Geophysical Survey Techniques Research Articles

Forensic geoscience non-invasive detection and characterisation of underground clandestine complexes, bunkers, tunnels and firing ranges

Recent events in conflict zones have emphasized that the successful detection and characterisation of buried clandestine complexes, bunkers and tunnels is vitally important for forensic investigators globally, to reduce or solve criminal activities, address national security threats and avoid potential terrorist attacks. However, this can often prove very difficult, particularly in urban areas, with potentially both below-ground non target items and above-ground infrastructures present, that can interfere with detecting target(s).Here we provide selected successful case studies where forensic geoscience techniques were used to detect and characterise buried clandestine complexes, bunkers and tunnels using different geophysical techniques. Generally, desktop studies assessing pre-existing information, including local geology, soils, historical/modern remote sensing, maps and photographs inform appropriate geophysical survey technique(s) selection. Subsequent near-surface geophysical techniques are then employed to produce accurate plans of sub-surface targets, with numerical modelling and correction for the interfering effects of above ground infrastructure, enabling the calibration of geophysical datasets to provide confidence in their respective interpretations.All forensic investigations are, of course, unique to every site, and thus require an individual approach to their respective ground conditions. Investigations should be both phased and iterative, with techniques tailored to local conditions: the selection of geophysical method(s) is crucial to improve successful detection rates of such important buried targets.

Reconstructing archaeological palaeolandscapes using geophysical and geomatic survey techniques: An example from Red Lily Lagoon, Arnhem Land, Australia.

Arnhem Land is a key region for understanding the Pleistocene colonisation of Australia, due to the presence of the oldest sites in the continent. Despite this, conventional archaeological survey has not been effective at locating additional pre-Holocene sites in the region due to a complex distribution of geomorphic units caused by sea level rise and coastal aggradation. This research uses geophysical and geomatic techniques to map the subsurface distribution of the geomorphic units in the Red Lily Lagoon region in eastern Arnhem Land. This reveals a complex Pleistocene landscape, which offers the potential to locate additional archaeological sites and so reveal more about the lifeways of the earliest Australians.

Assessment of slope stability on logged forest‐hill slopes using ground‐penetrating radar and electrical resistivity tomography

The use of ground‐penetrating radar (GPR) and electrical resistivity tomography (ERT) geophysical methods are considered a useful tool for assessing forest soil slope instabilities. These techniques provide a detailed survey regarding changes because of logging operations in an area, where the combination of soil, water, and rock conditions predisposes to the development of slope instabilities. In the current study, such geophysical survey techniques were applied at two localities, Bukovinka and Zemanův žleb near Křtiny (Southern Moravia, Czech Republic), during three sessions over 1 year. The selected survey sites were on different terrains but the same subsurface (sediments of Lower Marine Carboniferous). The condition for their selection was the absence of other major external influences apart from wood harvesting. The GPR and ERT measurements were carried out along the same lines, and changes in the geophysical profiles over time were observed. The results demonstrate the processes that lead to visible changes on both localities in the results of measurements. Both methods showed shifts in some soil properties configuration, separating partial rock masses or even incipient soil/rock movement. The ERT also detected a significant expansion of wet areas inside both studied rock masses, which may be because of the development of new channels for infiltration of rainwater. Considering that the only major external factor that could change the slopes drainage properties was the wood harvesting, it is reasonable to conclude that the observed changes are associated with the logging operations.

Imaging extensional fault systems using deep electrical resistivity tomography: A case study of the Baza fault, Betic Cordillera, Spain

Seismic hazard assessment of present-day tectonic faults may be improved by studying their structure and kinematics. These features have been mainly determined by surface geological studies of exposed fault traces, structural lateral segmentation or paleoseismic trenches. All these approaches rely mainly on two-dimensional analyses of surface outcrops, while knowledge of the faults at depth remain largely inaccessible. To improve on such limitations, geophysical methods can be applied to establish detailed information on fault morphology and segmentation at depth. This work analyzes new results of a deep electrical resistivity tomography survey acquired across the Baza Fault, a present-day tectonic fault that controls the geometry of the Neogene intra-mountainous Baza Basin (Betic Cordillera, Spain). Interpretation of our preferred resistivity model reveals its detailed structure down to approximately 1000 m depth. The survey shows a minimum 2 km wide complex normal fault system, with rotational tilting blocks bounded by potentially listric normal faults. This study presents subsurface evidence of the Baza fault (F3), coincident the main topographic scarps. However, the geophysical model and geomorphic evidence also support a fault branch (F1) that might be an additional active seismogenic source. The geophysical survey technique presented in this study provides essential data to improve assessment of the seismogenic potential of the Baza Fault.

