Mudline to Moho – Imaging from the Sea Floor to the Moho Discontinuity Offshore Argentina

Spectrum’s Oscar Ramirez, Genmeng Chen, Mike Saunders, Laurie Geiger, Milos Cvetkovic, Mark Roberts, and Richard Clarke demonstrate effective processing strategies that produce clear and consistent images of the Moho discontinuity and elements of the crystalline basement. This information was presented at SEG 2018 in Anaheim, California.

Introduction
Northern Argentina has a continental volcanic rifted margin segmented by major transfer zones which reflect the pre-existing Gondwanan structural fabric. The individual rift basins along this margin initially formed from intracratonic rifting during the middle Mesozoic, and evolved into a true passive margin following the separation of the South American plate from the African plate in the early Cretaceous.

The Argentine frontier exploration basin presents competing seismic imaging requirements due to the need for clear images of play fairways, prospects, and deep crustal structures necessary to define a tectonic framework. In the past these competing requirements would have resulted in a compromise between higher fold and record length. Modern acquisition and processing technologies and methodologies can be employed, which offer long record lengths necessary to image events as deep as 40 km, while retaining high fold to improve signal to noise ratios.

The presence of source rock is a key risk factor in this part of the basin with the Lower Cretaceous-Upper Jurassic syn-rift section presumed to contain Aptian age or older source rocks. This part of the section must be imaged clearly with the preservation of amplitudes to help in mitigating the source rock risk.

Results
We will now demonstrate the methodology on a 2D exploration dataset acquired offshore Argentina. 19,500 line km of data were acquired with a 12km streamer on a coarse 20km x 20km grid, covering an area of 210,000 km².

The data were acquired with a record length of 15 seconds and 2ms sample rate, using continuous recording and a nominal shot separation time of 10 seconds. The water depth varies from 50m to 5000m. There are strong, convergent currents in the survey area leading to large feather angles up to 48 degrees. In addition, water temperature variations from 7 to 17 degrees Celsius led to water column velocity variations that required a tomographic update for depth imaging.

The following  summarizes the acquisition parameters:
Source Interval: 25 m
Nominal Time Interval: 10 seconds
Record Length: 15 seconds
Source Depth: 8 m
Streamer Length: 12,000 m
Streamer Depth: 15 m

Continuous Recording / De-blending
The continuous recording and long record lengths enable the efficient acquisition of a dataset with both high fold and a long record length suitable for imaging deep targets, which is particularly important for understanding the basin architecture. The long offsets in the data also make it suitable for full-waveform inversion (Roberts, 2018), which offers an additional opportunity to search for velocity anomalies associated with hydrocarbon saturated sands.

The noise from the overlapping shots in the deeper section is mitigated by an effective multi-step de-blend methodology (Seher and Clarke, 2016). We first estimate the direct wave by creating a median stack of a number of shots, and then adaptively subtract it. Next, we estimate the reflected energy in the Radon domain and apply a direct subtraction. Finally we attenuate random noise using the Karhunen-Loève transform or vector median filtering (Seher, 2018).

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Mudline to Moho – Imaging from the Sea Floor to the Moho Discontinuity Offshore Argentina