable to analytical evaluation. The developed formulation is based on replacing the exponential term in the integrand of the field integral with its power series expansion with respect to the difference of the squares of the wavenumbers in air and in the conducting medium. As a result, the vertical magnetic field and the induced voltage are finally expressed as sums of the spherical Hankel functions depending on geometrical parameters and the wavenumbers. The derived solution exhibits advantages in terms of both time cost and accuracy, when compared to the conventional numerical schemes for evaluating the Sommerfeld-type integrals and to the previously published quasistatic solution to the considered problem.

The study of time domain denoising for the time-frequency electromagnetic method prospecting data

The time-frequency electromagnetic prospecting system has both the characteristics of frequency sounding and transient electromagnetic sounding system, can obtain apparent resistivity both in frequency domain and time domain, therefore, thus, more accurate and credible geoelectric information can be obtained. After analyzing, there are mainly three typical types of noise in time-frequency electromagnetic observed data, they are 50Hz power frequency and its harmonics interference noise, high-frequency random impulse noise and low frequency interference noise. In view of characteristics of these three types of noise and the time-frequency electromagnetic prospecting signal, we adopted frequency domain band stop filtering method remove the 50 Hz power frequency and its harmonics interference noise, proposed a segmentation and extension median filtering method and a fitting fixed extreme EMD method to remove the high frequency random impulse noise and the low frequency interference noise separately, and proposed a median filtering window size test selection method based on variance and skewness coefficient of sample signal. Moreover, we designed a denoising processing fl ow as handle 50Hz power frequency and its harmonics interference noise at fist step, handle high frequency random impulse noise at second step, handle low frequency interference noise at last step, it can give full advantages of the denoising handling methods. By theoretical analysis and verification by practical observation data experiments, the methods what we adopted and proposed and the processing flow we designed can effectively remove the three types of interference noises effectively and avoid method error, can preserve phase and amplitude information of effective signal accurately, finally, high quality and satisfactory time domain signal waveform can obtained as a result, and lays a good foundation for extracting accurate transient electromagnetic attenuation curve in time domain for the Time-Frequency electromagnetic prospecting data.

Validation of SMOS sea ice thickness retrieval in the northern Baltic Sea

The Soil Moisture and Ocean Salinity (SMOS) mission observes brightness temperatures at a low microwave frequency of 1.4 GHz (L-band) with a daily coverage of the polar regions. L-band radiometry has been shown to provide information on the thickness of thin sea ice. Here, we apply a new emission model that has previously been used to investigate the impact of snow on thick Arctic sea ice. The model has not yet been used to retrieve ice thickness. In contrast to previous SMOS ice thickness retrievals, the new model allows us to include a snow layer in the brightness temperature simulations. Using ice thickness estimations from satellite thermal imagery, we simulate brightness temperatures during the ice growth season 2011 in the northern Baltic Sea. In both the simulations and the SMOS observations, brightness temperatures increase by more than 20 K, most likely due to an increase of ice thickness. Only if we include the snow in the model, the absolute values of the simulations and the observations agree well (mean deviations below 3.5 K). In a second comparison, we use high-resolution measurements of total ice thickness (sum of ice and snow thickness) from an electromagnetic (EM) sounding system to simulate brightness temperatures for 12 circular areas. While the SMOS observations and the simulations that use the EM modal ice thickness are highly correlated (r2=0.95), the simulated brightness temperatures are on average 12 K higher than observed by SMOS. This would correspond to an 8-cm overestimation of the modal ice thickness by the SMOS retrieval. In contrast, if the simulations take into account the shape of the EM ice thickness distributions (r2=0.87), the mean deviation between simulated and observed brightness temperatures is below 0.1 K.

Evaluation of management practices in forest soil environments using a multi-frequency electromagnetic sounding system

Evaluation of management practices in forest soil environments using a multi-frequency electromagnetic sounding system

On the correlation of electrical conductivity and heat flow in Middle Valley, Juan de Fuca Ridge

The near‐surface electrical conductivity has been mapped within an area of Middle Valley, a sediment‐filled axial valley at the northern end of the Juan de Fuca ridge. The conductivity in the uppermost 20 m of sediment was determined by analyzing the magnetic field signal transmitted between a source coil and a receiver that were towed along the seafloor. The instrument is a version of a time domain electromagnetic (TDEM) sounding system. The heat flow pattern within Middle Valley, with a large central anomaly above a basement high, is reproduced by the conductivity measurements, the result of enhanced pore fluid electrical conductivity due to higher near‐surface temperatures in the high heat flow regions. The high correlation between heat flow and conductivity requires that porosity as inferred from Archie's law must be nearly uniform in the central part of the study area. Porosities derived from the conductivity measurements are in close agreement with measurements from the Ocean Drilling Project (ODP) boreholes. Higher near‐surface porosities are required in the eastern part of the valley to match the observed heat flow, consistent with the higher porosity seen at ODP site 855. A small region of apparently lower porosity was detected to the west of the center of the valley.

