Modern biomedical amplifiers have a very high common mode rejection ratio. Nevertheless, recordings are often contaminated by residual power-line interference. Traditional analogue and digital filters are known to suppress ECG components near to the power-line frequency. Different types of digital notch filters are widely used despite their inherent contradiction: tolerable signal distortion needs a narrow frequency band, which leads to ineffective filtering in cases of larger frequency deviation of the interference. Adaptive filtering introduces unacceptable transient response time, especially after steep and large QRS complexes. Other available techniques such as Fourier transform do not work in real time. The subtraction procedure is found to cope better with this problem. The subtraction procedure was developed some two decades ago, and almost totally eliminates power-line interference from the ECG signal. This procedure does not affect the signal frequency components around the interfering frequency. Digital filtering is applied on linear segments of the signal to remove the interference components. These interference components are stored and further subtracted from the signal wherever non-linear segments are encountered. Modifications of the subtraction procedure have been used in thousands of ECG instruments and computer-aided systems. Other work has extended this procedure to almost all possible cases of sampling rate and interference frequency variation. Improved structure of the on-line procedure has worked successfully regardless of the multiplicity between the sampling rate and the interference frequency. Such flexibility is due to the use of specific filter modules. The subtraction procedure has largely proved advantageous over other methods for power-line interference cancellation in ECG signals.

/pdf/removal-of-power-line-interference-from-the-ecg-a-review-of-2v5vi0o4vy.pdf

Removal of power-line interference from the ECG: a review of the subtraction procedure

We present a series of field experiments showing the transient electric fields generated by a seismic excitation of the subsurface. After removing the powerline noise by adaptive filtering, the most prominent feature of the seismoelectric recordings is the presence of electric signals very similar to conventional seismic recordings. In one instance, we identified small-amplitude precursory electromagnetic disturbances showing a polarity reversal on either side of the shotpoint. Concentrating on the dominant seismoelectric effect, we theoretically show that the electric field accompanying the compressional waves is approximately proportional to the grain acceleration. We also demonstrate that the magnetic field moving along with shear waves is roughly proportional to the grain velocity. These relationships hold true as long as the displacement currents are much smaller than the conduction currents (diffusive regime), which is normally the case in the low-frequency range used in seismic prospecting. Furthermore, the analytical transfer functions thus obtained indicate that the electric field is mainly sensitive to the salt concentration and dielectric constant of the fluid, whereas the magnetic field principally depends on the shear modulus of the framework of grains and on the fluid's viscosity and dielectric constant. Both transfer functions are essentially independent of the permeability. Our results suggest that the simultaneous recording of seismic, electric, and magnetic wavefields can be useful for characterizing porous layers at two different levels of investigation: near the receivers and at greater depth.

Seismoelectric wave conversions in porous media: Field measurements and transfer function analysis

[1] Lightning discharges generate broadband electromagnetic pulses with a peak component in the very low frequency (VLF; 3–30 kHz) range. VLF waves propagate through the Earth-ionosphere waveguide with relatively low attenuation, enabling the detection of these radio atmospherics at great distances from the lightning discharge. A new technique of long-range (≤6000 km) global lightning geolocation via sferic detection is presented. This new technique catalogs the dominant variation in expected received waveforms in a set of waveform banks, which are then used to estimate the propagation distance and accurately determine the arrival time. Using three sensors in a trial network, this new technique is used to demonstrate a median accuracy of 1–4 km, depending on the time of day. An overall cloud-to-ground (CG) stroke detection efficiency between ∼40 and 60% is estimated by correlating individual lightning stroke events to data from the National Lightning Detection Network (NLDN). Additional events reported by the trial network are shown to have a tight spatial clustering to storm clusters identified by NLDN, suggesting that many of the unmatched events correspond to weak cloud-to-ground strokes, M components, or cloud pulses. Exploiting an empirical correlation between peak VLF field strength and peak current values reported by NLDN, we also provide unvalidated estimates of the peak current and lightning channel polarity. The trial network does not distinguish between cloud and ground discharges, so these peak current estimates only relate to an Earth-referenced channel current for the subset of reported events that are return strokes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010JD013863

