Lessons from Nodal Acquisition in South Gippsland, Victoria  

Authors / CoAuthors

Fomin, T. | Costelloe, R.D. | Kemp, T.

Abstract

2D seismic vibroseis reflection data were acquired in the South Gippsland region of Victoria in June-July 2015 as part of a collaborative project between the Geological Survey of Victoria and Geoscience Australia. The purpose of the survey was to gain an understanding of the geometry and internal structure of the Cretaceous Strzelecki Group that includes up to 100m of volcanics (Older Volcanics) and the underlying Palaeozoic basement of the Melbourne Zone. The depth to the basement varies across the four transects from 400 m to 7 km maximum. The survey was designed to image shallow near surface groundwater targets and basin down to 7 km depth as well as deeper crustal structures. The potential deterioration of seismic data quality due to the presence of the shallow volcanics also required investigation. The Fairfield Zland cableless Nodal system was used with 15 m spacing of point receiver nodes rather than the cable acquisition system previously used by Geoscience Australia with an array of 6-geophone strings with 20m spacing. The advantages of using the nodal system are (a) more flexibility in design of the survey, especially in rugged terrains, or urban areas, and (b) significantly easier deployment of the equipment with much less downtime. A number of tests were carried out prior to the acquisition program to compare various sweep parameters and to optimise the acquisition parameters for a range of targets, both shallow and deep. These tests included linear sweeps with expanded bandwidth and non-linear customised high-dwell sweeps. High fold seismic reflection data (nominal fold 600) were collected to improve signal to noise ratio using two 50,000 lb vibroseis units for the source array. In an attempt to overcome the shallow volcanic layer issue data were recorded to long offsets up to 10 km, and not less than 4.5 km where the roads in the area were crooked. Despite the effort of collecting the high fold data and long offsets, the quality of seismic data was significantly degraded due to several reasons including acquisition issues, and, more likely, geology of the region that is not easy to image using conventional seismic techniques. Attenuation appears to have had a substantial effect on seismic data quality, resulting in a loss of energy. The most significant issue seems to be a high level of noise observed in the data, especially at far offsets. This noise is more likely related to environmental noise in the region and due to nodes not being fully buried. However, the seismic data previously acquired in the same area using a cable system also exhibits poor data quality. This suggests a `geological factor as a major issue for good quality imaging of the crust in this area. During the acquisition stage some general issues with the current nodal system were experienced, such as delays and problems of data harvesting and re-construction of shot gathers. This created a problem with real time monitoring of the data quality and resulted in difficulties in modifying acquisition parameters or calling shutdowns due to poor weather conditions. One example of lack of real time monitoring was a leap second event that caused timing errors that was identified only a few days after the event. The data processing is currently being undertaken and is expected to be completed in June 2016. The advanced processing stream includes Pre-Stack Time and Depth Migration, DMO, Common Reflection Surface and Post-Stack Migration processing. The preliminary results show good images of the basin in the shallower section but little reflectivity observed in middle and low crust.

Product Type

nonGeographicDataset

eCat Id

90091

Contact for the resource

Keywords

• External PublicationAbstract

• Vic

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

Publication Date

2016-01-01T00:00:00

Creation Date

Security Constraints

Australian Government Security ClassificationSystem    

Classification - unclassified

Legal Constraints

Creative Commons Attribution 4.0 International Licence    

Access - license

Use - license

Status

Purpose

Maintenance Information

unknown

Topic Category

geoscientificInformation

Series Information

Lineage

Unknown

Parent Information

Extents

[-39.0, -38.0, 145.5, 146.5]

Reference System

Spatial Resolution

Service Information

Associations

Downloads and Links

Source Information

Source data not available.