Multimode seismoelectric phenomena generated using explosive and vibroseis sources

Abstract

A field trial of seismoelectric surveying was carried out at a site underlain by 20 m of water-saturated clayey Champlain Sea sediments, renowned for their amenability to high resolution imaging by seismic reflection surveys. Seismically induced electrokinetic effects were recorded using an array of 26 grounded dipole electric field antennas, and two different seismic sources including an eight-gauge shotgun, and a moderate power (10 000 lb Minivib) vibrator. Despite the high electrical conductivity of the sediments, shot records show evidence of possible interfacial seismoelectric conversions caused by the arrival of P-waves at the base of the clay/top of bedrock and at the top of a layer of elevated porosity and conductivity within the clay at 7 m depth. However, the data are more remarkable for the fact that P-wave, S-wave, and PS/SP converted wave reflections evident in the seismic records all give rise to electrical arrivals exhibiting very similar moveout patterns in the seismoelectric records. Superficially, these electrical responses could be misinterpreted as simple coseismic seismoelectric effects associated with the arrival of reflected seismic waves at each dipole antenna on surface. However, their broader bandwidth, superior coherency and earlier arrival times compared to their corresponding seismic arrivals indicate that the electrical effects are generated by the arrival of seismic reflections below each dipole at the shallow intraclay interface 7 m below surface. Such quasi-coseismic arrivals have recently been predicted by full-waveform seismoelectric modelling and characterized as evanescent electromagnetic (EM) waves. In retrospect, they were also observed in earlier seismoelectric field trials, but not measured as clearly nor recognized as a distinct seismoelectric mode intermediate between interfacial and coseismic effects. We propose that the observed quasi-coseismic effect can be understood physically as a fringing field emanating from the travelling charge separation associated with a P-wave (direct or modeconverted) crossing a subsurface interface at an oblique angle. Such effects may be nearly indistinguishable from coseismic effects if the interface depth is small compared to the seismic wavelength, but recognition of the phenomenon contributes to an improved understanding of the seismoelectric wavefield, and will lead to improved interpretations. From a practical standpoint, the results of this field trial suggest that using electric field receivers to supplement geophones on surface could yield significantly higher resolution seismic reflection images in those areas where suitable near-surface layers exist for the generation of quasi-coseismic effects. The results also reinforce the importance of using multichannel recording to allow interfacial seismoelectric conversions originating at depth to be distinguished from stronger coseismic and quasi-coseismic arrivals originating in the near-surface by measurement of their arrival time versus offset (moveout) and amplitude versus offset b...

Numerical simulations to explain the coseismic electromagnetic signals: a case study for a M5.4 aftershock of the 2016 Kumamoto earthquake

“…These signals are interfacial radiation EM waves induced by nearly normal incidence of seismic waves on the porous media's interface, that is, the free surface of this water-saturated half-space model. Such kind of interfacial radiation EM waves have been confirmed in both numerical simulation (e.g., Haartsen and Pride 1997;Garambois and Dietrich 2002) and field experiment studies (e.g., Butler et al 2018).…”

“…They pointed out that, in retrospect, evanescent EM waves had been observed in earlier seismoelectric field trials, but not recognized correctly. The recognition of seismoelectric evanescent EM waves helps explain the origin of similar effects reported in previous studies and will contribute to improved interpretation of seismoelectric records (Butler et al 2018). In addition, evanescent EM waves can be a new form that the electrokinetic effect may take in practical application (Dietrich et al 2018).…”

“…The weak signals that show up around 6-14 s in the B θ component (Fig. 10b) are interfacial radiation EM waves (Haartsen and Pride 1997;Garambois and Dietrich 2002;Butler et al 2018). Their signal strengths are generally below the level of 0.0001 nT.…”

“…The discovery of seismoelectric evanescent EM waves has attracted the interests of seismoelectric exploration researchers. Butler et al (2018) analyzed seismoelectric experiments data obtained in Canada several years ago and consequently identified quasicoseismic EM signals, which were interpreted by them as the evanescent EM waves predicted by Ren et al (2016a). They pointed out that, in retrospect, evanescent EM waves had been observed in earlier seismoelectric field trials, but not recognized correctly.…”

Quantifying interface responses with seismoelectric spectral ratios

“…Garambois & Dietrich (2002) showed that IRs are developed over a Fresnel zone that is positioned directly beneath a source located at the Earth's surface. However recent studies suggest that for incident angles larger than the critical angle θ c = arcsin(v seis /v EM ), so called evanescent IRs are excited (Ren et al 2015(Ren et al , 2018Butler et al 2018). The evanescent IRs are carried along the interfaces by the seismic waves and can therefore be detected further away from the source.…”