Long-range lightning geolocation using a VLF radio atmospheric waveform bank

[1] We present a full-waveform modeling technique of the coupled seismoelectromagnetic wave propagation in fluid-saturated stratified porous media. Our simulation code uses the macroscopic governing equations derived by Pride [1994], which couple Biot's theory and Maxwell equations via flux/force transport equations. In this theory the coupling mechanism is explained by electrokinetic effects taking place at the pore level. The synthetic seismoelectrograms and seismomagnetrograms are computed by extending the generalized reflection and transmission matrix method and by using a discrete wave number integration of the global reflectivity obtained in the frequency wave number domain. Synthetic time sections and snapshots of the wave propagation are used to study the seismic, electromagnetic, and seismoelectromagnetic waves properties in fluid-saturated layered porous media. Two wave phenomena are investigated: (1) the electric and magnetic fields induced by the propagation of a seismic perturbation in a homogeneous porous medium and (2) the electromagnetic waves generated at depth when seismic waves propagate through a vertically heterogeneous porous medium. Concentrating on the second effect, we show that the zone which effectively contributes to the generation of EM disturbances along a plane interface coincides with the first Fresnel zone associated with a seismic-to-electromagnetic wave conversion. A numerical sensitivity study shows that the EM waves generated at depth by the passage of seismic waves through an interface are particularly sensitive to contrasts in porosity, permeability, fluid salinity, and fluid viscosity. Our numerical simulations highlight the potential of artificially generated seismoelectromagnetic converted waves for the characterization of the subsurface and its fluid content.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JB000316

Full waveform numerical simulations of seismoelectromagnetic wave conversions in fluid‐saturated stratified porous media

Recent studies have demonstrated that electroseismic phenomena in porous media have the potential to detect zones of high fluid mobility and fluid chemistry contrasts in the subsurface. However, there have only been a few field studies of these phenomena since they were first observed 60 years ago. None of these studies were able to support observations with an explicit comparison to results of full waveform modeling. In this paper, we demonstrate that the electroseismic phenomena in porous media can be observed in the field, explained, and modeled numerically, yielding a good agreement between the field and the synthetic data. We first outline the design of our field experiment and describe the procedure used to reduce noise in the electroseismic data. After that, we present and interpret the field data, demonstrating how and where different electroseismic signals originated in the subsurface. Finally, we model our field experiment numerically and demonstrate that the numerical results correctly simulate arrival times, polarity, and amplitude variation with offset behavior of the electroseismic signals measured in the field.

Electroseismic investigation of the shallow subsurface: Field measurements and numerical modeling

Field experiments carried out at a site near Vancouver, Canada have shown that a shallow lithologic boundary can be mapped on the basis of its seismoelectric response. As seismic waves cross the boundary between organic-rich fill and impermeable glacial till, they induce electric fields that can be measured at the surface with grounded dipole receivers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conversions. Two types of seismoelectric signals are observed. The primary response is distinguished by near simultaneous arrivals at widely separated receivers. Its arrival time is equal to the time required for a seismic P-wave to travel from the shotpoint to the fill/till boundary. On the surface, its maximum amplitude (about 1 mV/m) is measured by dipoles located within a few meters of the shotpoint. At greater distances, the amplitude of the primary arrival decays rapidly with offset, and secondary seismoelectric arrivals become dominant. They differ from the primary response in that their arrival times increase with dipole offset, and they appear to be generated in the immediate vicinity of each dipole sensor. Our studies show that the responses cannot be attributed to piezoelectricity or to resistivity modulation in the presence of a uniform telluric current. We infer that seismically induced electrokinetic effects or streaming potentials are responsible for the seismoelectric conversion, and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experiments were small in scale, the results are significant in that they suggest that the seismoelectric method may be used to map the boundaries of permeable formations.

