Exploration Boreholes Research Articles

In situ estimation of roof rock strength using sonic logging

Sonic travel time logging of exploration boreholes is routinely used in Australia to obtain estimates of coal mine roof rock strength. Because sonic velocity logs are relatively inexpensive and easy to obtain during exploration, the technique has provided Australian underground coal mines with an abundance of rock strength data for use in all aspects of ground control design. However, the technique depends upon reliable correlations between the uniaxial compressive strength (UCS) and the sonic velocity. This paper describes research recently conducted by NIOSH aimed at developing a correlation for use by the U.S. mining industry. From two coreholes in Illinois, two from Pennsylvania, and one each from Colorado, western Kentucky and southern West Virginia, sonic velocity logs were compared with UCS values derived from Point Load tests for a broad range of coal measure rock types. For the entire data set, the relationship between UCS and sonic travel time is expressed by an exponential equation relating the UCS in psi to the travel time of the P-wave in μs/ft. The coefficient of determination or R-squared for this equation is 0.72, indicating that a relatively high reliability can be achieved with this technique. The strength estimates obtained from the correlation equation may be used to help design roof support systems. The paper also addresses the steps that are necessary to ensure that high-quality sonic logs are obtained for use in estimating UCS.

Origin and stability of a permafrost methane hydrate occurrence in the Canadian Shield

Relatively little attention has been given to the stability of methane hydrates, formed during periods of past climate change, currently in areas of continuous permafrost. Although a large portion of the Canadian arctic is underlain by crystalline rocks, the occurrence, phase, and origin of alkanes in crystalline rocks under thick permafrost conditions (> 500 m) have not been reported. For the first time, composition and isotopic data for gases from a crystalline shield environment currently under permafrost conditions are presented. Gas and water samples were collected from exploration boreholes and seeps between 890 and 1130 m depths in the Lupin gold mine, Nunavut, Canada. Gases were methane-dominate (64–87%), with nitrogen (10–37%) the next largest component, and smaller amounts of ethane, propane, and carbon dioxide. Pressure and temperature measurements indicated gas hydrates were stable at the site prior to mining operations, a conclusion supported by noble gas and salinity determinations. Gas hydrate stability over the last 120 kyr glacial cycle was demonstrated by calculating transient subsurface pressure and temperature conditions utilizing the Memorial University/University of Toronto Glacial Systems Model (MUN/UofT GSM) and the Hydrogeosphere groundwater flow model. Model results also indicated glacial loading increased subsurface pressures, resulting in increased hydrate stability fields during glacial periods. Subglacial groundwater recharge would be limited by any significant formation of gas hydrates. Gas composition, combined with carbon and hydrogen isotopic determinations on methane (− 56 to − 42‰ VPDB and − 349 to − 181‰ VSMOW), carbon dioxide (− 55 to − 15‰ VPDB), ethane (− 37 to − 27‰ VPDB and − 330 to − 228‰ VSMOW), and propane (− 34 to − 27‰ VPDB and − 196 to − 172‰ VSMOW), indicated formation of natural gases by thermogenic processes, mixed with bacteriogenic gas, reasonable, given site geologic history. Methane hydrate formation affected gas composition and gas δ 2H values, complicating the interpretation. Gas production is not a modern process at this location, and the overall contribution to the global carbon budget is small. Glacial groundwater recharge models need to account for methane hydrate formation and dissipation due to changes in subsurface stability fields during glaciation and the effect of methane hydrate on hydraulic conductivity.

Low heat flux and large variations of lithospheric thickness in the Canadian Shield

