David Monk

Fresnel-zone binning: Fresnel-zone shape with offset and velocity function

The concept of the Fresnel zone has been explored by many workers; most commonly, their work has involved examining the Fresnel zone in the limiting case of zero offset and constant velocity. I have examined the shape of the Fresnel zone for nonzero offset and in the situation of constant velocity gradient. Finite-offset Fresnel zones are not circular but are elliptical and may be many times larger than their zero-offset equivalents. My derivation takes a largely geometric approach, and I suggest a useful approximation for the dimension of the Fresnel zone parallel to the shot-receiver azimuth. The presence of a velocity gradient (velocity increasing with depth) in the subsurface leads to an expansion of the Fresnel zone to an area that is far larger than may be determined through a more usual straight-ray determination.

Fresnel zone binning: Application to 3D seismic fold and coverage assessments

In a parallel article submitted to Geophysics, I have outlined the derivation of size and shape for Fresnel zones as a function of offset and velocity gradient. The fundamental conclusion of this analysis is that the Fresnel zone computed using a zero-offset assumption and constant velocity is a poor representation of the “area of influence” of the Fresnel zone in a practical sense. The relevance and practical application of these Fresnel zone sizes is described in terms of current “binning” in 3D seismic (particularly marine 3D), and the Fresnel zone analysis leads to an alternate method of assessing adequate coverage in a 3D survey.

Application of Electromagnetic Scanning to an Australian Frontier Basin With Complex Bathymetry

Apache is exploring a large block offshore Australia. Part of the area is covered with good quality 3D seismic and a number of prospects have been identified from this data. Additional potential prospects were inferred from 2D seismic in the relatively under-explored deep water area of the block. With the current high cost of 3D seismic and drilling programs, it was considered very important to high-grade the prospects and to correctly assess the prospectivity of the deep water area. Initially it was thought that traditional target-oriented CSEM surveys over each prospect would provide the required information. However, after further study, it was decided to cover the entire area with electromagnetic scanning. This approach provides a coarse 3D view of the entire area providing information not just about prospects identified from seismic, but potentially also revealing new hydrocarbon leads. A highly rugose seafloor in combination with a high resistive overburden of varying thickness appeared as a major concern for the scanning survey. A wide, deep submarine canyon provided both operational challenges and data processing issues. The rugosity of the seafloor and varying overburden thickness constitute significant local as well as regional variations to the background resistivity distribution, making it difficult to extract potentially hydrocarbon related anomalies from the scanning data. A novel approach to dealing with this problem was adopted, which takes advantage of complementary information from existing well logs and the available seismic data. A number of 1D inversions constrained by resistivity logs were performed at various locations across the survey. The results of the 1D inversions were then used to build a reference resistivity model that conforms to the bathymetry and the seismically derived overburden thickness. Detailed 3D simulation of the scanning survey for this resistivity model generated synthetic reference responses, which adequately account for most of the bathymetry and overburden related variations in the scanning data. Using these synthetic reference responses to normalize the scanning data, a number of interesting anomalies became apparent, one of which coincides with a known oil reservoir. The same anomalies had been masked by regional trends in previous results obtained by conventional single-receiver referencing. The results obtained significantly increased our confidence in the interpretation of the scanning data and highlight the increased value obtained from an integrated analysis with complementary geophysical data. Introduction Electromagnetic scanning is a reconnaissance application of seabed logging [1] which employs coarse grid acquisition to assess the prospectivity of large areas [2], [3]. The idea is to detect potentially hydrocarbon-bearing resistive formations independent of their seismic expression and high-grade those prospects already identified from seismic data. We present results from a recent scanning survey conducted in an Australian offshore basin as part of a major exploration campaign involving modern 3D seismic acquisition and the drilling of exploration wells. The survey covered an extensive area of 1650 square km where water depths range from approximately 100 m to more than 2500 m. The area hosts a producing oil reservoir and a number of exploration prospects previously identified from 2D and 3D seismic, which had been available over part of the survey area. The scanned basin is characterized by a highly rugose seafloor topography and a complex geology with rock properties that have challenged seismic interpretation in the past. From a seabed logging perspective, the complexity of the geology manifests itself mainly through a carbonate-dominated overburden of largely varying thickness, where resistivity data from wells indicate a sharp increase in resistivity with depth. The prospects are located in the stratigraphic sequence below the overburden at 400-3500 m depth below seafloor and range from about 10 to 20 square km in size.

