Abstract
Over the past few decades, the potential of collocated measurements of 6C data (3C of translational and 3C of rotational motion) has been demonstrated in global seismology using high-sensitivity, observatory-based ring laser technology. Proposed applications of 6C seismology range from tomographic reconstruction of near-receiver structure to the reduction of nonuniqueness in seismic source inverse problems. Applications to exploration problems have so far been hampered by the lack of appropriate sensors, but several applications have been proposed and demonstrated with array-derived rotational motion estimates. With the recent availability of, for example, fiber-optic-based high-sensitivity rotational motion sensors, widespread applications of 6C techniques to exploration problems are in sight. Potential applications are based on, for example, the fact that the extended set of combined translational and rotational motion observations enables carrying out array-type processing with single-station recordings such as wavefield separation and surface-wave suppression. Furthermore, measuring the rotational component (curl) of the seismic wavefield enables direct isolation of the S-wave constituents and could significantly improve S-wave exploration. Rotational measurements provide estimates of the spatial wavefield gradient at the free surface that allow carrying out analyses such as local slowness estimation and wavefield reconstruction. Furthermore, rotational motion measurements can help to resolve wavefield infidelity introduced by seismic instruments that are not well-coupled to the ground.
Highlights
Seismological observations of rotational motions induced by earthquakes and recorded with ring lasers (Figure 1; e.g., McLeod et al, 1998) as well as rotational motion measurements on exploration scale estimated from receiver arrays (e.g., Muyzert et al, 2012) are testimony that rotational motions induced by seismic waves are measurable and interpretable
It can be expected that the rapid advances in rotational sensor technology that have been observed over the past few years will further progress and enable accurate 6C measurements with affordable, portable, and robust devices in the near future
One attractive aspect of 6C measurements is that 6C data enable analyses with single-station recordings that conventionally require large receiver arrays
Summary
Seismological observations of rotational motions induced by earthquakes and recorded with ring lasers (Figure 1; e.g., McLeod et al, 1998) as well as rotational motion measurements on exploration scale estimated from receiver arrays (e.g., Muyzert et al, 2012) are testimony that rotational motions induced by seismic waves are measurable and interpretable. An example of rotational motion recorded by a ring laser following the Mw 7.9 earthquake in Nepal on 25 April 2015 is shown in Figure 2; Figure 3 displays an exploration-scale data example of rotational-motion measurements computed using geophone arrays and finite-difference techniques. Such rotational-motion measurements extend the set of seismic observables beyond conventional translational measurements and open up possibilities to improve existing methods as well as develop novel techniques for subsurface exploration at all scales.
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