R. Di Maio

A study of the correlation between electrical resistivity and matric suction for unsaturated ash-fall pyroclastic soils in the Campania region (southern Italy)

In the territory of the Campania region (southern Italy), critical rainfall events periodically trigger dangerous fast slope movements involving ashy and pyroclastic soils originated by the explosive phases of the Mt. Somma-Vesuvius volcano and deposited along the surrounding mountain ranges. In this paper, an integration of engineering-geological and geophysical measurements is presented to characterize unsaturated pyroclastic samples collected in a test area on the Sarno Mountains (Salerno and Avellino provinces, Campania region). The laboratory analyses were aimed at defining both soil water retention and electrical resistivity curves versus water content. From the matching of the experimental data, a direct relationship between electrical resistivity and matric suction is retrieved for the investigated soil horizons typical of an ash-fall pyroclastic succession. The obtained relation turns out to be helpful in characterizing soils up to close saturation, which is a critical condition for the trigger of slope failure. In such a regime, the water content and the matric suction have small variations, while electrical resistivity variations can be appreciated in a larger range of values. For this reason, besides suction measurements on very small soil volumes through classical tensiometers, our analyses suggest the direct monitoring of in situ electrical resistivity values as an effective tool to recognise the hydrological state of larger and more representative soil volumes and to improve early warning of dangerous slope movements.

A three-dimensional hydrogeological–geophysical model of a multi-layered aquifer in the coastal alluvial plain of Sarno River (southern Italy)

The coastal alluvial plain of Sarno River (Campania Region, southern Italy) is a very rich environment that has experienced a long history of changes due to both natural phenomena such as eustatic sea-level variations and deposition of volcanoclastic sediments, and human civilizations who populated this area since historical times. As a result, it is characterized by complex stratigraphic sequences and groundwater flow systems. The architecture of the multi-layered aquifer system in a sample area, located in a densely urbanized sector at the mouth of Sarno River, was reconstructed. Starting from the analysis of stratigraphic log data and laboratory geotechnical measurements, the lithostratigraphical-unit sequence was retrieved and a realistic three-dimensional (3D) model of the hydrogeological heterogeneity was obtained. The results of a detailed 2D electrical resistivity tomography survey were used to support the analysis of the spatial heterogeneity of the aquifer system in a sector characterized by lack of log data. The integration of hydrogeological and geophysical data allowed for the reconstruction of a 3D hydrogeophysical model of the multi-layered system, which electrically characterizes and geometrically identifies two aquifers. Finally, piezometric-level measurements validated the hydrogeological–geophysical model and showed the effectiveness of the methodology.RésuméLa plaine alluviale côtière de la Rivière Sarno (Campanie, Sud de l’Italie) est un milieu très riche qui a une longue histoire de transformations dues à la fois à des phénomènes naturels, tels les variations eustatiques et le dépôt de sédiments pyroclastiques et aux civilisations qui ont occupé cette zone depuis des temps historiques. Il en résulte des séquences stratigraphiques et des systèmes d’écoulement souterrain complexes. L’architecture du système aquifère multicouche a été reconstituée dans une zone test, localisée dans un secteur densément urbanisé à l’embouchure de la Rivière Samo. Partant de l’analyse de données de logs stratigraphiques et de mesures géotechniques en laboratoire, on a reconstitué la séquence lithostratigraphique unité et abouti à un modèle 3D réaliste des hétérogénéités hydrogéologiques. Les résultats d’une campagne de tomographie détaillée de la résistivité électrique 2D ont été utilisés pour conforter l’analyse de l’hétérogénéité spatiale du système aquifère dans un secteur caractérisé par un manque de données stratigraphiques. L’intégration des données hydrogéologiques et géophysiques a permis la restitution d’un modèle 3D du système multicouche, qui caractérise électriquement et identifie géométriquement deux aquifères. Finalement, des mesures de niveaux piézométriques ont validé le modèle hydrogéologique et géophysique et montré l’adéquation de la méthodologie.ResumenLa planicie costera aluvial del Río Sarno (Región de Campania, sur de Italia) es un ambiente muy rico que ha experimentado una larga historia de cambios debido tanto a fenómenos naturales, tales como variaciones eustáticos del nivel del mar y depositación de sedimentos volcánicos, como a las civilizaciones humanas que poblaron esta área desde tiempos históricos. Como resultado, se caracteriza por secuencias estratigráficas y sistemas de flujo de aguas subterráneas complejos. Se reconstruyó la arquitectura del sistema acuífero multicapa en un área de muestreo, localizada en un sector densamente urbanizado en la boca del Río Sarno. Comenzando por el análisis de datos de registros estratigráficos y medidas geotécnicas de laboratorio, se recuperó la secuencia de unidades litoestratigráficas y se obtuvo un modelo realístico tridimensional de la heterogeneidad hidrogeológica. Los resultados de un relevamiento tomográfico de resistividad eléctrica 2 D fueron usados para apoyar el análisis de la heterogeneidad espacial del sistema acuífero en un sector caracterizado por la carencia de datos de registros. La integración de datos geofísicos e hidrogeológicos permitió la reconstrucción de un modelo hidrogeológico 3D del sistema mulitcapa, el que caracteriza eléctricamente y identifica geométricamente dos acuíferos. Finalmente, medidas de niveles piezométricos validaron el modelo hidrogeológico–geofísico y mostraron la efectividad de la metodología.摘要(意大利南部Campania地区)Sarno河沿海冲积平原是一个非常富有的地区,由于海面升降变化和火山碎屑沉积物沉积及人类从很早以来就居住在此地,本地区经历过长期的变化。因此,地层序列和地下水系统非常复杂。重建了位于Sarno河口密集城市化地区样区中多层含水层系统的构造。从分析地层记录数据和实验室岩土测量数据入手,获取了岩性地层单元序列,得出了逼真的水文地质不均匀性三维模型。详细的二维电阻率断层摄影调查结果用来支持缺乏录井数据地区含水层系统的空间不均匀性分析。水文地质和地球物理数据的整合可以重建多层含水层的三维水文地质模型,这个模型电学上可以描述两个含水层,几何学上可以确定两个含水层。最后,测压水位测量结果验证了水文地质-地球物理模型,显示了方法的有效性。ResumoA planície costeira aluvial do rio Sarno (Campânia, sul de Itália) é um ambiente muito rico que foi alvo de uma longa história de alterações devidas a fenómenos naturais, tais como variações eustáticas do nível do mar e a deposição de sedimentos vulcaniclásticos, e civilizações humanas que povoaram esta área desde tempos históricos. Em resultado disso, caracteriza-se por sequências estratigráficas e por sistemas de escoamento de águas subterrânea complexos. Foi reconstruída a arquitetura do sistema aquífero multicamada numa área-amostra localizada numa zona densamente urbanizada na foz do rio Sarno. Partindo da análise de dados de diagrafias estratigráficas e de resultados geotécnicos laboratoriais foi concebida a sequência de unidades litoestratigráficas e foi obtido um modelo realístico tridimensional (3D) da heterogeneidade hidrogeológica. Foram usados os resultados de um levantamento tomográfico detalhado 2D de resistividade elétrica para apoiar a análise da heterogeneidade especial do sistema aquífero num sector caracterizado pela ausência de dados de diagrafias. A integração de dados hidrogeológicos e geofísicos permitiu a reconstrução de um modelo hidro-geofísico 3D do sistema multicamada, o qual caracteriza elétrica e geometricamente dois aquíferos. Finalmente, as medições de nível piezométrico validaram o modelo hidrogeológico–geofísico e mostraram a eficácia da metodologia.

