Paul A. Milligan

Imaging shallow objects and heterogeneities with scattered guided waves

Current surface seismic reflection techniques based on the common‐midpoint (CMP) reflection stacking method cannot be readily used to image small objects in the first few meters of a weathered layer. We discuss a seismic imaging method to detect such objects; it uses the first‐arrival (guided) wave, scattered by shallow heterogeneities and converted into scattered Rayleigh waves. These guided waves and Rayleigh waves are dominant in the shallow weathered layer and therefore might be suitable for shallow object imaging. We applied this method to a field data set and found that we could certainly image meter‐size objects up to about 3 m off to the side of a survey line consisting of vertical geophones. There are indications that cross‐line horizontal geophone data could be used to identify shallow objects up to 10 m offline in the same region.

Analysis and removal of multiply scattered tube waves

Because of irregularities in or near the borehole, vertical seismic profiling (VSP) or crosswell data can be contaminated with scattered tube waves. These can have a large amplitude and can interfere with weaker upcoming reflections, destroying their continuity. This type of organized noise cannot always be removed with filtering methods currently in use. We propose a method based on modeling the scattered tube‐wave field and then subtracting it from the total data set. We assume that the scattering occurs close to the borehole axis and therefore use a 1-D impedance function to characterize borehole irregularities. Estimation of this impedance function is one of the first steps. Our method also accounts for multiply scattered tube waves. We apply the method to an actual VSP data set and conclude that the continuity of reflected, upcoming events improves significantly in a washout zone.

Hydrophone VSP imaging at a shallow site

We evaluated the capabilities of vertical seismic profiling (VSP) for imaging the complex heterogeneous unconsolidated sedimentary structures at a shallow site. We deployed a 24‐level hydrophone array with 0.5-m level spacing down a preexisting poly vinyl chloride (PVC) cased well. Data acquisition time was quick. Only 15 multioffset shot points using a hammer‐on‐plate source were needed to acquire reflection data between the water table at 3 m and the bedrock at 35 m to produce a depth section image. This image extended 9 m from the receiver well, yielding resolutions between fresh‐water‐bearing sands and impermeable muds and clays of better than 1 m. Depth accuracy of the image was confirmed by good correlation with cone penetrometer logs. We used conventional wavefield separation and VSP-CDP mapping techniques to image the data. Tube waves, created by seismic arrivals at cross‐sectional area changes in the borehole fluid column, were the primary source of coherent noise in the data. The tube‐wave arriv...

The use of VSP techniques for fault zone characterization: An example from the San Andreas Fault

Vertical seismic profiling (VSP) technology is increasingly being used in the fields of earthquake seismology and tectonics. This is motivated in part by the growing number of oil field microseismic monitoring surveys, but more so by projects that involve drilling deep wells for monitoring crustal activity at depth. Examples of these projects are the San Andreas Fault Observatory at Depth (SAFOD), the Nankai Trough Seismogenic Zone Experiment, the Gulf of Corinth Rift Laboratory, and the Taiwan Chelungpu Fault Drilling Project, and other projects by the International Continental and Ocean Drilling Programs (ICDP and IODP). These projects require instrumentation and surveying in deep and possibly hot borehole environments. With higher resolution than surface seismic data, images from 2D and 3D VSP data contribute to better characterization and interpretation of complex reservoirs at smaller scales. The location of receivers in the low-noise borehole environment yields higher signal-to-noise ratios, higher ...

Crosswell electromagnetic and seismic imaging: An examination of coincident surveys at a steam flood project

We studied crosswell electromagnetic and seismic images of three oil‐saturated intervals within a southern California heavy oil field undergoing steam flood. The crosswell survey is located in a portion of the field where one well is in a “cold spot,” resulting in differing steam propagation within the three units. Log analysis shows linear or second‐order polynomial relationships (with correlation coefficients greater than 0.7) between electrical conductivity and water saturation, porosity, and clay content; whereas only a weakly linear relationship can be found between velocity and temperature in the lower unit studied. Crosswell seismic data is used to produce a velocity tomogram and a reflection section, and crosswell electromagnetic data is used to produce a conductivity section and derived porosity and water saturation. The seismic velocities from the tomograms show lateral variations consistent with the lateral variations in temperature seen in observation wells on either side of the crosswell sect...

Optimized 3D VSP Survey Geometry Based On Fresnel Zone Estimates

Summary We present a method to optimize the acquisition geometry of 3D VSP data, that is, seismic data recording surface sources in large-aperture borehole receiver arrays. By estimating the Fresnel volume of specular rays at the target depth of interest, we are able to adapt the shooting geometry to the varying size of the Fresnel zone with offset from the well. Such an adaptive pattern of varying shot spacing helps to reduce illumination artifacts close to the receiver well and, at the same time, reduces the overall number of shotpoints, and thus, effort required to obtain the same image volume. We show a real data example where our approach has successfully been applied to optimize the shotpoint geometry.

3-D Velocity Imaging in the Shallow Subsurface using Multi-well, Multi-offset, VSP Data: A Case Study from the Lawrence Livermore National Laboratory Site

A vertical seismic profiling (VSP) method was developed to produce compressional (p-wave) velocity sections in a 3-D survey volume, down to a maximum depth of 57m. The VSP method consisted of a multilevel hydrophone tool deployed down several wells. Multiple offset surface shotpoints were recorded at multiple azimuths around each well head, using an impact source. The VSP data were processed and inverted to produce p-wave interval velocities. These velocity sections were then projected into a model of the survey volume, together with borehole log information and other geologic information, to produce a 3-D visualization which aided the location and interpretation of aquifer boundaries below the water table. The final image displayed several low velocity zones, and these were attributed to partially saturated pore spaces, possibly both natural, or pump induced by air suction. Low velocity layers appeared above several actively pumped aquifers, and it was concluded that the multi-offset, multi-azimuth, VSP ...

Anisotropic Statics For a 2D Line Over a Mountainous Overthrust Belt In Bolivia

The most important discovery made during this study was the existence of near-surface anisotropy, leading to azimuth dependent velocities and arrival times for refracted waves. A method was developed to process first-break picks from these refraction arrivals, and create a set of up-line and down-line static corrections for each shot station and receiver station in a surface consistent manner. Correct application of these azimuth dependant statics to gathers of CMP traces were used to effectively ’beam-steer’ the stack to create or destroy reflection horizon coherency depending upon the horizon dip.

A Dutch Day in the Field at the Richmond Field Station

Current surface seismic reflection techniques based on the common-mid-point (CMP) reflection stacking method can not be readily used to image small objects in the first few meters of the weathered layer.

Near-surface Exploration For Treasure-filled Tunnels In Grano-diorite

A seismic exploration program for shallow tunnels (-28 meter) in the Philippine province of Neuva Viscaya has identified target areas that correlate at several boreholes with the known location of subsurface voids.Two surveys were conducted: i 3D VSP in the summer of 1995, and a fairly unconventional 3D geometry utilizing subsurface sources and surface geophones in February of 1997. Analysis of both datasets by volumetric projection of anomalously low-amplitude raypaths the identification of a target zone at 28 meters. In March of 1997, miners directed by survey results working offshoots to a horizontal tunnel intersected a fracture zone at 28 meters depth. The target zone defined by the anomalously low-amplitude seismic data is co-located with this fracture.