How Good Seismic Resolution Needs To Be For Detecting Thin Beds?
Authors: Paternoster Benoit Jacques , Cauquil Eric Charles , Saint-Andre Cyril 
Published: May 2011

Increasing seismic resolution is often the ultimate objective assigned to new surveys or to the reprocessing of existing data. This is especially true for HR2D surveys or 3D data (re)processing dedicated to the assessment of shallow hazards prior to the drilling operations. Over pressured and/or gas saturated thin sedimentary bodies to be drilled through represent one kind of such geo-hazard. Thus, it is paramount to improve the detection of thin beds and be able to identify their thickness and extension as accurately as possible in order to ensure safe operations.

Increasing seismic resolution through the maximization of the high frequency content may not always be the best strategy. An alternate attitude can be the improvement of the signal-to-noise ratio as this proves to be a limiting factor for achieving good detection.

Under careful control of the seismic wavelet characteristics (amplitude, frequency content and phase) and provided the noise level of the seismic data is sufficiently low, useful information can be extracted from instantaneous phase. It can serve the purpose of detecting beds much thinner than the conventionally accepted detection limit of a tenth of a wavelength.

Introduction

When facing a new surface seismic volume, geophysicists and more generally interpreters often wonder about the minimal thickness of a sedimentary body that can be analyzed with the given dataset. It is often deemed insufficient or limited for the interpretation to be carried out. The next question that follows is about how should the data be reprocessed or a new survey be acquired in order to achieve the desired objective.

Behind the simplicity of the question: " How good seismic resolution needs to be for detecting thin beds??? hide fundamental wave propagation and signal issues. To start with, resolution and detection are two distinct issues, which require proper understanding to be dealt with. Even if they are not independent notions, they cannot be deduced one from another.

Abundant literature has described the phenomenon of " seismic tuning?? where top and bottom seismic responses of a thin layer are located so close from one another that they interfere and mix into a complex response. The mixing is a direct consequence of the band limited nature of the seismic wavelet which can be propagated through the earth's subsurface. It has the detrimental impact of providing the interpreter with the vision of a layer whose apparent thickness does not correspond to the thin layer's true thickness: the layer top and base interfaces are not resolved. Further, the intensity of the seismic response is distorted to a point that it may not be discerned from the background response: the thin layer might become undetected.

This paper is not another theoretical paper on seismic tuning discussing resolution and detection issues in the ideal case of noise free data. It presents and discusses observations made on real data leading to the idea that high resolution data are not necessarily data with a high power of detection. It rather emphasizes the importance of getting good quality data from an appropriate processing flow. As such, it complements paper OTC-21522-PP from the same authors addressing the issue of using exploration or reservoir 3D data to generate seismic volumes dedicated to Shallow hazard assessment (" SHAZ?? cubes).