Simulations of landslide hazard scenarios by a geophysical safety factor

Soil response to rainfall is a complex phenomenon that requires modeling of many sources of heterogeneity, whose variations can be relevant on various timescales and whose precise description requires a large amount of data inputs. Due to the great complexity of the problem, many simplifying assumptions are usually made in modeling landslides triggered by rainfall. As regards rainfall-induced shallow landslides, conventional approaches base slope stability analyses on the infinite slope model combined with hydrological models, which provide the time evolution of groundwater pressure head and volumetric water content. On the other hand, the response of geophysical quantities to water changes depends also on the variations in mechanical and hydrological properties. For this reason, we attempt a different approach to the problem of slope stability assessment by shifting the focus on the analysis of variations in geophysical properties. In this paper, starting from experimental resistivity data acquired in a test area, we perform a series of numerical simulations to study how changes in soil resistivity spatial distributions may affect the size of unstable areas. We use a simple cellular automaton whose states are defined by the values of a local and time-dependent geophysical factor of safety, which depends on soil electrical resistivity and slope inclination. We studied the probability of occurrence of rainfall-induced shallow landslide events by driving the system to instability through a decrease in electrical resistivity values. Numerical simulations are performed by varying number and intensity of the applied perturbations. Hazard scenarios obtained by in situ distributions of resistivity values are compared with those coming from initial random distributed resistivity values. Our results suggest possible critical rates of resistivity changes for triggering instability in the investigated area and point out the crucial role of resistivity variations in prediction of larger events.

3D geophysical imaging for site-specific characterization plan of an old landfill

As it is well-known, the characterization plan of an old landfill site is the first stage of the project for the treatment and reclamation of contaminated lands. It is a preliminary in-situ study, with collection of data related to pollution phenomena, and is aimed at defining the physical properties and the geometry of fill materials as well as the possible migration paths of pollutants to the surrounding environmental targets (subsoil and groundwater). To properly evaluate the extent and potential for subsoil contamination, waste volume and possible leachate emissions from the landfill have to be assessed. In such perspective, the integrated use of geophysical methods is an important tool as it allows a detailed 3D representation of the whole system, i.e. waste body and hosting environment (surrounding rocks). This paper presents a very accurate physical and structural characterization of an old landfill and encasing rocks obtained by an integrated analysis of data coming from a multi-methodological geophysical exploration. Moreover, drillings were carried out for waste sampling and characterization of the landfill body, as well as for calibration of the geophysical modeling.

Interpolation of Induced Polarization Transients by the Generalized Cole-Cole Relaxation Model

We consider a Generalized Cole-Cole (GCC) model as representative of relaxations of electric voltages in time domain Induced Polarization.