Geophysical Study of the Diendorf-Boskovice Fault System (Austria)

We describe here the results of the characterization of subsurface structures in an area of the south-eastern edge of the Bohemian Massif, in Austria by high-resolution geophysical survey techniques and advanced analysis methods of potential fields. The employed methods included potential field multiscale techniques for source-edge location and characterization of sources at depth. Our results confirmed the presence of already known structures: the location of the Diendorf Fault and the Moldanubian Shearzone are clearly recognized in the data at the same location as on the geological maps, even where the Diendorf fault is covered with sediments of the Molasse Basin. In addition, we detected several geological contacts between different rock types in the Bohemian Massif west of the Diendorf Fault. From our results, we were also able to quickly identify and image, without a priori information, previously unknown structures, such as faults with-depth-to-the top of about 500 m and magmatic intrusions about 400 m deep.

An Assessment of Geophysical Survey Techniques for Characterising the Subsurface Around Glacier Margins, and Recommendations for Future Applications

Geophysical surveys provide an efficient and non-invasive means of studying subsurface conditions in numerous sedimentary settings. In this study, we explore the application of three geophysical methods to a proglacial environment, namely ground penetrating radar (GPR), seismic refraction and multi-channel analysis of surface waves (MASW). We apply these geophysical methods to three glacial landforms with contrasting morphologies and sedimentary characteristics, and we use the various responses to assess the applicability and limitations of each method for these proglacial targets. Our analysis shows that GPR and seismic (refraction and MASW) techniques can provide spatially extensive information on the internal architecture and composition of moraines, but careful survey designs are required to optimise data quality in these geologically complex environments. Based on our findings, we define a number of recommendations and a potential workflow to guide future geophysical investigations in analogous settings. We recommend the initial use of GPR in future studies of proglacial environments to inform (a) seismic survey design and (b) the selection of seismic interpretation techniques. We show the benefits of using multiple GPR antenna frequencies (e.g., 25 and 100 MHz) to provide decimetre scale imaging in the near surface (e.g., < 15 m) while also enabling signal penetration to targets at up to ∼40 m depth (e.g., bedrock). This strategy helps to circumvent changes in radar signal penetration resulting from variations in substrate conductivity or abundant scatterers. Our study also demonstrates the importance of combining multiple geophysical methods together with ground-truthing through sedimentological observations to reduce ambiguity in interpretations. Implementing our recommendations will improve geophysical survey practice in the field of glacial geology and allow geophysical methods to play an increasing role in the interpretation of glacial landforms and sediments.

Boulder displacements along rocky coasts: A new deterministic and theoretical approach to improve incipient motion formulas

Several impacts of high-energy marine events – tsunamis and storms – which have occurred along the Mediterranean coasts, have determined boulder displacements in different ways, spanning from sliding and overturning to saltation. The dynamics of these transport modes to the incipient motion of boulders, strictly connected to the pre-setting scenarios, has been extensively studied considering the boulder size and the hydrodynamic parameters of high-energy marine events, such as the wave flow velocity. Notwithstanding this, known hydrodynamic relationships provide some unrealistic flow values concerning the real wave flow velocity registered along the Mediterranean coasts. In this work, a new deterministic and theoretical approach has been developed to assess the minimum flow velocities required to move coastal boulders. The proposed approach is based on the introduction of modified parameters, which consider the real dimensional features of the boulders assessed by means of modern geophysical survey techniques, into the incipient motion formulas. To accurately define these parameters, we created a database composed of boulders located along the coasts of Apulia and Sicily (Italy) that were surveyed in previous studies through photogrammetric and Terrestrial Laser Scanner techniques. Results highlighted that the incipient motion formulas, revised with the new proposed parameters, provided minimum flow velocity values for boulder transport initiation, which were significantly lower compared to those calculated using the previous approach. Thus, the proposed modifications are a step forward towards defining the dynamics of boulder displacement.

Crude oil exploration in Africa: socio-economic implications, environmental impacts, and mitigation strategies.