Ten new heat flux determinations have been made using measurements in 22 mining exploration boreholes located at latitudes higher than 51°N in the Canadian Shield. They provide data in poorly sampled regions near the core of the North American craton where one expects the lithosphere to be thickest. The new heat flux values are all smaller than 34 mW m−2 and are among the lowest recorded so far in the shield. For all the new sites, there is no relationship between heat flux and heat production in surface rocks. In the Canadian Shield, heat flux variations occur at wavelengths <100 km and are mostly of crustal origin. Local averages in two 250 × 250 km windows located on Archean areas at high latitudes on either side of James Bay are 29 mW m−2 and 31 mW m−2, the lowest values found so far at this scale in the Canadian Shield. S wave traveltime delays derived from tomographic models provide the additional constraints needed to resolve differences of deep lithospheric thermal structure. There is no significant correlation between average surface heat flux and traveltime delays within the Canadian Shield, confirming that variations of the surface heat flux are mostly of crustal origin. Traveltime delays cannot be explained by variations in crustal heat production only and require variations of heat supply to the lithosphere and/or radiogenic heat production in the lithospheric mantle. These variations are associated with changes of lithospheric thickness that may be as large as 80 km. The heat flux at the base of the Superior lithosphere is constrained to be 11 ± 2 mW m−2.

美国密苏里州西南部伯林顿灰岩地层控制水流的染色示踪研究

Dye-tracing methods are utilized in an area of southwestern Missouri, USA, in order to evaluate factors influencing groundwater flow in the karst lithology. Four new dye traces were conducted in the Burlington Limestone formation of the surficial Springfield Plateau aquifer. Dye traces were conducted with two large sinkholes with mapped caves as their drainage base, one with fluorescein and one with rhodamine-WT. Two other traces, both using fluorescein dye, were performed where dye was introduced into a groundwater-level monitoring well and an exploratory borehole. Results of these four traces indicate that structural geologic control—as expressed by joints, fractures, faults, and photolineaments—does not account solely for the observed dye-trace results. The available data suggest that significant influence on groundwater movement is exerted by bedding planes within the low dip-angle limestone formation or by lithologic variability within the formation (such as silt- or clay-rich layers that have not yet been clearly identified). Average linear tracer velocities from each of the four traces indicate groundwater movement is primarily through conduits that flow slower than open channel surface waters in the region, but significantly faster than typical groundwater flow through porous media.

Isotope-geochemical features of Devonian rocks in the Pripyat Trough: Famennian suprasalt deposits

This work presents the results of isotope studies of the Upper Devonian (Famennian) suprasalt complex carried out with core material of exploratory borehole 71 drilled at the northwestern margin of the Pripyat Trough. Lithological features of the rocks point to a high degree of postsedimentary transformation of rocks. Significant role in these transformations was played by evaporites and related brines and the presence of hydrocarbons in the sequence. Such a combination provides insight into the behavior of isotope systems under conditions of the geochemical transformation of sedimentary material. The combined application of several isotope (δ13C, δ18O, δ34S, and 87Sr/86Sr) methods made it possible to consider the complex mechanisms of compositional and isotopic transformation of rocks in the course of their lithogenetic alterations. The source material can be deduced with a high probability from the Rb-Sr systematics of the clayey component of marls.

SUSPECTED PRESENCE OF HIBBINGITE IN OLIVINE PYROXENITE ADJACENT TO THE UG2 CHROMITITE, BUSHVELD COMPLEX, SOUTH AFRICA

Core from exploration boreholes at the Karee platinum mine of the western Bushveld Complex, in South Africa, became iron-stained and friable upon surface exposure. This alteration mostly occurs in pyroxenites that are immediately above and below the UG2 chromitite layer and adjacent to the Merensky Reef. Chemical analyses of such rocks have Cl concentrations from 0.08 to 0.16 ± 0.02 wt%. Analyses of comparable fresh rocks, Merensky pyroxenite and also iron-rich ultramafic discordant pipes, give values of less than 0.04 wt% Cl. The mineral hibbingite, Fe2Cl(OH)3, has been tentatively identified, on the basis of its petrographic features and qualitative electron-microprobe analysis, as veins 0.1 mm thick that cut, and have sharp contacts with, serpentine that occurs in olivine grains. These textural relationships show that this alteration occurred below ±500°C. There is no evidence for any other whole-rock geochemical changes, except Cl.

