Deghosting and Designature
PGS deghosting and designature techniques aim to create a simple and robust seismic wavelet to ensure reliable subsurface information can be extracted from the recorded data.
The recorded wavefields can be described as the convolution of several independent components. To obtain the earth’s impulse response, all seismic acquisition effects need to be removed.
Recorded signature = SGF * gs * gr * R
Where SGF is the ghost-free source signature, gs is the source ghost, gr is the receiver ghost and R is the instrument filter.
PGS offers a complete set of deghosting solutions comprising both acquisition based and processing based techniques or a combination of both. Combining GeoStreamer® receiver-side deghosting with the GeoSource® source-side deghosting provides the most accurate results since no simplifying assumptions about the sea-surface conditions are made.
Source designature (SGF) is the process of collapsing the far-field signature into a desired output wavelet (generally a zero-phase wavelet). The far-field signature estimation can be improved by making use of near-field hydrophone recordings.
Receiver deghosting (removing gr) accounts for the fact that the sea-surface reflects seismic energy back into the water layer such that any seismic event is recorded twice: first as an up-going wavefield that has been reflected by the subsurface, and then as a ghost, which is the down-going field that has been reflected by the sea-surface. Hence, a doublet response is being recorded. Receiver deghosting effectively compensates for this doublet effect, creating a single event response.
The receiver deghosting process also takes into account variations in receiver depth and emergence angle.
The seismic energy emitted by the seismic sources consists of two constituents: one component propagates directly downwards into the subsurface, the second component, often referred to as the source ghost, first propagates upwards to the sea-surface before it is reflected at the sea-surface again to propagate as a down-going wavefield.
Source deghosting (gs) is applied to generate single event responses of the sources from the doublet responses recorded because of the presence of reflections from the sea-surface.
Acquisition or Processing-based Deghosting
Two types of method are offered to address the ghosts, acquisition- and processing-based solutions:
- Acquisition-based solutions use real physical measurements such as multi-component sensor recordings, as in GeoStreamer, to eliminate the receiver ghost and multi-level source arrays as in GeoSource to remove the source ghost.
- Processing-based solutions are designed to work for single-sensor streamer recordings and conventional source arrays. As such, these solutions suffer from the intrinsic limitation of the initial data recordings. They are also based on simplifying assumptions about the shape and reflecting properties of the sea-surface and require the precise knowledge of the source and receiver depths, so that the ghost functions can be modeled.