We evaluate feasibility of virtual source redatuming with a synthetic 1D elastic dataset using receivers buried at 30 m. This work is to help validate an onshore field experiment in Saudi Arabia. In this study the receivers are located within the complex heterogeneous near surface. A major challenge is that our model contains a large number of highly contrasting layers and as a result is strongly contaminated with both surface and internal multiples. We demonstrate that virtual source redatuming is feasible under these conditions and delivers a reliable image of the target horizon. We examine various pre-processing options that can address multiples and improve virtual source imaging. The effects of source aperture and sampling are demonstrated on the final images. An optimal selection of the crosscorrelation time gate is made by observing the effects of different ghost arrivals on the resulting stacks. Using analysis of the correlation gathers, we quantify improvements introduced by up-down wavefield separation using land dual-sensor summation and justify the selection of a larger summation aperture.
Summary A new imaging application is presented using sonic waveform data to locate a nearby borehole. Conventional methods for locating one borehole from another borehole generally involve passive magnetic and active electromagnetic (EM) sensing to detect the presence of magnetic or conductive casing. The deeper reading EM methods do not work well when the two boreholes are orthogonal. In this case, acoustic full-waveform sonic data is used to identify the location, distance and direction to a target vertical borehole from a near horizontal sidetrack well. The target borehole was located at a distance of 9.25 ft with an accuracy of less than 0.5 ft, which was subsequently confirmed by drilling. In addition, acoustic data allowed to image up to 100 ft above and below the observation well at a resolution better than gamma ray and electric logs from the target well.
Summary A series of land surface vibrator repeatability tests were conducted in Saudi Arabia as a part of a feasibility experiment for permanent monitoring. While post-stack repeatability of 15% to 20% was achieved, pre-stack seismic repeatability is difficult to accurately quantify. If we can understand the main controlling factors behind pre-stack vibrator repeatability, we stand a chance to improve upon these results. This study focuses on pre-stack repeatability metrics for field data acquired using a surface vibrator and a combination of surface and cemented buried geophones. A series of six 2D surveys were repeated as well as daily and hourly sweep tests. Observations suggest that the main factors affecting seismic repeatability include vibrator geometry errors (as small as 0.5 m), and how the vibrator interacts with the near surface. It was observed that the initial sweeps acquired with a vibrator show significant time and amplitude variability as measured by both the surface and deep cemented sensors, whereas data recorded from later sweeps appears more repeatable. This initial “warming up stage” followed by a more stable sweep was observed on all repeat acquisition tests, even when sweep sequences were only one hour apart. This effect may be caused by ground compaction, with some partial rebound within a short time following termination of the sweep sequence. Due to all these factors, it is clear that land seismic data acquired using a surface vibrator has some inherent non-repeatability, even when the source positioning errors are minimal.
Summary We present results of a feasibility study evaluating various configurations for seismic monitoring in a desert environment. The experiment, conducted in an onshore field in Saudi Arabia, involved drilling and instrumenting 80 shallow receiver holes located along a 2D line and shooting multiple repeat surveys with a dense carpet of vibrator points. In each shallow hole colocated geophones and hydrophones were permanently cemented at 10, 20 and 30 m below surface. A small cluster of 12 surface geophones is placed next to each hole for comparison purposes. It was essential to design a processing sequence that was optimized for imaging and repeatability. The best stack image and repeatability were obtained using data from the receivers located at the deepest level. Post-stack repeatability of around ~ 15% normalized root-mean-square amplitude (NRMS) is obtained over a large portion of the 2D line. Virtual source redatuming of buried receiver data offered additional imaging improvements. Both conventional and redatumed images show significant improvement when adaptive dual-sensor summation was also utilized. Despite a very complex near surface, dense and frequent shooting to permanently cemented buried sensors delivers repeatability approaching marine data.
Summary A seismic field acquisition test was conducted in an onshore field in Saudi Arabia. The effects of near-surface complexity (in the form of sand, karsts, topography), environmental noise as well as large surface temperature variations are illustrated and quantified by 4D attribute analysis using permanent seismic sources and buried geophones. We show that burying receivers dramatically improves the wavelet amplitude stability as well as naturally reduces the effect of man-made surface noise.
The near-surface layers comprise sand from 3 m to more than 20 m thick in this area overlying layered limestones and marls with karsting of limited lateral extent from 20 m to at least 50 m deep. Data quality from the permanent sources at this depth was inadequate for imaging using such low-fold acquisition, but the best data seems to come from the deepest buried sensors at 50 m. In addition, more sensitive sensors may be used to properly record weak high-frequency signal in this buried quiet environment.
Summary We apply virtual source redatuming to land seismic data with shallow buried receivers and evaluate potential improvements in post-stack repeatability associated with redatuming. In particular we focus on examining how redatuming can handle three typical sources of land seismic non-repeatability: changes in acquisition geometry, variable source coupling and daily/seasonal changes in the near surface. We use a synthetic model derived from log and well data and acquisition parameters inspired by a recent field experiment conducted in one of the onshore fields in Saudi Arabia. We demonstrate that, in all cases, the virtual source method improves repeatability of redatumed stacked images even when the density of virtual sources is substantially smaller than that of the surface sources.
Summary Hydrophones show a lot of promise as permanent single sensors for land seismic monitoring due to their potential to reject shear wave energy; however, a key issue in desert environment is their performance in dry rock above the water table. To investigate this issue a feasibility study was conducted over a producing oil field in Saudi Arabia. Results presented here include comparable images of colocated geophones, hydrophones and two distinct dual sensor applications. Conclusions indicate dry hydrophones can produce similar or slightly better images to geophones while dual-sensor summation delivers superior results over either individual dataset. Though challenges such as hydrophone coupling remain, the results are encouraging enough to seek improved dry hydrophone packaging and deployment methods, which can address both image and repeatability requirements.