Rapid, high-accuracy electromagnetic soundings using a novel four-axis coil to measure magnetic field ellipticity

A unique high-resolution electromagnetic sounding system, utilizing a four-axis coil and a new calibration technique, has been designed, constructed, and tested. The resulting measurements have an accuracy of approximately ±0.1%. The four-axis coil, which includes three approximately orthogonal coils and a calibration coil, is designed to automatically collect ellipticity data at 20 discrete frequencies in the range 30 Hz–30 kHz. The approximately orthogonal coils detect three components of the magnetic field and the calibration coil injects a calibration signal into the receiving system at the same time that the data are being recorded. A three-dimensional mathematical rotation program was developed to calculate ellipticity, removing the need to know the absolute coil orientation. Equations were formulated for the angular relationships between the approximately orthogonal coils. The actual intercoil angles were computed using a nonlinear optimization analysis applied to the measured data. The system was tested by measuring the ellipticity at one point on the earth's surface for various orientations of the coil. In addition, a close match between calculated and measured ellipticity profiles was obtained in a field test over a known target.

First measurements from a deep-tow transient electromagnetic sounding system

An electromagnetic sounding system has been developed to map the shallow electrical conductivity structure of the deep sea floor. The instrument consists of a magnetic source and several colinear magnetic receivers forming an array which is towed along the seafloor. The source generates a time varying magnetic field; the shape of the resulting magnetic field waveform at the receivers depends on the electrical conductivity below the seafloor between the receivers and the source. The instrument can be towed systematically over a study area under acoustic transponder or GPS navigation to construct a map of the electrical conductivity. Towing speeds of greater than 1 m s−1 (2 knots) can be achieved without adversely effecting data quality. The instrument is sufficiently robust to survive continual contact with thinly sedimented, abrasive basalt. We present the first results from a deployment in August, 1990 near the Cleft Segment of the Juan de Fuca Ridge along an 8 km track to the west of the spreading center. Unforeseen problems with the instrument restricted the utility of the measurements for constructing detailed vertical conductivity profiles, but the measurements were adequate to determine an average conductivity in the upper 25 m, at more than 70 stations. The conductivity was found to vary from 0.1 to 0.4 S/m along the track.

Sea‐ice thickness measurement using a small airborne electromagnetic sounding system

The evaluation of a small electromagnetic induction sounding system for use in airborne measurement of sea‐ice thickness is discussed, as are the results from arctic field testing. Also outlined are the system noise and drift problems encountered during arctic field evaluation, problems which adversely affected the quality of the sounding data. The sea‐ice sounding results indicate that for ice floes with moderate relief it should be possible to determine thickness to within 5 percent, but that because of sounding footprint size and current model algorithm constraints, steep‐sided pressure ridge keels cannot be well defined. The findings also indicate that with further system improvement the day of routine sea‐ice thickness profiling from an airborne platform is close at hand.

An active source electromagnetic sounding system for marine use

Instrumentation has been developed for carrying out active source electromagnetic sounding experiments in the deep oceans. Experiments of this type are directly and uniquely sensitive to the presence of molten or partially molten material, to temperature structure and to the porosity of upper crustal rocks such as those that accommodate hydrothermal circulation systems. Electromagnetic sounding experiments therefore represent an extremely desirable addition to the existing range of geophysical techniques for studying geological processes in thermally, hydrothermally or magmatically active regions—for example, at oceanic spreading centres.

Airborne electromagnetic sounding of sea ice thickness and sub-ice bathymetry

A study was made in May 1985 to determine the feasibility of using an airborne electromagnetic sounding system for profiling sea ice thickness and the sub-ice water depth and conductivity. The study was made in the area of Prudhoe Bay, Alaska. The multifrequency airborne electromagnetic sounding system consisted of control and recording electronics and an antenna. The electronics module was installed in a helicopter, and the 7 m long tubular antenna was towed beneath the helicopter at about 35 m above the ice surface. For this electromagnetic system, both first-year and second-year sea ice could be profiled, but the resolution of ice thickness decreased as the ice became rough. This decrease was associated with the large footprint of the system, which effectively smoothed out the sea ice relief. Under-ice water depth was determined, as was seawater conductivity. The results of the feasibility study were encouraging, and further system development is therefore warranted.

The deep transient electromagnetic sounding technique: First field test in Australia

A test survey was carried out in the Sydney Basin using a wire source, loop receiver, deep transient electromagnetic sounding system. A field system was designed and built, the data acquisition system automated to simplify field operation, and current US processing software adapted to the hardware. A test survey was conducted in the Sydney Basin over horizontally layered strata with resistivity contrasts. Three hundred transients were recorded at eight stations within two days. After processing and interpretation, the geoelectric models obtained from the inversions qualitatively correlate with the basin geology, to a depth of 2 km. These measurements suggest that the deep transient electromagnetic sounding method could be successfully used for deep mineral and oil and gas exploration problems in Australia.