Development of the first coal seam gas exploration program in Indonesia: Reservoir properties of the Muaraenim Formation, south Sumatra

Abstract The Late Miocene Muaraenim Formation in southern Sumatra contains thick coal sequences, mostly of low rank ranging from lignite to sub-bituminous, and it is believed that these thick low rank coals are the most prospective for the production of coal seam gas (CSG), otherwise known as coalbed methane (CBM), in Indonesia. As part of a major CSG exploration project, gas exploration drilling operations are being undertaken in Rambutan Gasfields in the Muaraenim Formation to characterize the CSG potential of the coals. The first stage of the project, which is described here, was designed to examine the gas reservoir properties with a focus on coal gas storage capacity and compositional properties. Some five CSG exploration boreholes were drilled in the Rambutan Gasfield, south of Palembang. The exploration boreholes were drilled to depths of ~ 1000 m into the Muaraenim Formation. Five major coal seams were intersected by these holes between the depths of 450 and 1000 m. The petrography of coal samples collected from these seams showed that they are vitrinite rich, with vitrinite contents of more than 75% (on a mineral and moisture free basis). Gas contents of up to 5.8 m 3 /t were measured for the coal samples. The gas desorbed from coal samples contain mainly methane (CH 4 ) ranging from 80 to 93% and carbon dioxide (CO 2 ) ranging from 6 to 19%. The composition of the gas released into the production borehole/well is, however, much richer in CH 4 with about 94 to 98% CH 4 and less than 5% CO 2 . The initial results of drilling and reservoir characterization studies indicate suitable gas recovery parameters for three of the five coal seams with a total thickness of more than 30 m.

Hydrothermal activity in the Tulancingo–Acoculco Caldera Complex, central Mexico: Exploratory studies

Mineral alteration and fluid inclusion studies of drill cuttings and core samples indicate that the sedimentary basement rocks and the volcanic rocks associated with Tulancingo–Acoculco Caldera Complex have been the site of two distinct and major hydrothermal events. The complex, located in the eastern portion of the Trans-Mexican Volcanic Belt, is formed by the Pliocene Tulancingo Caldera and the younger (Pleistocene) Acoculco Caldera, which developed within the older depression. The volcanic rocks are underlain by Cretaceous sedimentary rocks of the Sierra Madre Oriental. The earliest important hydrothermal event occurred during the emplacement of Mid-Tertiary granitic intrusions that metamorphosed the sedimentary rocks; these intrusives are not exposed at the surface. However, granitic rocks were encountered at the bottom of exploratory borehole EAC-1, drilled within the Caldera Complex. The second main event occurred during the formation of the Tulancingo and Acoculco Calderas. Both episodes lead to secondary mineralization that reduced the permeability of the reservoir rocks. A possible third hydrothermal event may be associated with the recent magmatic activity within the Acoculco Caldera.Thermal logs from well EAC-1 display a conductive thermal gradient with maximum temperatures exceeding 300 °C at 2000 m depth. Although there are no active thermal springs in the area, there is extensive fossil surface hydrothermal alteration and cold gas discharges with high He 3/He 4 ratios.

Evaluation of coal structure and permeability with the aid of geophysical logging technology

Coal structures and the associated permeability were studied by a combined investigation of geophysical logging data and well tests from coal reservoirs of Huainan and Huaibei coalfields in China. A large amount of the site observations on coal sections of mining faces and the macroscopic investigations on textures of drilling coal cores were carried out, providing experimental information for classification of coal structures. Geophysical log was implemented in 600 boreholes for exploration of coal and coalbed methane (CBM) resources, including 10 CBM wells which were used for permeability tests. The permeability tests were conducted by using injection/fall-off method. The results, together with the log data obtained from geophysical log technologies, have been analysed by using geological statistics, permitting correlation of the permeability of coal seams to the log responses. The correlation developed in this study provides better evaluation of coal reservoir permeability, resulting in an improved CBM exploration in the given coalfields.

