A. F. McClymont

Detailed images of the shallow Alpine Fault Zone, New Zealand, determined from narrow-azimuth 3D seismic reflection data

Previous high-resolution seismic reflection investigations of active faults have been based on 2D profiles. Unfortunately, 2D data may be contaminated by out-of-the-plane reflections and diffractions that may be difficult to identify and eliminate. Although full 3D seismic reflection methods allow out-of-the-plane events to be recognized and provide superior resolution to 2D methods, they are only rarely applied in environmental and engineering studies because of high costs. A narrow-azimuth 3D acquisition and processing strategy is introduced to produce a high-resolution seismic reflection volume centered on the Alpine Fault Zone (New Zealand). The shallow 3D images reveal late Quaternary deformation structures associated with this major transpressional plate-boundary fault. The relatively inexpensive narrow-azimuth 3D acquisition pattern consisting of inline source and receiver lines was easily implemented in the field to provide 2- by 4-m CMP coverage over an approximately 500- by 200-m area.The narrow...

Integrating Geophysical and Geotechnical Engineering Methods for Assessment of Pipeline Geohazards

Because pipelines can cover extensive distances through diverse terrain, they are subject to various geohazards, including slope failure and earthquake damage, which can have costly environmental and monetary impacts over their designed operational lifetime. Here, we show how geophysical investigative techniques can be used to complement other geotechnical investigation methods to provide a detailed understanding of site geology to best inform geohazard assessments. We pay particular attention to how multiple geophysical methods can be used to obtain spatially continuous measurements of subsurface physical properties, and layer and structural geometries. The geophysical data can then be used to either interpolate or extrapolate geotechnical engineering properties between and away from boreholes and excavations, or optimize the locations of subsequent boreholes or excavations. To demonstrate the utility of our integrated approach of incorporating geophysical methods to geohazard assessments, two case studies are presented. The first case study shows how electrical resistivity tomography (ERT), seismic refraction tomography (SRT) and multichannel analysis of surface wave (MASW) datasets are used to constrain the thickness and extent of potentially sensitive glaciomarine clay layers that are subject to slope instability and structural failure along a proposed pipeline route near Kitimat, British Columbia (BC). A second case study describes how high-resolution ground-penetrating radar (GPR) and seismic reflection surveys are used to locate and characterize fault strands that may cause future ground deformation at a proposed pipeline crossing of the Tintina/Rocky Mountain Trench fault in northeastern BC. INTRODUCTION Geohazards are a particular group of hazards which can be defined as natural phenomena or hazards of a geological, geotechnical, hydrological or tectonic origin. For pipeline projects it is critical to properly characterize all potential geohazards that can threaten initial construction activities and potentially impact the pipeline and associated areas throughout the designed operational lifespan. Geohazard assessments should include a comprehensive understanding of local subsurface geological and geotechnical conditions. Conventional assessments of this type have typically utilized intrusive drilling and investigative excavation methods to identify and map local soil and rock structures, characteristics and engineering properties. In this contribution, we show how the integration of geophysical methods to geohazard and geotechnical assessments can help reduce costs, site impacts, and optimize planned investigations, all while providing an improved comprehensive and spatially extensive understanding of subsurface conditions. Over the past 20 years, advancements in near-surface geophysical survey methods (as opposed to more conventional deeper looking methods used for oil and gas exploration) have led to their more frequent use in geotechnical engineering applications. In the case of pipeline projects, the use of geophysical survey methods to design trenchless pipeline crossings of environmentally sensitive water bodies has almost become routine [1, 2]. With increasing scrutiny of the potential environmental impacts associated with pipeline projects and the need to minimize expensive cost overruns during construction, it is now essential to incorporate new technologies, like

Current Land and Waterborne Geophysical Methods for Guiding Horizontal Directional Drilling and Trenching Along Pipeline Right-of-Ways

