The Synthesis of geophysical information integrates some datasets from different geophysical techniques, each with its advantages and limitations, to reduce uncertainties and provide coherent subsurface information through interpretations that justify and incorporate all geophysical models.

Traditionally, geophysical datasets (from different information sources) have been analyzed independently, only comparing results, because many geophysical parameters do not have a mathematical formulation that relates them to each other. However, several research processes are currently being conducted to achieve a “fusion” of the data, which allows the integration and joint interpretation of any type of multidisciplinary data and its application in different fields of the Earth sciences.

The ICGC (Cartographic and Geological Institute of Catalunya) works along these lines, creating this synthesis of geophysical information to show the multi-parametric geophysical content of specific strategic areas of our territory, with the aim of becoming a consultation tool and a visualization tool that improves knowledge of the subsurface. It helps professionals who interact with the subsurface, or the public, to take informed decisions. 

Areas of interest

The ICGC has a special interest in studying the various sedimentary basins of Catalunya, mainly Neogene basins, considered strategic points in the territory for several reasons. First, these are highly populated areas that require a solid understanding of the subsurface to address issues related to environmental control, civil engineering, or geological and seismic risks assessment. In addition, their geological morphology makes them relevant for underground storage, exploration, and exploitation of natural resources, such as the research of alternative energy sources (geothermal), in the current context of mitigating climate and energy crisis.

To characterize the different basins, the ICGC proposes to apply a methodology that it has developed in recent years. This methodology is based on the use of a wide range of geophysical techniques in the same area (basin) to reduce uncertainties about the distribution of subsoil materials, the sediment thickness, the bedrock depth, the discontinuities or the definition of the geological structures of the basins and their margins, and to represent it visually in these geophysical information syntheses.

The basins selected for the study during the period 2022-2023 are the following:

• la Seu d'Urgell.
• la Cerdanya.
• el Vallès (sector central).

In addition to these basins, there are plans to produce similar syntheses for sensitive and environmentally significant areas, such as the Ebro Delta. Extensive geophysical information is already available in this area thanks to the EBROADMICLIM project (2014-2018). Other areas of interest for the coming years are: the rest of the Vallès-Penedès basin, Girona-La Selva basin, Baix and Alt Empordà basin, Baix Camp basin, Conca de Barberà, and Baix Ebre basin (Figure 1).

Figure 1. Areas of interest in the context of the geophysical database (clicking on thefigure allows you to download Cerdanya, Vallès and La Seu d’Urgell Geophysical information synthesis.

Objectives

The main objective is to develop and disseminate a synthesis of geophysical information that integrates all the geophysical data available from the ICGC for a specific basin or sector of a sedimentary basin, primarily Neogene basins. Depending on the geophysical techniques applied in each case, the synthesis of geophysical information will present images that characterize the subsoil with different geophysical properties of materials through quantifications of several physical properties and parameters of the medium, such as electrical resistivity, P-wave velocity, S-wave velocity, the density of the materials, soil thickness, or the depth of the soil-rock contact. Thus, a dynamic and flexible work methodology is proposed, adapting it to the specific characteristics of each basin.

Geophysical methods

To prepare this geophysical synthesis, data from geophysical methods in which the ICGC has expertise are analyzed, both in field data acquisition and in data processing. Thus, the geophysical data obtained from the following methods have been used:

• Magnetoteluric method, This method provides 2D geoelectrical models that show the electrical resistivity value to a depth of approximately 3.000 meters to determine the presence of faults, fractures and discontinuities, fluid circulation, thickness and depth of the subsurface lithology (defining contacts between materials with sufficient contrast in the physical parameter of electrical resistivity) and estimating the depth of the bedrock.
• Gravimetric method, aporta mapes que s’obtenen a partir del processat de la mesura de la gravetat relativa en diferents punts de la zona d’estudi. Habitualment, els observables es distribueixen en forma de malla i els valors es mostren en forma de mapes (Anomalia de Bouguer i Anomalia residual) que defineixen la geometria de les conques sedimentàries amb la determinació d’estructures geològiques, localització de falles i fractures, gràcies a la variació de la densitat dels materials del subsol.
• Passive Seismic with Combined Techniques of the H/V Spectral Ratio and the Seismic Array. The spectral ratio technique provides a map of the soil fundamental frequency (f0) and an estimated map of the depth of the seismic bedrock. The seismic array shows vertical profiles (1D) of the physical parameter Vs, which indicates the degree of compaction of the materials. It also builds the equation or empirical adjustment that relates the soil fundamental frequency (f0) to the depth at which the soil-rock contact is detected (Figure 2). An important area of application of these techniques is urban zones, due to their negligible environmental impact and their fast implementation. Its use extends to the fields of seismic engineering, civil engineering, geotechnics, and urban geology.

Imatge

Figure 2. Seismic bedrock estimation (bottom right) from soil fundamental frequency (bottom left) using the empirical relationship established by ICGC (2021) (top right), estimated from vertical seismic array models (top left) and deep boreholes.

• Electrical Tomography. This method provides 2D geoelectrical models of the most superficial and local parts of the study areas (analyzing the first 150 meters of depth). Defines subsurface materials using the physical parameter of electrical resistivity to delineate contacts between subsurface materials (boundaries), bedrock, water table, and saline intrusion; it also detects cavities or old water mines, identifies fractured zones and discontinuities, and locates contaminated areas.
• Refraction Seismic Tomography. This method provides 2D models of the subsoil based on the physical parameter of P-wave velocity, indicating the degree of compaction and hardness of the materials (ripability of materials). It identifies the contact between soft soil and rock, the degree of rock alteration, the location of faults and fractures, and characterizes geological structures, if there is sufficient contrast in the physical parameter of P-wave velocity.
• Reprocessing of Old Reflection Seismic Data. This method provides a 2D image of the main reflectors of the environment (contacts between materials), detects faults and fractures, identifies discontinuities between materials, and defines the geological structure at greater depths.
• Multiparametric Correlation. It involves the integration of several geophysical parameters in a single 2D profile. In this case, correlate the 2D model of the magnetotelluric method with the profile that defines the bedrock geometry (seismic bedrock) derived from the H/V spectral ratio technique and seismic array (Figure 3). Finally, a gravimetric profile is modeled by defining layers with different densities, which match with the contacts of the previous methods. The model shows the fit between the gravimetric data measured in the field and the calculated data (red line in Figure 3)..

Imatge

Figure 3. Bottom: 2D geophysical integration. Electrical resistivity model (MT), bedrock geometry (H/V), and definition of subsurface layers based on their density (gravimetry). Top: Fit between measured and calculated residual anomaly data (gravimetry).

Summary document

All these contributions are in the synthesis of geophysical information, distributed according to the example shown in Figure 4. The synthesis contains the spatial location of field measurements based on the geological map at a scale of 1:250.000 and its legend. The remaining space in the synthesis is filled with surfaces of the quantified parameters, which vary according to the specific information of each study site. Each synthesis is also accompanied by an appendix describing the information necessary to understand the origins of the product.

Imatge

Figure 4. Example of the arrangement of the different elements that make up the geophysical information synthesis document.

Imatge

Example of geophysical information synthesis: Conca del Vallès (central sector) (Download PDF, Catalan)

Synthesis of geophysical information

Scale	Title	Edition	Year	Download  GeoPDF	e-Shop  Printed	Miniature
-	la Cerdanya	1	2023	3,7 MB	-	Imatge
-	Conca del Vallès (sector central)	1	2023	3,4 MB	-	Imatge
-	la Seu d'Urgell	1	2023	2,2 MB	-	Imatge