Crude oil exploration is a source of significant revenue in Africa via trade and investment since its discovery in the mid-19th Century. Crude oil has bolstered the continent’s economy and improved the wellbeing of the citizenry. Historically, Africa has suffered from conflicts due to uneven redistribution of crude oil revenue and severe environmental pollution. Advancements in geophysical survey techniques, such as magnetic and gravity methods, to seismic methods, have made the commercial exploration of crude oil possible for some other countries in Africa apart from Nigeria, Angola, Algeria, Libya, and Egypt. The occurrence of organic-rich, oil-prone Type I, II, and mixed II/III kerogens in sedimentary basins and entrapment within reservoir rocks with intrinsic petrophysical properties are majorly responsible for the large deposits of hydrocarbon in Africa. The unethical practices by some multinational oil corporations have resulted in social movements against them by host communities and human rights groups. The unscrupulous diversion of public funds, award of oil blocks, and production rights to certain individuals have impaired economic growth in Africa. The over-dependence on crude oil revenues has caused the economic recession in oil-producing countries due to plummeting oil prices and global pandemic. Most host communities of crude oil deposits suffer from a lack of infrastructure, arable soils, clean water, and their functioning capabilities are violated by crude oil exploratory activities, without adequate compensations and remedial actions taken by oil companies and the government. Thus, this review examines crude oil exploration in Africa and provides insight into the environmental and socio-economic implications of crude oil exploration in Africa. Furthermore, this report highlights some recommendations that may ensure ethical and sustainable practices toward minimizing negative impacts and improving the quality of life in affected communities.

Mapping of groundwater potential using integrated geophysical techniques at forestry research institute of Nigeria, Ibadan, South Western Nigeria

Water is an essential commodity for life survival on Earth. Groundwater exists below the surface in the soil pores, fractures within rocks, fissures, and other weak geological features or zones. The aim of this research was to delineate groundwater potential within the Forest Research Institute of Nigeria (FRIN) and hence determining the possible areas suitable for siting borehole for sustainable potable water supply. Four electromagnetic traverses were carried out and ten vertical electrical sounding (VES) points were identified for detailed probing using the Schlumberger configuration. Wenner array survey was also carried out along two traverses. The VES data collected was processed using curve matching and Computer software called Winresist while Wenner array was processed using RES2DINV. The results from the interpretation of the four (4) EM profiles revealed low conductivity zone with a value ranging from 4.6 to 19.7 mS m.-1 The results of VES give a maximum of four subsurface geoelectrical layers with five curve types, which are K, Q, AK, HK, and KH. The weathered basement has a resistivity value ranging from143.8 to 450 Ωm and depth to basement ranging between 13.9 m and 39.4 m. The interpretation of the ten VES points obtained suggested that three VES points (VES 2, 5, and 7) are suitable for borehole drilling. The results of the 2D resistivity value ranges from 17.5 to 747 Ωm with a varying depth between 3.25 and 15.9 m. The results of the integrated geophysical survey techniques have proven to be an effective method for groundwater delineation in the study area.
 Keywords: Aquifer, Borehole, Electromagnetic, Geoelectric, and Groundwater

Electrical Resistivity Tomography for Mapping Groundwater Resources in Different Environ

A major share of water supply to meet the ever increasing demands for domestic, industrial and irrigation usages is derived from groundwater resources which are distributed over the entire globe. These resources are invisible from ground surface. The need for their delineation with high precision has led to the continuous advancement in the geophysical survey techniques and interpretation theory. Electrical resistivity tomography (ERT) is an outcome of such advancement in the conventional electrical resistivity methods used for exploration of this sub-surface natural resource on the basis of distinctive reduction in the electrical resistivity value of groundwater bearing geological formations/structure in comparison to their host environs devoid of water. Objective of this paper is to present an overview of the application of ERTin delineation of groundwater resources in complex geological setup of problematic areas, sources of geothermal water, and delineation of groundwater pockets in mining areas in order to prevent natural disasters such as landslide in open cast mines with the help of related case studies. This kind of knowledge about the occurrence of groundwater/geothermal resources is essential to achieve the preset objectives of their management.