Megasource time‐domain electromagnetic sounding methods

The Colorado School of Mines time‐domain electromagnetic (EM) sounding system makes use of a grounded length of cable powered with high‐amplitude current square waves to generate an EM field for probing the earth. The vertical component of magnetic induction is detected at a sounding site located at a relatively large distance compared to the desired depth of investigation. With a source moment of a million ampere meters or greater, offset distances of several tens of kilometers can be achieved easily, providing depths of investigation of up to 10 km. The recorded induction field versus time curves are routinely interpreted by comparison with computer‐generated theoretical curves for a layered earth. Megasource EM surveys have been carried out at The Geysers in northern California and near Yakima in central Washington, providing apparently meaningful information on the electrical structure in these areas at depths as great as 10 km.

Development and applications of a wide band electromagnetic sounding system using a pseudonoise source : Duncan, P M; Hwang, A; Edwards, R N Geophysics, V45, N8, Aug 1980, P1276–1296

Development and applications of a wide band electromagnetic sounding system using a pseudonoise source : Duncan, P M; Hwang, A; Edwards, R N Geophysics, V45, N8, Aug 1980, P1276–1296

The development and applications of a wide band electromagnetic sounding system using a pseudo‐noise source

A very wide band (0.03 Hz to 15 kHz) electromagnetic (EM) system has been developed. It was used over the band 1 Hz to 10 kHz to determine the electrical structure of the earth’s crust from depths of a few meters to over 40 km. A direct current of from 1 to 5 A was reversed through a long wire bipole transmitter in a pseudo‐random binary sequence (PRBS). Depending upon the frequencies selected, a sensitive one‐component flux gate magnetometer or an air‐core coil was used to monitor temporal changes in the vertical component of the magnetic field at a recording site. The measured signal was crosscorrelated digitally in real time with an exact copy of the transmitted waveform in order to obtain a good signal‐to‐noise ratio at distances up to 5 times the length of the bipole. The output crosscorrelogram was deconvolved from the system input, the autocorrelogram of the transmitted waveform, using a Wiener least‐squares filter to give the impulse response of the earth. This was then transformed into frequency domain to yield directly the phase and amplitude transfer function, which was inverted in a routine manner with a Marquardt algorithm to find layer conductivities and thicknesses. Two field experiments are described. The first demonstrates the viability of the technique to sound a simple layered section of interbedded shale and limestone. It was a relatively shallow sounding to a depth of about 500 m in the sedimentary basin of southern Ontario. The second experiment was designed to search for a conductive zone in the lower crust. A bipole over 20 km long carried a transmitter current of only 5 A, yet field data were obtained over the range 35 to 70 km. The primary magnetic fields never exceeded 10 mgamma and were often considerably smaller. The interpretation indicates a conductive layer of resistivity less than 270 Ω-m at a depth of about 20 km.

A study of topographic and misorientation effects in multifrequency electromagnetic soundings

A theoretical analysis has been made on the effects of rugged topography and misorientation of loops on the observed results in the case of a new ground multifrequency electromagnetic sounding system which uses a horizontal loop as the transmitter. The system measures the ratios of the amplitudes of the vertical and the horizontal magnetic fields at a receiver for several frequencies. Three possible sources of error have been considered, namely, the effects of a sloping ground, the effects of an altitude difference between the transmitter and the receiver loops and the effects of a tilt of the axis of the transmitter loop away from the vertical. The results have been presented in terms of the percentage variations of the ratio parameters with the variation of a normalized frequency parameter. It has been shown that the errors are maximum in the low frequency range where the observed ratio values may be too low. It is also clear that in the case of an altitude difference, it is important to know which loop is at a higher altitude. When the axis of the transmitter loop is not vertical, the values of the measured ratios are dependent on the direction of the tilt, i.e., whether the tilt is towards or away from the receiver. The study indicates that it may be possible to apply a first order correction to the field data if information about the topography and the geology of the area is known.

ELECTROMAGNETIC DEPTH SOUNDING EXPERIMENT ACROSS SANTA CLARA VALLEY

An electromagnetic depth sounding experiment with a horizontal loop carrying an oscillating current was carried out in Santa Clara Valley, California. The field data are interpreted in terms of the polarization parameters of the magnetic polarization ellipse. The chosen parameters are tilt‐angle, ellipticity, and the modulus of wavetilt. The electrical discontinuities deduced are in general agreement with well data and with a geologic section based on resistivity soundings. The results clearly reveal an intermediate, highly resistive layer, which is a permeable stratum for groundwater recharge. It is concluded that a portable electromagnetic sounding system, measuring only tilt‐angle and ellipticity, should easily locate highly resistive gravel deposits bounded by conductive clay beds. It is theoretically obvious that the system is useful in those regions where it is extremely difficult to inject current into the ground.