Behavior of iron-group elements, oxybarometry, and genesis of unique chromite deposits in the Kempirsai massif

Ultramafic rocks and high-Cr chromite ore from the Almaz-Zhemchuzhina deposit, the largest in the Main ore field of the Kempirsai massif, have been studied. The detailed mineralogical and geochemical examination of deep structure test and exploratory boreholes allowed us to establish the rough stratification of ultramafic rocks and to demonstrate the position of unique chromite deposits in the generalized vertical section of the southeastern Kempirsai massif. From top to bottom, a barren harzburgite-lherzolite series gives way to an ore-bearing dunite-harzburgite complex with the largest chromite deposits, including the unique Almaz-Zhemchuzhina deposit, in its upper portion and then to pyroxene-free dunite densely impregnated with chromite in the upper part and containing sparsely disseminated chromite at its base. The lower unit is composed of a barren lherzolite-harzburgite series transformed into blastomylonites near the contact with dunite, suggesting a tectonically doubled section in the southeastern part of the massif. The synore asymmetric geochemical zoning developed in the course of formation of chromite deposits as a result of removal of oreforming iron-group elements from the underlying and wall ultramafic rocks into the overlying rocks. Host rocks with disturbed initial proportions of Cr, Fe, Ni, and Mn, together with orebodies, made up ore-bearing zones no less than 1 km in thickness and subdivided into supra-, inter-, and subore subzones. The subore and wall rocks are characterized by partial loss (wt %) of Cr2O3(0.1), NiO (0.04), FeOtot(0.5), and MnO (0.02) and their removal into the interore and supraore (0.03 NiO) subzones. Thus, the subore ultramafic rocks served as a source of ore-forming components, while the interore zone with orebodies occurring therein served as a zone of discharge of these components. Using Mossbauer spectroscopy, the crystal chemistry of iron ions was studied in a representative selection of Cr-spinel samples from rocks and ores of the southeastern and western blocks (the Almaz-Zhemchuzhina and Geophysical XII deposits). The degree of iron oxidation in the samples varies from 8 to 33%. In most cases, a difference in degree of iron oxidation is established in stoichiometric approximation and from Mossbauer data. In other words, the integral stoichiometry of ferrous and ferric ions is disturbed. Such a disturbance may be related not only to partial inversion of the Cr-spinel structure but also to local heterogeneity of the mineral at the micro- and nanolevels with clustering of cations and formation of their associates. An empirical correction of the olivine-Cr-spinel geothermometer and oxybarometer has been performed. The inverse correlation between oxygen fugacity and degree of depletion of ultramafic rocks indicates that these rocks were formed in a closed system with participation of a water-methane fluid. Along with stratification of ultramafics, this correlation testifies to a powerful asthenospheric source of reduced fluids. The retention of low oxygen fugacity in central portions of orebodies does not rule out that after a break this source participated in the formation of unique chromite deposits in the Kempirsai massif.

Strike-slip faulting as a trigger mechanism for overpressure release through piercement structures. Implications for the Lusi mud volcano, Indonesia

Piercement structures such as hydrothermal vent complexes, pockmarks, and mud volcanoes, are found in various geological settings but are often associated with faults or other fluid-focussing features. This article aims to investigate and understand the mechanisms responsible for the formation of piercement structures in sedimentary basins and the role of strike-slip faulting as a triggering mechanism for fluidization. For this purpose four different approaches were combined: fieldwork, analogue experiments, and mathematical modeling for brittle and ductile rheologies. The results of this study may be applied to several geological settings, including the newly formed Lusi mud volcano in Indonesia (Mazzini et al., 2007). Lusi became active the 29th of May 2006 on the Java Island. Debates on the trigger of the eruption rose immediately. Was Lusi triggered by the reactivation of a fault after a strong earthquake that occurred two days earlier? Or did a neighbouring exploration borehole induce a massive blow-out? Field observations reveal that the Watukosek fault crossing the Lusi mud volcano was reactivated after the 27th of May 2006 earthquake. Ongoing monitoring shows that the frequent seismicity periodically reactivates this fault with synchronous peaks of flow rates from the crater. Our integrated study demonstrates that the critical fluid pressure required to induce sediment deformation and fluidization is dramatically reduced when strike-slip faulting is active. The proposed shear-induced fluidization mechanism explains why piercement structures such as mud volcanoes are often located along fault zones. Our results support a scenario where the strike-slip movement of the Watukosek fault triggered the Lusi eruption and synchronous seep activity witnessed at other mud volcanoes along the same fault. The possibility that the drilling contributed to trigger the eruption cannot be excluded. However, so far, no univocal data support the drilling hypothesis, and a blow-out scenario can neither explain the dramatic changes that affected the plumbing system of numerous seep systems on Java after the 27-05-2006 earthquake. To date (i.e. April 2008) Lusi is still active.