Integrated Design Assessment Optimizes Layout and Engineering of Timor Gas Field

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19391, “An Integration of Geology, Geohazards, Geophysics, and Geotechnics for Pre-FEED Site Layout and Engineering Design of an Offshore Gas Field in the Timor Sea,” by Pattarapong Chullabrahm, Korn Saranyasoontorn, and Maythus Svasti-Xuto, PTTEP, et al., prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission. The complete paper presents an integration of geology, geohazards, geophysics, and geotechnical assessments for a design of an offshore gas production facility and an associated export pipeline. The gas field described in the paper is off the northwest coast of Australia in the Timor Sea at a water depth of approximately 130 m. Introduction The gas field is envisioned to include a subsea production system, a fixed central processing platform (CPP) for separation and treatment, and a floating storage and offloading (FSO) vessel for condensate. Processed gas will be transported by a new gas export pipeline (GEP) that connects the CPP with a tie-in point on an existing export pipeline. A schematic of the infield facilities is shown in Fig. 1. Regional Geology and Geohazards To understand the seabed geology, the associated geohazards, and their effect on the proposed development, an initial desktop study into the seabed and shallow subsurface geology was undertaken. The main output from the desktop study was a preliminary geological ground model that helped determine the most suitable geophysical and geotechnical survey techniques to obtain accurate information needed to model the ground conditions further in preparation for engineering design. Regional Geology. The field development lies within the Vulcan sub-basin of the Bonaparte Basin, which contains sediments to a depth of 15 km. The sub-basin is a southwest/northeast-trending extensional depocenter comprised of a series of fault bound horsts, grabens, and terraces and is active tectonically. The resulting deformation is dominated by extensional fault structures (e.g., normal faults). During the late Pliocene epoch and early Quaternary period, sedimentation of the Bonaparte Basin was dominated by shallow water carbonates in a tropical ramp setting, including the development of isolated, intrashelf carbonate buildups that are often referred to as pinnacles or shoals. Avoiding these steep-sided shoals is an objective in selecting a preferred pipeline route; however, acknowledgement of other, unmapped seabed features such as terraces, banks, and valleys must also be considered. The base-case GEP route, called the Southern Bypass route, has a length of approximately 347 km and is proposed to link the field to the tie-in point. A more-direct route between the field and the tie-in point could shorten this route significantly (up to 70 km). This provides an opportunity for a significant expense savings for GEP construction but only if a technically feasible route through a major subsea canyon—the Lambert Shelf Valley (LSV)—can be found. This canyon is up to 100 m deeper than the surrounding seabed and has steep-sided slopes ranging between 30 and 50°. Geohazards. The LSV has the potential to influence the proposed seabed infrastructure. The geohazards associated with this canyon have a direct effect on the GEP. The following geohazards required investigation: Slope instability Turbidity currents Hard ground Mobile bed forms Pockmarks and shallow gas Soft soils Liquefaction

Subsurface structure of St. Elmo submarine volcanic mound (Austin, Texas) and its volcanic conduits imaged using geophysical methods

Geophysical survey techniques including electrical resistivity imaging and magnetics were utilized to study the late-Cretaceous submarine volcanic mound of St. Elmo railroad cut located in south Austin, TX. The St. Elmo site cut exposes a sequence of late Cretaceous volcaniclastic rocks in contact with carbonate rocks. Resistivity imaging results provided subsurface evidence that the St. Elmo submarine mound has high resistivity limestone blocks scattered randomly within the resistivity sections. These limestone blocks are interpreted to be erratic blocks of Austin Chalk, which were probably torn from the walls of the volcanic conduits (vents) and ejected. Magnetic anomalies correlate well with the resistivity anomalies, strengthening the interpretation of the resistivity data. Resistivity results from the residential site, not far from the St. Elmo volcanic mound, are interpreted as a conduit which is similar to the conduit anomalies observed on the St. Elmo railroad cut site.

Integrated ERT, Seismic, and Electrical Resistivity Imaging for Geological Prospecting on Metro Line R3 in Qingdao, China

Metros are mass rapid-transit systems that have helped solve commuting problems for residents of metropolises throughout the world. However, adverse geological structures, leading to problem such as water inrush and mud gushing, pose a major threat to metro construction. Geophysical survey techniques provide quick and relatively inexpensive ways to detect adverse geological structures ahead of the tunnel face. Left line of the R3 metro tunnel in Qingdao, China will pass an aquifer sand layer under the Wolong River. To map the geological conditions below the river, we proposed a comprehensive prospecting method that would combine the merits of surface electrical resistivity imaging (S-ERT), seismic ahead prospecting, and tunnel-face and borehole electrical resistivity imaging (TBERI). The prospecting scheme was effectively revealed in a numerical model analysis, in which the stratum distribution was reconstructed using S-ERT. Field tests validated the synthetic results. The ahead prospecting scheme was able to reconstruct the distribution of an aquifer sand layer. Based on the results obtained, we believe that, the proposed scheme could play an important role in metro tunnel construction.