/pdf/measurement-of-the-seismoelectric-response-from-a-shallow-23k5qvtvu6.pdf

Measurement of the seismoelectric response from a shallow boundary

Harmonic noise generated by power lines and electric railways has plagued geophysicists for decades. The noise occurs as electric and magnetic fields at the fundamental frequency of power transmission (typically 60 Hz in North America) and its harmonics. It may be recorded directly during time‐domain measurements of electric and magnetic felds, or indirectly, by geophone cables during the acquisition of seismic data.

Subtraction of powerline harmonics from geophysical records

We describe a procedure for the simultaneous estimation of multiple stationary sinusoidal contaminants in a time series and demonstrate its application to the cancellation of powerline noise in seismic and seismoelectric records. An estimate of the noise in each record is obtained by seeking a linear combination of sinusoids that are harmonics of one or more fundamental frequencies; this combination must fit the data in a least‐squares sense. The algorithm accommodates estimation windows of arbitrary length, including windows shorter than one fundamental period, without cross‐contamination between harmonic estimates.Provision is made for refining the fundamental frequencies, for which small errors (e.g., 0.01 Hz in 60 Hz) can result in significant residual noise near the ends of the record (beating). Our strategy is novel in that it uses all specified harmonics and iteratively searches frequency space for a best fit using numerical derivatives. This determination is very robust for at least two fundamenta...

Cancellation of multiple harmonic noise series in geophysical records

An underground test of a seismoelectric prospecting method for massive sulfides was performed at the Mobrun Mine (Rouyn‐Noranda, Quebec) in June 1991. The method is based upon the conversion of seismic energy to high‐frequency pulses of electromagnetic radiation by sulfide minerals. The delay between shot detonation and detection of the electromagnetic radiation gives a one‐way traveltime for the acoustic wave to reach the zone of seismoelectric conversion, which when combined with P‐wave velocity allows the shot‐to‐ore zone distance to be calculated. A 0.22-kg explosive charge located within 50 m of the orebody provided the seismic excitation, and the resulting electromagnetic emissions were received by electric dipole and induction‐coil antennas. First‐arrival information from a 35‐shot survey above an orebody, the 1100 lens, provides firm evidence that short duration pulses of electromagnetic radiation are produced by the passage of acoustic waves through the orebody. The survey also demonstrated that ...

Field trials of a seismoelectric method for detecting massive sulfides

Field trials of a Soviet inspired seismoelectric method at an underground zinc mine in British Columbia, Canada, have demonstrated its effectiveness and confirmed some of the claims by the inventors of the method. The deposit investigated is a massive sulphide orebody principally made up of sphalerite, grading about 15% Zn in the area investigated. This deposit is a difficult target for standard geophysical techniques as it is virtually indistinguishable from the andesite/rhyolite host rock. However, we were able to demonstrate that the massive sulphide ore produces high frequency electromagnetic emissions when a strong seismic wave passes through the orebody. An explosive seismic source (less than 1 kg) and wide bandwidth, 1?5000 kHz, EM sensors connected to a computer data acquisition system were used to produce and collect the seismoelectric signals. Our experiments show that there was a substantial increase in high frequency EM activity when seismic waves passed through the orebody. Thus, EM signal arrival time and shot point location could be used to locate the source of the EM signals. To demonstrate this capability shotpoints were placed around a known portion of the orebody in order to image the extent of the ore zones. A two dimensional image of the zones creating the EM signals was constructed from data gathered at 14 shot positions. The location and extent of a large anomaly in the image corresponds well with the drill-inferred position of the orebody. Another smaller anomaly appears to be coincident with the known location of a pillar of ore left behind by previous mining.

R. Don Russell

Papers

Measurement of the seismoelectric response from a shallow boundary

Subtraction of powerline harmonics from geophysical records

Cancellation of multiple harmonic noise series in geophysical records

Field trials of a seismoelectric method for detecting massive sulfides

Underground tests of the radio pulsed effect seismoelectric method at the Lynx Mine, Canada