Geoarchaeological studies associated with remedial measures at Silbury Hill, Wiltshire, UK

Silbury Hill, near Marlborough, Wiltshire, UK, is one of the largest prehistoric mounds in Europe, characterised by uncertain structure and unusual burial conditions. Internal collapses occurred in autumn 2000 as a result of the incomplete refilling of tunnels dug in the last few centuries. Planning for the repairs has provided an opportunity to examine samples from recent exploratory boreholes and the 1960s excavations. This paper brings together the results of these investigations, comparing the materials from inside the hill with nearby soil profiles. The truncated nature of the old ground surface and the extensive multiple iron pans found inside the hill are discussed, along with their implications for construction and post-depositional processes.

Hydraulic conductivity changes and influencing factors in longwall overburden determined by slug tests in gob gas ventholes

This study presents the results of core-log analyses from the exploration boreholes, the analyses of face advance rates, and the results of downhole monitoring studies performed in gob gas ventholes for calculation of changes in hydraulic properties in the longwall overburden at a mine site in southwestern (SW) Pennsylvania section of Northern Appalachian Basin. In the first part of the study, coal measure rocks in overburden strata were analyzed and the locations where possible fractures and bedding plane separations would occur were evaluated. In the second part, the hydraulic conductivities were computed by two different slug test analyses methods using the water level changes measured in gob gas ventholes as longwall face approached. Hydraulic conductivities were analyzed with respect to the changes in overburden depth, the locations of the borehole, and mine face advance rates. These data were used to interpret the potential productivities of the gob gas ventholes as a result of fracturing and changes in hydraulic conductivities. The general results showed that the probability of fracturing and bedding plane separations in the overburden increase between strong and weak rock interfaces. Also, the probability of bedding plane separations increases as the interface is close to the extracted coal seam. Evaluation of slug tests showed that the hydraulic conductivity developments in the boreholes and their potential production performances are affected by the underground strata and the roof materials. In situations where the roof material is stiff and thick, the development of high permeability fractures around the borehole will be less. Results also indicated that borehole location with respect to face position affects the fracturing time and permeability evolution as well. Greater overburden depths generally cause earlier fracturing as longwall face approaches, but eventually result in lower hydraulic conductivities and potentially less effective boreholes. Increasing mining rates also resulted in generally lower hydraulic conductivities in the overburden. The results of this study were intended to improve the interpretation of gob gas venthole performance and to provide better siting of these boreholes.

Transport of 234U in the Opalinus Clay on centimetre to decimetre scales

Abstract The Opalinus Clay formation in North Switzerland is a potential host rock for a deep underground radioactive waste repository. The distribution of 238U, 234U and 230Th was studied in rock samples of the Opalinus Clay from an exploratory borehole at Benken (Canton of Zurich) using MC-ICP-MS. The aim was to assess the in situ, long-term migration behaviour of 234U in this rock. Very low hydraulic conductivities of the Opalinus Clay, reducing potential of the pore water and its chemical equilibrium with the host rock are expected to render both 238U and 230Th immobile. If U is heterogeneously distributed in the Opalinus Clay, gradients in the supply of 234U from the rock matrix to the pore water by the decay of 238U will be established. Diffusive redistribution separates 234U from its immobile parent 238U resulting in bulk rock 234U/238U activity disequilibria. These may provide a means of estimating the mobility of 234U in the rock if the diffusion rate of 234U is significant compared to its decay rate. Sampling was carried out on two scales. Drilling of cm-spaced samples from the drill-core was done to study mobility over short distances and elucidate possible small-scale lithological control. Homogenized 25-cm-long portions of a 2-m-long drill-core section were prepared to provide information on transport over a longer distance. Variations in U and/or Th content on the cm-scale between clays and carbonate-sandy layers are revealed by β-scanning, which shows that the (dominant) clay is richer in both elements. Samples were digested using aqua regia followed by total HF dissolution, yielding two fractions. In all studied samples U was found to be concentrated in the HF digestion fraction. It has a high U/Th ratio and a study by SEM-EDS points to sub-μm up to several μm in size zircon grains as the main U-rich phase. This fraction consistently has 234U/238U activity ratios below unity. The minute zircon grains constitute the major reservoir of U in the rock and act as constant rate suppliers of 234U into the rock matrix and the pore water. The aqua regia leach fraction was found to be enriched in Th, and complementary to the HF fraction, having 234U/238U activity ratios above unity. It is believed that these U activity ratios reflect the surplus of 234U delivered from the zircon grains. Some cm-spaced samples show bulk rock 234U/238U activity ratios that are markedly out of equilibrium. In most of them a striking negative correlation between the total U content and the bulk rock 234U/238U activity ratios is observed. This is interpreted to indicate net 234U transfer from regions of higher supply of 234U towards those of lower supply which is, in most cases, equivalent to transfer from clayey towards carbonate/sandy portions of the rock. In contrast, the 25 cm averaged samples all have uniform bulk rock 234U/238U activity ratios in equilibrium, indicating U immobility in the last 1–1.5 Ma on this spatial scale. It is concluded that the small-scale lithological variations which govern U spatial distribution in the Opalinus Clay are the major factor determining 234U in situ supply rates, regulating its diffusive fluxes and controlling the observed bulk rock 234U/238U activity ratios. A simple box-model is presented to simulate the measured bulk rock 234U/238U activity ratios and to give an additional insight into the studied system.