Reducing infill requirements using Fresnel zone binning and steerable streamers

Typically, most conventional marine 3D seismic surveys are acquired with infill to ensure adequate subsurface seismic reflection coverage in areas where seismic surveys may have been affected by adverse currents, sub-optimal streamer feather angle matching or source/streamer separations. For a typical 3D seismic survey, infill shooting may be as much as 25% or more of the total cost of prime seismic acquisition. By combining the use of an alternative method for assessing coverage with the use of the lateral control steerable streamer systems, it is possible to reduce infill to at least single digit figures for some surveys where considerably more infill was anticipated.

Introduction to special section: Monitoring reservoir and overburden changes

Time-lapse geophysical technologies, including 4D seismic methods, are key enablers for improved hydrocarbon recovery and more cost-effective field operations. By analyzing differences in multiple geophysical surveys and by integrating those analyses with conventional reservoir monitoring data, time

Efficient Wave Field Sampling in Vibroseis Operations.

Summary It has been observed on multiple occasions that after full processing, residual organized noise depends on source and receiver line intervals. An explication of this observation is proposed and illustrated by a recent single vibrator experiment in Egypt.

Physics and the hunt for black gold

In 1924 geologist Ludger Mintrop predicted that the rock beneath a patch of land in Orchard, Texas, should contain a geological feature known as a salt dome – a block of salt that often traps hydrocarbons. The prediction was significant because Mintrop made it by sending sound waves into the Earth and recording the reflections – the first time that anyone had ever tried to use sound to look for reserves of oil or gas. He was vindicated through subsequent drilling, and the basis of his technique is still used by energy companies to find new reserves of oil and gas without first having to drill expensive holes in the ground.

Using thumpers as a seismic source. Why an old technique is now ready for use.

The idea of generating seismic records using a weight drop system is not new (see for example Neitzel 1958). However, the method has generally not been sufficiently accurate or controllable to when seismic is to be used for detailed high resolution seismic. In this paper we examine seismic data resulting from a new breed of source which appears to be more than adequate for the demands now placed on seismic data interpretation.

Evalutation of sea bottom features from seismic calibration data: A case history

Introduction During 1992 a seismic survey was conducted in the West Delta area of the gulf of Mexico. The water depth in the area of the survey varied from approximately 20 feet to over 200 feet, and for this reason, seismic acquisition using a bottom cable technique was selected. Additionally the use of hydrophones and geophone was chosen, because of the potential to minimize ghost and reverberation effects associated with the variable water layer. It was known prior to the commencement of the survey, that in some areas, the “hard” water bottom was overlain by soft mud flows with remarkable physical properties including velocity in some cases of less than 1000 feet/sec. ( ref. Meeder et.al. (1988) ).It was anticipated that the mud flows may present significant problems in both acquisition and processing of the seismic data, and an attempt was made prior to the seismic survey, to map the sea floor features as part of a hazard survey, and some aspects of the seismic acquisition and processing were specifically aimed at trying to resolve the sea floor features. Calibration Data. Vertical incidence “calibration” data was collected as a separate part of the acquisition effort. Calibration data offers one approach to the processing of hydrophone and geophone data for bottom referenced systems, and can be used to determine a scalar, appropriate for application to the geophone component of the seismic data, prior to summation with the hydrophone data, in order to optimally attenuate ghost energy and water bottom reverberations. (ref. Barr and Sanders 1989 ). Other methods of optimally combining bottom referenced hydrophone and geophone data have been proposed, but in this case the calibration data offered an early opportunity to resolve some of the sea bottom features. Note that while the conventional seismic data was in general noisy for a variety of reasons, the calibration data was of very high quality, since only the direct arrivals are examined, and in this seismic data set the source is positioned as close as possible to the receivers ( vertically over each receiver on the bottom ). An example of the calibration data for a set of receivers is shown in Figure 1. Seismic data The quality of seismic data acquired in the survey area was extremely variable. In some areas of the survey, where there were large accumulations of soft mud on the bottom, the seismic records were so weak that only direct arrivals on the near traces were visible when displayed at the same gain as other records where the data amplitudes appeared normal, the mud layers led to large absorption of the seismic energy. Additionally there were at least eight different types of noise regularly observed in the seismic data, and the shallow mud layers resulted in to trace to trace statics which on occasions approached 100 msec.