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Seismic inversion is routinely used as the basis for estimation of hydrocarbon reservoir properties from volumes of seismic data calibrated by well log data. Its use in shallow site investigation and geohazard applications has been rare. This paper presents the results of seismic inversion trials on two conventional site survey datasets. The aim is to demonstrate the additional value that can be extracted from existing seismic/well log datasets by applying advanced geoscience methods. We also discuss the further improvements that can potentially be achieved by using available, but not currently widely used, high resolution 3D seismic and shallow geotechnical borehole wireline logging methods. The first trial utilised standard high resolution 2D site survey seismic data, in conjunction with well logs from two North Sea oil wells. A simple P-wave-only seismic inversion was performed. The results show a clear correlation between P impedance anomalies and shallow gas reservoirs, and enable discrimination of seismic artefacts to be achieved with greater confidence. The second trial was a P wave inversion of a high resolution 2D dataset, using high resolution wireline logging of geotechnical boreholes to constrain the inversion. The results show the benefits of seismic inversion for interpretation of shallow geohazards. The results of the trials also indicate the potential benefits of improved wireline logging of shallow (down to 500m) boreholes and wells, for geohazard, geotechnical and drilling hazards purposes. Introduction. Considerable effort and resources are invested into acquiring 2D or 3D high resolution multi-channel digital seismic data, for assessment of shallow geohazards, drilling hazards and for geotechnical purposes. However, this data is not fulfilling its whole potential. Conventionally, interpretation is limited to horizon picking and simple amplitude analysis. The purpose of our study is to take the analysis of the high resolution seismic data a step further. We are trying to apply a well-established technique, seismic inversion, that has been used mainly for reservoir characterisation, to get the maximum out of our data. The use of seismic inversion for reservoir characterisation is well-established and widely accepted as an essential method. There has also been some application of inversion of exploration 3D seismic data to specific geohazard problems, notably gas hydrates (1, 2). An example of the inversion of high resolution seismic data and integration with geotechnical data has been demonstrated by the GEOSIS project (3). Two examples are shown here, showing how converting high resolution seismic data to acoustic impedance can improve geohazard interpretation. A few problems have been encountered during the studies. A first problem is the lack of log data that needs to be used in the calibration of the results and the generation of the low frequency model. A second problem is confirmation of the results because the hazardous areas that we want to highlight tend to be avoided when drilling. Considering all the difficulties of running such a project, the results are encouraging.

The paper discusses the use of 3D exploration seismic data for geohazard evaluation purposes. as based on a body of data from i] number of recent cases on the European continental shelf, Comparison is made with conventional high-resolution 2D site survey data, An assessment of the current impact of the technique on the market in Europe, which takes account of theviews of several operators. Introduction Over the past few years [here has been increasing interest in the use of exploration 3D seismic to improve the detection of shallow gas and other drilling hazards. The Fugro/Geoteam companies in Europe, especially Geoteam AS in Oslo, have been contracted on a range of projects to interpret existing exploration 3D seismic data for assessment. This has been undertaken using data from several North Sea area for various clients. the majority to date on the Norwegian continental shelf. Often the 3D data has been used in conjunction with acquisition and integration of conventional 2D high resolution (FIR) seismic site survey data, or other complementary seismicdata. but in some cases the shallow gas geohazard as assessment has been made, at the client's request. using only 3D seismic data. To optimise the exploration 3D seismic data, reprocessing work is currently underway on one central North Sea site to attempt to improve resolution To obtain a snapshot of the wider picture of the use of geohazard 3D in Europe, some views of operators have been sought. and the market impact this represents is discussed. A related activity has been the performance of a small number of trials conducted in full high-resolution 3D seismic acquisition for geohazard objectives. This approach is currently not standard geohazard survey practice and still in the development stage. it merits separate discussion and is outside the scope of this paper. Geoteam Experience Approach. A 3D seismic geohazard evaluation is typically commissioned before a site survey, to enable better planning and optimisation of the later data acquisition. It can, however, be performed alongside the interpretation of the 2D site survey data. to produce an integrated report. 3D evaluation is also leading, particularly in Norway, to a reduction in the amount of l-fR2D acquired. A complementary HR2D survey can then, if the site is suitable, be as limited as a crossed pair of lines at the location. 3D evaluation has also been used alone, if the area is sufficiently familiar, has suitable geology and has been densely sampled by previous drilling. Methodology. With reference to the usual types of supporting geological information (previous wells, regional geological information etc.) the 3D data is interpreted by picking significant horizons. from sectional and time slice views, and producing isochrons (i.e. structure maps) and reflection strength maps of these horizons. Modern workstations are utilised for the interpretation: Charisma in Norway, and Western OASIIS in the U.K.

We present a selective review of geohazards that have influenced recent offshore development projects. The examples have been chosen to show how recent developments in geophysical interpretation have addressed the problems posed by various types of geohazards. Three types of geohazards are considered: environmental issues, geohazards affecting sub-sea foundation design and deepwater geohazards. Environmental issues may not be conventionally regarded as "geo" hazards; however, many environmental issues present significant constraints on offshore developments and must be treated in the same way as traditional geohazards. They are features of, or on, the seabed, which must be located, mapped and characterised so that they can be avoided or mitigated. Examples include coral reefs, herring spawning grounds, algal mats and many other types of benthic biology. Geohazards affecting sub-sea foundation design are relatively well known (shallow channels, lateral variations in soil properties, hardgrounds etc.) and there has been a progressive improvement in the geophysical interpretative methods available to characterise these hazards. Deep-water geohazards have been widely discussed and the range of hazards that may be encountered is widely appreciated, but there is still scope for new surprises. Our examples show how geophysical interpretation can characterise some of the major hazards such as slope instability, complex topography and mud volcanoes. Developments in geophysical interpretation include increasing use of digital processing and interpretation of "analogue" datasets, use of seismic velocities to characterise soil types, tight integration of geophysical and geotechnical datasets, interpretative use of visualisation software. Introduction The types of seabed and shallow sub-seabed constraints on offshore developments have expanded over the last ten years. This has been caused by several factors including the move into deepwater with its many associated geohazards, the rise in environmental constraints and the increase in sub-sea developments with smaller, lighter structures being more sensitive to very shallow soil conditions. There has been a concurrent improvement in site survey data acquisition and introduction of new methods such as swathe bathymetry. There has also been a continuing development in the way we look at the data, how we interpret it and how the resultsare presented. Environmental issues as geohazards In addition to the standard geohazard identification scope of work for site surveys, many oil companies are including an environmental scope of work as part of the survey work. The information required is usually obtained from the standard survey programme, but in some cases, site specific surveys are carried, often in conjunction with seabed sampling and ROV inspection. The following describes some of the issues that FSL have encountered and attempted to resolve through remote sensing. Cold Water Coral Formations Deep-water coral (e.g. Lophelia Pertusa) accumulations are extremely sensitive to changes in their local environment and therefore oil companies are now taking great care in avoiding and preserving these unique communities. The accumulations of coral identified in recent surveys within the Norwegian North Sea can be grouped into two main categories based on shape, size and location.

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