Understanding the Chena Hot Springs, Alaska, geothermal system using temperature and pressure data from exploration boreholes

Chena Hot Springs is a small, moderate temperature, deep circulating geothermal system, apparently typical of those associated to hot springs of interior Alaska. Multi-stage drilling was used in some exploration boreholes and was found to be useful for understanding subsurface flow characteristics and developing a conceptual model of the system. The results illustrate how temperature profiles illuminate varying pressure versus depth characteristics and can be used alone in cases where staged drilling is not practical. The extensive exploration activities helped define optimal fluid production and injection areas, and showed that the system could provide sufficient hot fluids (∼57 °C) to run a 400-kWe binary power plant, which came on line in 2006.

Reservoir rock properties of coal measure strata of the Lower Monongahela Group, Greene County (Southwestern Pennsylvania), from methane control and production perspectives

Abstract The methane emission rate into an underground mine environment from overburden strata during longwall mining is impacted by reservoir and geomechanical characteristics of the coal measure rocks in the overlying strata, as well as the presence of any coal seam. The reservoir characteristics and how they change during mining potentially affect the performance of gob gas ventholes, which consequently impacts the efficiency of methane control in the mining environment. This study presents reservoir and elastic properties of coal measure rocks in the Lower Monongahela Group in Greene County, southwestern Pennsylvania, of the Northern Appalachian Basin. Since the source of methane in this region from underground mining is located between the Sewickley coal and the Pittsburgh coal, a specific emphasis was given to this interval. Core analyses were performed in the laboratory to determine rock porosity and permeability. Geophysical logging data (gamma, density, sonic) obtained from two exploration boreholes were used for evaluating formation boundaries, shale contents, log porosities, and geomechanical properties of formations. Permeability was also calculated using density-log data and empirical equations and compared with laboratory measurements and slug tests performed in isolated intervals of boreholes. The results presented in this study can be used as data sources for reservoir studies related to the production and control of methane.

Modeling of ECBM recovery from Amasra coalbed in Zonguldak Basin, Turkey

Zonguldak coal basin is the only productive hard coal basin of Turkey. The eastern part of the basin is called as Bartin–Amasra District, which has deeper coal seams. The depth and difficulty of mining these coal seams make this district an important candidate for coalbed methane (CBM) recovery. However, there is not enough reservoir data for modeling purposes. In this study, the lithologic information collected for coal mining industry was used to determine the correlations and the continuity of the coal seams. The lithologic information was examined and the depths of the coal seams and the locations of the exploration boreholes were used to perform a reliable correlation using a new method. As a result of the correlation study, 63 continuous coal layers were found. A statistical reserve estimation of each coal layer for methane was made by using Monte Carlo simulation method. The initial methane in place found in the coal layers both in free and adsorbed states were estimated using probabilistic simulations resulted in possible reserve (P10) of 2.07 billion m3, probable reserve (P50) of 1.35 billion m3 and proven reserves (P90) of 0.86 billion m3. Among the determined continuous coal layers, coal layer #26 was selected for a preliminary investigation of the applicability of enhanced coalbed methane (ECBM) recovery and CO2 storage. The scarcity of coal seam reservoir data required a sensitivity study for the effects of reservoir parameters on operational performance indicators. The effects of adsorption, coal density, permeability, cleat porosity and permeability anisotropy parameters were examined using the Computer Modeling Group's (CMG) GEM module.