Mapping the possible buried archaeological targets using magnetic and ground penetrating radar data, Fayoum, Egypt

Magnetic and ground penetrating radar (GPR) techniques are perhaps the most convenient and popular geophysical survey techniques for detecting the buried archaeological targets, because of their high speed and quality of information. The results of the two used techniques have been integrated in Dawwar Jabal, Fayoum Governorate, Egypt to detect the buried archaeological remains and their depths.The magnetic survey shows various high positive magnetic anomalies, some of these anomalies are related to the possible archaeological targets and the others are due to the mudstones which may include iron minerals. The results of the GPR survey after processing shows scattering and strong reflection of the subsurface oscillations along some profiles, which may reflect the existence of the possible buried objects. Meanwhile, no evidence of scattering or clear reflection for the signals are recorded along the other profiles which suggest that there is not any possibility of archaeological targets.The integration of the obtained results from the two used techniques indicates that there is some agreement in mapping the locations of the possible buried archaeological targets and their depths (1–7.5 m) for most of the detected targets. However, few targets are observed only on the data of one of the used techniques and disappeared along the other. This is related to the limitation of each technique. Therefore, such combination of the two mentioned techniques to detect the archaeological targets yields more information about the locations and depths of the buried targets than is obtain if each technique is used individually.

The Messapic Site of Muro Leccese: New Results from Integrated Geophysical and Archaeological Surveys

The regular application of geophysical survey techniques to evaluate archaeological sites is well established as a method for locating, defining, and mapping buried archaeological materials. However, it is not always feasible to apply a range of different methods over a particular site or landscape due to constraints in time or funding. This paper addresses the integrated application of three geophysical survey methods over an important archaeological site located in south Italy. In particular, it is focused on the results achieved from a past geophysical survey and the ongoing excavations performed by archaeologists in the site of Muro Leccese. Muro Leccese (Lecce) is one of the most important Messapian archaeological sites in southern Italy. The archaeological interest of the site was generated since the discovery of the remains of Messapian walls (late 4th–3rd centuries BC). With the aim of widening the archaeological knowledge of the Messapian settlement, several integrated methods, including magnetometry, ground-penetrating radar, and electrical resistivity tomography were used on site to fulfill a number of different research objectives. Since the most important targets were expected to be located at shallow soil depth, a three-dimensional (3D) ground-penetrating radar (GPR) survey was carried out in two zones, which were labeled respectively as zone 1 and zone 2, and were both quite close to the archaeological excavations. The GPR investigations were integrated with a 3D electrical resistivity tomography (ERT) survey in zone 1 and with a magnetometric, in gradiometry configuration survey in zone 2. The integration of several techniques allowed mapping the structural remains of this area and leading the excavation project. The geophysical results show a good correspondence with the archaeological features that were found after the excavation. Current work on the geophysical survey data using different codes for the processing of the data and merging different datasets using a Geographic Information System allowed achieving a user-friendly visualization that was presented to the archaeologists.

Surveying Batavia’s Graveyard: Geophysical controlled experiments and subsurface imaging of archaeological sites on an Indian Ocean coral island

We conducted geophysical surveys on Beacon Island in the Houtman Abrolhos archipelago offshore Western Australia, to investigate areas of archaeological interest related to the 1629 Batavia shipwreck, mutiny, and massacre. We used three complementary near-surface geophysical survey techniques (total magnetic intensity, electromagnetic induction mapping, and ground-penetrating radar) to identify anomalous target zones for archaeological excavation. Interpreting near-surface geophysical anomalies is often complex and nonunique, although it can be significantly improved by achieving a better understanding of site-specific factors including background conditions, natural variability, detectability limits, and the geophysical response to, and spatial resolution of, buried targets. These factors were not well-understood for Beacon Island nor indeed for the Australian coastal environment. We have evaluated the results of controlled experiments in which we bury known targets at representative depths and analyze the geophysical responses in terms of an ability to detect and resolve targets from natural background variability. The maximum depth of detectability of calibration targets on Beacon Island is limited to approximately 0.5 m due to significant variations in background physical properties between a thin ([Formula: see text]) and highly unconsolidated dry sand, shell, and coral layer of variable thickness overlying a sea-water-saturated sandy half-space. Our controlled measurements have implications for calibrating and quantifying the interpretation of geophysical anomalies in areas of archaeological interest, particularly in coastal and sandy-coral island environments. Our geophysical analyzes contributed to the discovery of archaeological materials and five historical burials associated with the 1629 Batavia shipwreck.

