Integrated geophysical profiles and H/V microtremor measurements for subsoil characterization

Abstract

Mapping bedrock structure beneath overburden is crucial for understanding geological and hydrogeological processes. Acquiring this information is generally done using well drilling or geophysical surveys; but these studies are expensive and require large periods of acquisition and processing time. In addition, geophysical data acquisition can be logistically challenging in urban zones with limited available areas for instrumentation deployment. Under favourable conditions (1D structure and high acoustic impedance contrast) the H/V microtremor technique can provide estimates of bedrock depth. This technique is used to obtain the soil resonance frequency in seismic microzonation studies. It is based on the computation of the horizontal to vertical spectral ratio of microtremor recordings acquired at a single station. The soil resonance frequency is related to soil shear‐wave velocity and thickness. Here we investigate the capability of combining microtremor and traditional exploration geophysical techniques (electrical resistivity and seismic tomography) to obtain an empirical relationship relating soil resonance frequency and overburden thickness. Subsequently we propose to extend microtremor measurements to adjacent areas that have not been covered by geophysical surveys. This methodology has been applied at a test site located in a granitic environment where alluvial/colluvial sediments cover the granite weathering profile. This area is characterized by urban development and sectors having rugged topography. A priori, this area has suitable conditions to apply the H/V microtremor technique. Overburden thickness has been estimated to range between 20–50 m. The proposed methodology has been validated at the test site, encouraging us to apply the H/V method as an exploration tool in similar geological environments.

Characterization of a complex near-surface structure using well logging and passive seismic measurements

“…Since the 1990s, several authors have introduced the H / V method as suitable for exploration studies (e.g. Ibs-von Seht and Wohlenberg, 1999;Delgado et al, 2000;Benjumea et al, 2011). These studies benefit from the relationship between the soil resonance frequency (υ 0 ) and bedrock depth (H ) to delineate bedrock geometry on basins (Gabàs et al, 2014).…”

Seismic velocity model and near‐surface geology at Mycenaean Tiryns, Argive Basin, Peloponnese, Greece

“…The use of the gradient approach in SRT imaging yields rather smooth increments of velocity with increasing depth. Nevertheless, velocity gradient plots help interpret the depth to bedrock and lithologic boundaries in the subsurface (Benjumea et al ). The seismic velocities in the resulting v p and v s tomograms are consistent throughout the area, allowing lithologic correlations.…”

“…However, while the soil–bedrock boundary remains steady along the v p contour of 1400 m/s, the boundary for v s is found at velocities between 600 m/s and 900 m/s. This probably indicates an irregularly thick weathering profile in the bedrock formation across the area due to both lateral and vertical discontinuities, which are typical in highly fractured karstic limestone terrains (Sheehan, Doll, and Mandell ; Barton ). The two longest east–west trending composite profiles outside Tiryns are an example (Fig.…”

“…Both v p and v s on near‐surface rocks decrease with increasing joint/fracture density, joint/fracture roughness, type of viscoelastic material and particle size filling the joint/fracture network, and rock quality designation parameter (Leucci and De Giorgi ; Barton ; Wu, Zhu, and Zhao ). Fractures (and other forms of natural discontinuities) cause velocity anisotropy and attenuation anisotropy (Barton ; Best, Sothcott, and McCann ). In particular, a high joint/fracture density attenuates high‐frequency components in the spectrum of the waveform (Leucci and De Giorgi ).…”

“…However, fractures filled with fine‐grained sand or clay/silt exhibit lower specific fracture stiffness and smaller wave transmission coefficient and vice versa (Wu et al ). From the geotechnical engineering point of view, typical v p values of soft and hard near‐surface limestones vary between 1700 m/s and 2400 m/s and among 2800 m/s‐6400 m/s, respectively (Barton ). In Tiryns,v p values in the bedrock vary from 1400 m/s to 3500 m/s and are interpreted as a transition from soft to medium‐hard limestone, with a rock mass quality index between very poor and poor (Barton ).…”