Turning High Resolution Geophysics Upside-down: Application of Seismic Inversion to Site Investigation and Geohazard Problems
Authors: Hamilton I.W. , Hartley B. , Angheluta C. , Digby A. 
Published: October 2008

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.