High-resolution Geophysical Imaging Research Articles

High-resolution geophysical imaging of reptile burrows (San Salvador rock iguana, the Bahamas): implications for ichnology and conservation ecology

Abstract Neoichnological research of terrestrial tracemakers in coastal settings provides important palaeoenvironmental information about their context within the subaerial facies. Here we present the first geophysical dataset of reptile burrows in a carbonate substrate and use it to help visualize parts of the burrows of the Bahamian (San Salvador) rock iguana ( Cyclura rileyi ). High-resolution 800 MHz ground-penetrating radar (GPR) images within an enclosure on San Salvador Island were employed to discriminate between the electromagnetic signal response from subsurface anomalies related to air-dominated voids or live animals within burrows. The dielectric contrast between the carbonate substrate and open burrows was sufficient to identify the majority of the 15–20 cm-wide subsurface extensions of the inclined tunnels in the upper 30–40 cm. Whereas limestone clasts induced some interference, it is possible to differentiate their high-amplitude diffractions from those produced by the iguana burrows. Our research indicates that GPR imaging is a viable, rapid, non-invasive method of visualizing animal burrows, with implications for neoichnology, palaeoichnology, and conservation ecology of semi-fossorial species. Furthermore, the critically endangered status of Bahamian land iguanas, as well as ongoing threats from natural and introduced pressures, highlights the need for research into their ichnological record.

Coherent diffraction imaging for enhanced fault and fracture network characterization

Abstract. Faults and fractures represent unique features of the solid Earth and are especially pervasive in the shallow crust. Aside from directly relating to crustal dynamics and the systematic assessment of associated risk, fault and fracture networks enable the efficient migration of fluids and therefore have a direct impact on concrete topics relevant to society, including climate-change-mitigating measures like CO2 sequestration or geothermal exploration and production. Due to their small-scale complexity, fault zones and fracture networks are typically poorly resolved, and their presence can often only be inferred indirectly in seismic and ground-penetrating radar (GPR) subsurface reconstructions. We suggest a largely data-driven framework for the direct imaging of these features by making use of the faint and still often underexplored diffracted portion of the wave field. Finding inspiration in the fields of optics and visual perception, we introduce two different conceptual pathways for coherent diffraction imaging and discuss respective advantages and disadvantages in different contexts of application. At the heart of both of these strategies lies the assessment of data coherence, for which a range of quantitative measures is introduced. To illustrate the versatility and effectiveness of the approach for high-resolution geophysical imaging, several seismic and GPR field data examples are presented, in which the diffracted wave field sheds new light on crustal features like fluvial channels, erosional surfaces, and intricate fault and fracture networks on land and in the marine environment.

Seismology on small planetary bodies by orbital laser Doppler vibrometry

The interior structure of small planetary bodies holds clues about their origin and evolution, from which we can derive an understanding of the solar system’s formation. High resolution geophysical imaging of small bodies can use either radar waves for dielectric properties, or seismic waves for elastic properties. Radar investigation is efficiently done from orbiters, but conventional seismic investigation requires landed instruments (seismometers, geophones) mechanically coupled to the body.We propose an alternative form of seismic investigation for small bodies using Laser Doppler Vibrometers (LDV). LDVs can sense motion at a distance, without contact with the ground, using coherent laser beams reflected off the body. LDVs can be mounted on orbiters, transforming seismology into a remote sensing investigation, comparable to making visual, thermal or electromagnetic observations from space. Orbital seismometers are advantageous over landed seismometers because they do not require expensive and complex landing operations, do not require mechanical coupling with the ground, are mobile and can provide global coverage, operate from stable and robust orbital platforms that can be made absolutely quiet from vibrations, and do not have sensitive mechanical components.Dense global coverage enables wavefield imaging of small body interiors using high resolution terrestrial exploration seismology techniques. Migration identifies and positions the interior reflectors by time reversal. Tomography constrains the elastic properties in-between the interfaces. These techniques benefit from dense data acquired by LDV systems at the surface, and from knowledge of small body shapes. In both cases, a complex body shape, such as a comet or asteroid, contributes to increased wave-path diversity in its interior, and leads to high (sub-wavelength) imaging resolution.

Definition of small-scale volcanic structures by Electrical Resistivity Tomography: the Trentaremi cone, an example from the Campi Flegrei Caldera (Italy).

Small-scale buried structures in volcanic environments could present an extreme relevance, due to their link to the eruptive activity of individual monogenetic edifices, but require, to be detected, a high-resolution geophysical imaging. In such context, the Electrical Resistivity Tomography is often able to recover primary knowledge about the heterogeneity of the buried layers, despite the reduced investigation depth. In this paper an example of such matter is proposed, concerning the Posillipo hill, in the southeastern sector of the Campi Flegrei Caldera (Italy), where an electrical resistivity survey has been carried out in the roman age Seiano tunnel. In such a way, details of the Trentaremi tuff cone buried structure have been revealed, with a maximum depth of exploration of about 100 m. The obtained results have been compared with geological observations on the exposed rocks, interpreting their underground continuation. The geophysical image reconstructs the inner structure of the Trentaremi cone. At the same time, its relationships with respect to the deposits belonging to Neapolitan Yellow Tuff eruption have been highlighted. Reconstructing this stratigraphic sequence, indications about the past volcanic dynamics have been obtained, presenting valuable implications for the future activity forecasting.

