Introduction

Electroseismic phenomena have been shown to produce two forms of energy: the interface response and the coseismic field Butler et al. (1996); Garambois and Dietrichz (2001); Haines and Guitton (2002). As explained in detail by Pride and Haartsen (1996), these phenomena depend on the pressure-induced flow of pore fluid relative to the grain matrix as a P-wave passes through a fluid-saturated porous medium. The pore fluid carries with it a small amount of electric charge due to the electric double layer that exists at the grain/fluid boundary. This mechanism produces an electric charge separation within the P-wave. The associated electric field is termed the ``coseismic field''. When the P-wave encounters an interface in mechanical or elastic properties, the charge distribution is disrupted and made asymmetrical; this results in what can be approximated as an oscillating disk of electric dipoles at the first Fresnel zone. The corresponding electric field is termed the ``interface response'' and can be observed at the Earth's surface or other remote location. It shows reversed polarity on opposite sides of the shot point, virtually zero moveout (V_em >> V_p), and the amplitude pattern of a dipole.

A third type of electroseismic energy is predicted by Equation 144 of Pride and Haartsen (1996):

 

$$E_x = T^F_{E,s_{em}} \left (\frac{i}{\omega s_{em} x} - \fr... ...ac{e^{i \omega s_{em} x}}{4 \pi x} 6 \cos \theta \sin \theta F$$ (1)

where  

$$T^F_{E,s_{em}} = \frac {i \omega \mu \rho_f L}{G(s^2_{em} - s^2_s)}$$ (2)

(with original typographic error corrected). It was first reported in the literature by Haines et al. (2004). We refer to this form of electroseismic energy as the ``direct field'' because it is analogous to the seismic direct wave. The direct field is the electric field of a vertical electric dipole created at the location of an impact source (Figure 1). The impact source (a sledgehammer strike, for instance) creates an asymmetric fluid-pressure distribution (enhanced pressure beneath the source and decreased pressure above) which results in a similarly asymmetrical charge distribution. This charge distribution has a strong vertical dipole component, so the measured field shows reversed polarity on opposite sides of the shot point and the amplitude pattern of a dipole. The dipole is localized at the shot point and begins at the time of the source impulse and continues until the earth has relaxed to its original state.

 

direct_field Figure 1 Cartoon diagram of the electroseismic direct field, created at the location of an impact source.

Electroseismic data may also demonstrate the existence of a similar-looking, but entirely unrelated, electric field. This is the field of a conductor moving with velocity ${\bf v}$ within the Earth's magnetic field ${\bf B}$, described by Lorentz's equation:

 

$${\bf E} = {\bf v} \times{\bf B}.$$ (3)

If a metal hammer plate is in good electrical contact with the soil then the electric field resulting from the hammer impact may be observed at nearby electrode dipoles during the time that the hammer plate is moving, typically not more than 10 ms. The field can often be discriminated from electroseismic fields by the fact that it shows non-reversed polarity on the two sides of the shot point.

In this contribution we provide a detailed analysis of these observations and further examples of them from recently-acquired field data.