In Western Canada, oil and natural gas pipeline projects are being considered that will move hydrocarbons from the Prairie Provinces and British Columbia, to the Pacific Ocean, the Atlantic, and even potentially the Arctic. Along the proposed right-of-ways, the pipeline engineers will encounter challenging and varied terrain, including discontinuous permafrost, creek and river crossings, glaciomarine clays, thick muskeg, and other subsurface conditions that require specialized engineering planning in advance of construction. Geophysical surveys, in support of geotechnical investigations, provide continuous subsurface information to help inform design challenges associated with the many terrain challenges. Some geophysical surveys to be considered include electrical resistivity tomography (ERT), induced polarization (IP), seismic refraction, seismic reflection, multi-channel analysis of surface waves (MASW), ground penetrating radar (GPR), and borehole geophysics. Typically, a combination of several geophysical surveys along with drilling information, are optimal for the costeffective site characterization of problematic segments of proposed pipeline right-of-ways. BACKGROUND The landlocked nature of Western Canadian oil and gas production prohibits Canadian commodities from being sold at world market prices [0]. As such, over the last 5 years, approximately 20 liquefied natural gas (LNG) projects on the Pacific Coast, and four different bitumen pipeline projects directed toward three different oceans have progressed through various stages of evaluation [1,2,3]. All of these projects involve traversing terrain that requires detailed characterization in order to carry out predictive geohazard assessments, and prepare preventative geotechnical studies essential to avoiding pipeline failures. Near surface geophysics has been used as a guide for intrusive geotechnical investigations, pipeline construction [4] and as an aid to horizontal drilling for pipeline construction [5]. Typical areas of concern include river and creek crossings, discontinuous permafrost, lava flows and associated lava tubes, glaciomarine clay deposits, thick muskeg (i.e. bog or organic material), active faulting, intensive fracturing or jointing, unusually deep or shallow bedrock, and deeply incised deposits of highly permeable sediments. Geophysical surveys have a role in assessing each of these categories of subsurface conditions. Some techniques, such as electrical resistivity tomography (ERT), are applicable to assessing both bedrock and overburden conditions. Other techniques have very specific niche applications, such as ground penetrating radar (GPR), which is particularly well suited to imaging muskeg thickness. This paper will briefly discuss various geophysical methods (see Butler [6] for more details of each technique) of interest and their potential applications, in the context of pipeline planning and design. OVERVIEW OF GEOPHYSICAL METHODS While most geophysical methods may offer some insight toward addressing the above noted subsurface scenarios, the techniques that are practicably and effectively applied in the field are few. This is simply a result of there being relatively few physical properties that can be economically, rapidly, and confidently measured in the field, and which also provide meaningful site characterization information. Fortunately, most 1 Copyright

Geophysics and Geologic Hazards

Sinkholes in Florida pose significant geotechnical, engineering, and hydrogeological challenges for using the land in constructive ways. In some instances, the sinkholes may prove unstable, thus limiting the overburden stress that can be applied. Additionally, the sinkholes may provide a conduit for accelerated contaminant transport from surface activities. In this case study, we use electrical resistivity tomography (ERT) to understand the scope of sinkhole activity under a planned landfill. As part of their application, the landfill permit applicant submitted a dense network of parallel, twodimensional electrical resistivity profiles as described in the following. We provided an alternative, three dimensional analysis of this data set to enhance detection of subsurface sinkhole targets. Eighty five parallel resistivity lines spaced 6m (20ft) apart were coalesced into a large three-dimensional resistivity model to map the 14 hectare (35 acre) site. The results revealed that resistive sand-filled sinkholes could extend at least 30m (100ft) below ground surface with a diameter that ranged from 30 to 100m (100-300ft). The host conductive limestone was shown to have a complex undulating topography with eroded pinnacles. Using cone penetrometer technology (CPT), the edge of the limestone pinnacles were also shown to have significant raveling, which coincided with a narrow range of resistivity values. The implications of the correlation between direct characterization using CPT and indirect characterization with ERT suggest that raveling could cover as much as 17% of the site. Based on these findings, the site was determined to be ill suited for landfill construction.

Resistivity/Induced Polarization/Self-Potential Methods and Applications

Modern multielectrode and multichannel resistivity systems have made it relatively easy and rapid to collect time domain induced polarization (IP) data in near surface surveys. This paper will examine a wide variety of applications through case studies in a variety of geological settings in Western Canada. Case studies will show various applications and complementary features of IP surveys including distinguishing salt water from conductive clays, identifying faults, locating deeply buried structures underneath active facilities, and distinguishing landfilled debris from leachate. IP data sets will be correlated with other data sets including resistivity, seismic reflection, and borehole geophysical parameters.