Avoiding pitfalls in shallow seismic reflection surveys

Abstract

Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air-coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common-midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting nearsurface reflection data correctly often requires more than a simple scaling-down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section therefore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing.

Interlobate esker architecture and related hydrogeological features derived from a combination of high-resolution reflection seismics and refraction tomography, Virttaankangas, southwest Finland

“…Additionally, the complex depositional environment such as esker cores, MUKH structures and fan lobe channels as well as the fractured bedrock lead to lateral velocity variations. Even though in shallow seismic surveys reflections from the water table are not always common (Steeples and Miller 1998), one can notice these reflections here distinctly in the shot gathers (e.g. Fig.…”

“…The wireless sensors spaced along the entire length of each profile allowed the energy from the refracted waves to be recorded as far as 1 km notably improving both the investigation depth for the refraction tomography and the fold coverage at depth for the reflection data. For the reflection data processing, a simple and conventional algorithm was chosen to avoid generating processing artefacts (Black et al 1994;Steeples and Miller 1998). The algorithm is based on signal enhancement in the pre-stack data and CMP (common midpoint; 2-m spacing) stacking after normal moveout corrections.…”

Shallow subsurface imaging using high resolution seismic reflection methods

“…which often make seismic reflections subtle and noisy. Hence, as compared to conventional processing techniques, more attention must be paid when we process the high-resolution data e.g., ground roll, removing of air-blast noise, spatial aliasing and refraction muting (Steeples et al, 1998;Steeples, 2000). …”

Looking Ahead of a Tunnel Boring Machine with 2-D SH Full Waveform Inversion

“…Due to the limited space on a TBM, only a few source and receiver positions can be used. Conventional imaging techniques for near-surface exploration suffer from various pitfalls (Steeples and Miller, 1998). Incomplete aperture and inadequate velocity analysis cause artifacts in the images.…”