Local and “Global” Perspectives on the Middle Woodland Southeast

During the Middle Woodland period, from 200 BC to AD 600, southeastern societies erected monuments, interacted widely, and produced some of the most striking material culture of the pre-Columbian era, but these developments are often overshadowed by the contemporaneous florescence of Hopewell culture in Ohio. I argue that the demonstrable material links between the Middle Woodland Southeast and Midwest demand that we cease to analyze these regional archaeological records in isolation and adopt multiscalar perspectives on the social fields that emerged from and impacted local Middle Woodland societies. In synthesizing recent research on Middle Woodland settlement, monumentality, interaction, and social organization, I make explicit comparisons between the Middle Woodland Southeast and Ohio Hopewell, revealing both commonalities and contrasts. New methodological approaches in the Southeast, including geophysical survey techniques, Bayesian chronological modeling, and high-resolution provenance analyses, promise to further elucidate site-specific histories and intersite connectivity. By implementing theoretical frameworks that simultaneously consider these local and global dimensions of Middle Woodland sociality, we may establish the southeastern Middle Woodland period as an archaeological context capable of elucidating the deep history of the Eastern Woodlands as well as long-standing issues surrounding middle-range societies.

Supporting renewable energy projects using high resolution hydrographic and geophysical survey techniques, Garden Island, Western Australia

Supporting renewable energy projects using high resolution hydrographic and geophysical survey techniques, Garden Island, Western Australia

Subsurface structure of a maar–diatreme and associated tuff ring from a high-resolution geophysical survey, Rattlesnake Crater, Arizona

Geophysical survey techniques including gravity, magnetics, and ground penetrating radar were utilized to study the diatreme and tuff ring at Rattlesnake Crater, a maar in the San Francisco Volcanic Field of northern Arizona. Significant magnetic anomalies (+1600nT) and a positive gravity anomaly (+1.4mGal) are associated with the maar. Joint modeling of magnetic and gravity data indicate that the diatreme that underlies Rattlesnake Crater has volume of 0.8–1km3, and extends to at least 800m depth. The modeled diatreme comprises at least two zones of variable density and magnetization, including a low density, highly magnetized unit near the center of the diatreme, interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization. Magnetic anomalies and ground penetrating radar (GPR) imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization, likely produced by energetic emplacement of hot pyroclastic density currents. GPR profiles on the tuff ring reveal long (~100m) wavelength undulations in bedding planes. Elsewhere, comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse. Interpretation of these geophysical data suggests that Rattlesnake Crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed. The positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor. These dense magnetized bodies have excess mass of 2–4×1010kg, and occupy approximately 5% of the diatreme by volume. Magnetic anomalies on the crater floor are elongate NW–SE, suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation. GPR imaging of the tuff ring also suggests that substantial land-slip may have occurred on the western rim, perhaps causing part of the tuff ring to collapse into the crater. Strong radar reflections indicative of well-developed weathering horizons are present as well. The techniques employed at Rattlesnake Crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option.

Gradiometry survey and magnetic anomaly testing of Castros de Neixón, Galicia, Spain

During the summer of 2011, a geophysical survey with subsequent magnetic anomaly testing was conducted in Northwest Iberia, in the province of Galicia, Spain, the most extensive evaluation of its kind to be performed on a Castro Culture hill fort with distinct Bronze and Iron Age occupations. The investigation focused on determining the spatial extent, occupation, and use of the multicomponent Bronze and Iron Age hill fort site(s) of Castros de Neixón.A Geoscan Research FM36 Fluxgate Gradiometer was used successfully to measure and identify magnetic anomalies interpreted as potential archaeological features within the occupation areas.Over 400 monopole and dipole magnetic anomalies were detected, many of which have been interpreted as representing pits, ditches, palisades, hearths and a possible corral. A parallel defensive ditch with zig-zag entrance was located north of the occupation at Castro de Neixón Grande. Subsurface testing of a promising monopole anomaly revealed a large storage pit with artifacts dating to the Castro period. New research since the 1990s has opened up traditional discussions linking the characteristic architecture of defensive walls and round structures (after which the Castro Culture was named) to a warrior-elite society, to more complex interpretations related to the intensification of agriculture and corresponding increase in sedentism. These new interpretations integrate domestic and agricultural features into Castro subsistence practices, including storage pits. Our results support these interpretations, revealing the extent to which storage pits are a dominant feature type at Castros de Neixón. Future research and subsurface excavations directed at understanding under-investigated features located via geophysical survey techniques at sites like Castros de Neixón may clarify questions surrounding Castro Culture social organization.