Land seismic acquisition testing strategies and results – Southern Chad, Africa 2013-2015

In order to improve imaging in areas with surface laterites/near surface scatters, over the past four years, 23 seismic acquisition tests were incorporated into the regular Glencore seismic acquisition programmes in Chad. Seismic acquisition testing provided tremendous benefit by improving subsurface imaging, increasing operational efficiency and reducing costs. These tests included eight vibroseis parameter tests, four maximum offset/record length tests, three sweep effort tests, three 2D Swaths (adding 50 km of high-resolution subsurface data), two noise tests, one receiver array test, one sweep force test and one 3D Cross-Spread test, which provided targeted 3D imaging. Most of the acquisition tests were incorporated into the regular seismic acquisition production with little delay or additional cost, so that any potential improvements could be applied immediately. This paper will focus on the tests and results in two areas: Region 1, which has near surface laterites and only fair-to-good data quality; and Region 2, where mixed laterite / laterite free zones resulted in good-to-excellent data quality. Region 1 area is covered in red, iron-rich clay soils with areas of crystalline laterite at the surface. Laterite is a soil and rock type which is rich in iron and aluminium, and is commonly formed in hot and wet tropical areas. Laterites are typically rusty-red due to high iron oxide content. Laterite may form chunks and boulders in the subsurface which may act as scatterers. Additionally, vibrator coupling deteriorates in areas with hard laterite at the surface. Data quality in Region 2, which is medium to highly structured, is excellent except under large continuous areas of laterite, where the data quality is only good. Away from laterite, 80 Hz continuous reflectors were seen as deep as 3 seconds with excellent imaging. Under laterite, frequencies and signal-to-noise ratio were significantly reduced, with only 50 Hz along reflectors at 2 seconds.