Geological disconformity found at Bimini, Bahamas, reveals 120K-yr-old land surface at shallow depth

In a 1971 Nature paper (Harrison 1971) entitled “Atlantis Undiscovered — Bimini, Bahamas,” I reported on beachrock, marble, and cement materials lying offshore at shallow depth to the west of North Bimini. I concluded that these materials were too young to reflect a supposed ancient culture. In March 2007, J. Eagle and I cored three exploratory boreholes to 50-ft depths in the Bimini Inlet (Fig. 1). The rocks encountered were far too old to fit with a curve for Holocene sea-level rise in the broader Caribbean (includes the Bahamas). Instead, C-14 and fossil-coral age dating, and stratigraphic information, indicate that a significant disconformity is present beneath the Bimini Islands. A reasonable implication of the missing rocks is that during a period of time covering perhaps 800,000 years — or a minimum of 120,000 years at the least — an ancient land surface in the Bimini area could have been above sea-level during the Pleistocene continental glaciations. This old land surface would have been available for human habitation for upwards of the last 120,000 years, at a minimum.

The use of microgravity to detect small distributed voids and low-density ground

Microgravity is established as a technique for the detection of natural and man-made cavities. However, past published examples have concentrated on substantial natural cavities or mine workings. Two cases are described for which the voids present are relatively small and discontinuous, and where much of the affected ground is characterized by low-density ground rather than large open voids. Here we use the term ‘low-density ground’ to encompass ground that has been disturbed by the collapse or partial collapse of material into a void such that the affected ground shows an anomalously low density in comparison with the surrounding unaffected ground. Case study 1 was undertaken with Mouchel Parkman on behalf of Hertfordshire County Council. A doline had opened up within a school playground. The collapse was expected to be related to natural voids within the chalk at 7–10 m depth. The low-density areas identified in the microgravity survey, in addition to some control locations, were subsequently proved by dynamic probing. The investigation accurately identified voided and poorly consolidated ground, and provided the basis for an assessment of the risks of imminent and potential future collapse. Case study 2 was undertaken with Laing O'Rourke, who were principal contractor for the construction of the A590 bypass, Cumbria. Exploratory boreholes confirmed the presence of loose material in the glacial till overburden associated with suspected voids within the limestone bedrock. The microgravity survey identified areas of low subsurface density, which were subsequently targeted by boreholes. A detailed ground model constructed from borehole data allowed a synthetic gravity map to be calculated for comparison with the measured gravity. Poorly consolidated ground, depth to bedrock and surface topographic effects could be isolated and a clear interpretation of subsurface produced. The microgravity technique is shown to be an effective tool for the investigation of poorly compacted ground and small shallow voids in complex and disturbed ground conditions. When followed by targeted intrusive investigation such surveys can yield a great deal of information that would not otherwise be available to the engineer.

A geographical information system managing geotechnical data for Athens (Greece) and its use for automated seismic microzonation

This article presents a geographical information system (GIS) which manages geotechnical data obtained from detailed geotechnical surveys as well as from in situ observations in Athens, Greece. Thoroughly examined data from more than 2,000 exploratory boreholes and trial pits located in the wider area of Athens have been incorporated using a relational database system. From the analysis of these results, thematic maps are compiled to illustrate the distribution of engineering geological information (e.g. the depth of the “Athens schist” head). In addition, a methodology for an automated GIS-aided seismic microzonation study is outlined and is being employed taking into account the aforementioned geotechnical and engineering geological information, as well as existing seismological data to estimate the variability of seismic ground motion for the southern part of Athens.