Problems associated with PZ summed data

PZ summation is a technique commonly used for to remove water layer multiples and receiver ghosts. It is based on the combination of pressure (scalar) recorded by hydrophones and vertical velocity (vector) recorded by geophones. After PZ summation, seismic data contains only up-going energy. The up-going events, such as primaries and source-side multiples, are recorded on a hydrophone (P) and a vertical geophone (Z) with the same polarity. While the down going events such as the direct arrival and receiver ghost are recorded on a hydrophone (P) and a vertical geophone (Z) with opposite polarity (Figure 4.4). Thus, PZ summation suppresses the receiver-side ghost and multiples, but not the source side effects.

Figure 4.4 Schematic showing the polarity of different arrival on a geophone and hydrophone: (a) the up going primary is recorded as the same polarity, (b) the direct arrival as opposite polarity, (c) the receiver-side ghost as opposite polarity, (d) the source multiple as the same polarity and (e) the receiver multiple as opposite polarity.

There are several issues associated with the PZ summed data available, and thus only the inversions using the pressure data are shown.

1. PZ summed data is not physical, and cannot be readily simulated, as it has only up-going waves, while down-going waves are removed. Receiver side ghosts and multiples are partially suppressed, while source side effects are still present in the data. Most inversion and modelling algorithms either use an absorbing boundary or a free surface. However, using an absorbing boundary, all multiples are attenuated. Conversely, using a free surface, all multiples are predicted provided the water layer is accurate. Simulation of the PZ summed data though, requires a free

(a) Up-going primary

surface for modelling the source side ghosts and multiples, and an absorbing boundary to suppress the receiver side ghosts and multiples. This is not straightforward to simulate. Both the hydrophone and geophone responses can be simulated, and PZ-summation repeated for the synthetic data, before the calculating a residual dataset for the back-propagation stages of the inversion. This though may be very costly. Alternatively further preprocessing may be necessary to remove the source side multiples such that an absorbing boundary condition can be imposed, or no hydrophone data with multiples should be used such that a free surface condition can be imposed. Free-surface multiples increase the non-linearity of the problem, and hence its removal may seem appealing.

2. The OBC data were acquired with offsets up to 11 km. At short offsets, both reflections and refractions are dominant, but at offsets larger than 6 km, the wide-angle turning rays are dominant.

While it is relatively easy to suppress the surface-related multiples and receiver ghosts for the near-normal-incidence reflection, it is not straightforward for the wide-angles. Almost all existing PZ summation schemes are sufficient for sub-critical reflections, but inadequate for wide-angle arrivals which are an essential ingredient in a successful inversion scheme to update the macro-velocity. The PZ data offsets were clipped to a maximum offset of 6.2 km due to the inadequacy of this technique at larger offsets. Diving rays that travel 1.5 - 2 km deep are typical of this offset range. Thus, FWI using the PZ summed data could aim to improve the imaging of the shallow gas cloud in the overburden rocks, but not the deeper reservoir, as only reflected waves are present at these depths (3km) with 6 km offsets. FWI of the reservoir would be useless, but improved imaging of the overburden would potentially allow for improved reflection imaging of the reservoir.

3. Lower frequencies are consistently recorded by the hydrophones and not the geophones. Thus, the PZ summation used a low-cut filter to suppress the lower frequencies, and the summed data does not contain frequencies less than 5 Hz. Extremely low-frequency data may be noisy, but are essential for FWI as they allow for a more reliable update of the background velocity, and for the mitigation of the effects due to the use of a poor starting model.

5. The source wavelet used for the acquisition of the data is unknown. Source inversion is not yet incorporated into the 3D code developed by the FWI group at Imperial and will in any case obscure systematic velocity errors. Thus, an estimation of the source wavelet is needed. The best estimate of the source wavelet for the PZ summed data was obtained by applying minimum-phase spiking deconvolution to a shot gather, followed by a linear moveout, horizon flattening on first arrival picks and subsequent stacking. The wavelet though was extracted for offsets ranging from 3700 to 6200 only, and was inadequate at shorter offsets. At short offsets, short period multiples on the source side should be incorporated into the source signature, as the source side effects are still present in the data.

The deconvolution suppressed the short period multiples. Thus, at short offsets the signature used was inaccurate and ringy. Also the true source signature most likely varied with angle while the

the test inversions (not discussed in this thesis) excluded data with offsets less than 1800 m. This was seen as insufficient to obtain useful updates in the shallow overburden of the velocity model.

6. Any amplitude irregularities should also be edited prior to inversion, using surface consistent gain applications (Vigh et al., 2009). The amplitudes of the PZ summed data were altered in order to match the geophone and hydrophone data. Amplitudes are true for any given trace, but not correctly scaled relative to other traces, that is, they are matched in time but not spatially. Thus the amplitude-offset relationship has been lost or partially lost with PZ summation.

FWI using the PZ summed data was initially tested. However, due to the aforementioned reasons it was not pursued further and the PZ summed FWI models are not presented in this thesis. All the inversion results shown in the following section use the raw pressure data.