The hydrogeology of Adama-Wonji basin and assessment of groundwater level changes in Wonji wetland, Main Ethiopian Rift: results from 2D tomography and electrical sounding methods

Rise of groundwater level becomes an emerging concern at Wonji irrigation field, Main Ethiopian Rift. An integrated study based on geophysical resistivity methods is conducted at Wonji wetland to understand the link between irrigation water and the shallow aquifer system as well as to confirm the current concern of groundwater rise. The study was also intended to improve the uncertainty of understanding the hydrogeology of Wonji wetland including the extent and direction of groundwater–surface water interaction. The vertical and horizontal contacts between the different geological series of the Wonji area are resolved with 2D high-resolution geophysical imaging. Results from both VES and 2D tomography show low resistivity layers under Wonji irrigation field with narrow ranges in resistivity variation which corresponds to a homogeneous saturated layer. The geoelectric sections reveal two fault systems running NW–SE and N–S directions which impede lateral groundwater flow. Furthermore, groundwater is converged towards the Wonji irrigation site strained by these fault systems. The geophysical results show strong link between irrigation water and the shallow unconfined aquifer as well as among the local and regional flow systems.

A structural and geophysical approach to the study of fractured aquifers in the Scansano-Magliano in Toscana Ridge, southern Tuscany, Italy

Fresh water availability has recently become a serious concern in the Italian Apennines, as various activities rely on a predictable supply. Along the ridge between Scansano and Magliano in Toscana, in southern Tuscany, the situation is further complicated by contamination of the nearby alluvial aquifers. Aquifers locally consist of thin fractured reservoirs, generally within low-permeability formations, and it can be difficult to plan the exploitation of resources based on conventional techniques. An integrated study based on geological data investigated the link between tectonics and groundwater circulation, to better define the hydrological model. After the regional identification of fault and fracture patterns, a major structure was investigated in detail to accurately map its spatial position and to understand the geometry and properties of the associated aquifer and assess its exploitation potential. The subsurface around the fault zone was clearly imaged using ground probing radar, two-dimensional and three-dimensional resistivity tomography, and three-dimensional shallow seismic surveys. The vertical and horizontal contacts between the different geological units of the Ligurian and Tuscan series were resolved with a high degree of spatial accuracy. Three-dimensional high-resolution geophysical imaging proved to be a very effective means of characterising small-scale fractured reservoirs.

Assessing fault displacement and off-fault deformation in an extensional tectonic setting using 3-D ground-penetrating radar imaging

High-resolution geophysical imaging can aid in the study of active faults by providing subsurface information about the geologic structure and deformational patterns present. We collected ground-penetrating radar (GPR) data across the active Emigrant Peak Fault in order to image and quantify fault displacement and assess off-fault deformation in an alluvial fan setting. The Emigrant Peak Fault in Fish Lake Valley, Nevada, is part of the northwest-striking, right-lateral Walker Lane-Eastern California Shear Zone that accommodates a portion of the North America-Pacific plate boundary strain. The normal fault displacement along this northeast-striking fault plays an instrumental role in transferring right-lateral deformation through a regional releasing bend. The GPR surveys span the Emigrant Peak Fault and adjacent alluvial fan and consist of a high-resolution 50 MHz 3-D grid and a longer 25 MHz 2-D line. The 3-D grid covers an area of 115 m by 500 m straddling the fault, while the 2-D GPR data form a 1500 m long line perpendicular to the active trace of the Emigrant Peak Fault. Depth of imaging ranges between ~ 25 m for the 50 MHz data and ~ 40 m for the 25 MHz data. GPR data exhibit structural geometries dominated by NW-dipping normal faulting and corresponding antithetic faults. Near the main fault strand, alluvial fan strata dip consistently to the NW while the stratigraphic architecture becomes more complex in the down-dip direction, including colluvial wedges and small graben structures. At the main fault, GPR imaged reflectors showing offset of 1–2 m, which is in agreement with the minimum displacement observed at the youngest terraces along the fault scarp. Radar data reveal considerable off-the-main-fault deformation and complex structures not evident in surface observations, aerial photography, and differential GPS digital elevation models. Rapid rejuvenation of the active alluvial fan likely erases any surface expression of deformation away from the main fault. This study provides further evidence that 3-D subsurface imaging is necessary for accurate determination of fault displacement and off-fault deformation of active fault zones.

Geophysical imaging with backpropagation and zeroth‐order phase approximation

A promising technique for high resolution geophysical imaging has been proposed which is based on the inversion of an exact solution to the reduced wave equation. The method is known as backpropagation because it propagates a received waveform back into the host formation mapping wavespeed variations. The implementation of backpropagation is complicated by the fact that it requires a complete characterization of the wave source and the wave propagation properties of a homogeneous host medium. The imaging procedure can be simplified by utilizing a zeroth—order phase approximation which allows implementation with first arrival times. Numerical experiments and a field study suggest that this approach is a practical means for high resolution subsurface imaging.