High‐frequency reflections in granite? Delineation of the weathering front in granodiorite at Piñon Flat, California

Abstract

We analyze data from two orthogonal seismic lines 336 m in length collected at Piñon Flat, California, over weathered granodiorite bedrock. Each line was made up of 10 reversed segments 84 m in length. We analyzed the first arrivals from these data and found dramatic variations in velocity along the profiles. An upper layer (approximately 2-m thick) known from trenching to be composed of soil and sandy grus had measured velocities ranging from 400 to 700 m/s. Velocities inferred from refraction analysis of first arrivals of the reversed lines revealed a heterogeneous lower layer below the soil with measured velocities of 1600–2700 m/s by a depth of 15 m. We interpret these data to be measuring velocities of a deeply weathered unit characterized by granodiorite corestones embedded in a matrix of saprolite. The most remarkable feature of these data emerged from attempting to process the same data as reflection data. Simple bandpass filtering in the 250–400 Hz band revealed a bright, impulsive arrival with three characteristic properties: (1) irregular velocity moveout that is inconsistent with that expected from a layered earth model, (2) the arrival is at a nearly constant time‐depth on all data, and (3) the arrival tends to be followed by a ringing coda whose frequency varies from trace to trace. This arrival ties exactly with a velocity discontinuity measured in a borehole located on one of the profiles that we interpret as the base of the weathered layer. We suggest this arrival is a specular reflection from a weathering front that occurs along horizontal sheeting joints at a fixed depth below the surface. This surface acts as an effective mirror for high‐frequency seismic waves which are then channeled upward through an intact, high-Q path of unaltered blocks of granodiorite to define the observed signals at the surface.

Structural control of weathering processes within exhumed granitoids: Compartmentalisation of geophysical properties by faults and fractures

“…This is illustrated by experiments carried out at depth through boreholes or galleries far from any weathering (Jamtlid et al, 1984;€ Hayles et al, 1999;Jeanne et al, 2012). Other field studies demonstrated the sensitivity of seismic methods to alteration fronts (Lee and de Freitas, 1990;Radzevicius and Pavlis, 1999). Therefore, combining seismic and electric methods is appropriate to characterise weathering of fractured crystalline rocks.…”

“…Acworth,1987;Neuman, 2005;Roques et al, 2014). For example, the depth of present day and paleo-weathering fronts have been estimated as a few metres (Virgili et al, 1974), several tens of metres (Radzevicius and Pavlis, 1999;Dewandel et al, 2006), hectometres (Genter and Traineau, 1996;Olesen et al, 2007), or up to a kilometre (Luthi, 2005). The factors governing deepening, and therefore lateral and depth extents of weathering in the vicinity of major structures, still require investigation.…”

“…The factors governing deepening, and therefore lateral and depth extents of weathering in the vicinity of major structures, still require investigation. Even though geophysical surveys have been able to characterise weathering fronts in poorly faulted formations (Lee and de Freitas, 1990;Radzevicius and Pavlis, 1999;Carvalho et al, 2000;Fonseca et al, 2006;Carvalho et al, 2010;Olona et al, 2010), to our knowledge only a few such surveys have been focused on the deepening of weathering fronts in the vicinity of structures such as faults (Wyns et al, 2004;Dewandel et al, 2006;Chandra et al, 2010, and references therein). In this paper, we focus on two faults outcropping in weathered granitoids of the Catalan Coastal Ranges (Spain).…”

Subsurface mapping in the Iberian Pyrite Belt using seismic reflection profiling and potential-field data

“…Kim et al 1994;Radzevicius and Pavlis 1999;Juhlin et al 2002;Juhlin and Stephens 2006;Eaton 2003;Schmelzbach et al 2007;Malehmir et al 2012a;Heinonen et al 2013;Ehsan et al 2014). The mining sector is one of the fields where the seismic reflection method has shown great utility to complement the more generally applied gravimetric, magnetic, electrical and electromagnetic methods.…”

Modelling of the Neves Corvo Area

“…In the last decades, due to the advances in electronics such as 24 bit A/D converters and processing techniques, the seismic reflection method has gained a dramatic increase in the number of applications to hard-rock exploration (e.g. Kim et al 1994;Radzevicius and Pavlis 1999;Eaton 2003). The mining industry is one of the fields where the seismic reflection method has shown great utility to complement the more generally applied gravimetric, magnetic, electrical and electromagnetic methods in VSC.…”