Why doesn’t my geotechnical data tie my geophysical surveys?
Duality of P-wave velocity in the near seabed explains why geotechnical boreholes mistie shipboard and AUV reflection data
Svitzer Surveys offers repicked velocity logs for use in soil properties analysis and tying of time sections to depth data. The majority of marine survey data, pinger, multibeam and reflection seismic, use reflected acoustic signals. These are ideal for mapping the seabed and deeper features, however the near-seabed presents a problem where sediments are unconsolidated or fluid-charged.
Foley’s paper from the 2015 SEG Near Surface Asia Pacific meeting “P-Wave Velocity Variations in Near Seabed” explains the mechanism of ambiguous velocities in saturated, unconsolidated, porous sediments, see Seabed Misties with borehole data. Essentially for a short distance in the seabed p-wave velocities split between fluid and sediment frame transport mechanisms, with the latter being the slower. Soil data will not match the fluid column velocities, they match the sediment grain matrix velocities. If you are trying to tie soil properties to p-wave reflection data with a 1,500m/s or 4,850f/s velocity and are not getting a match try again with a slower velocity. A good example that illustrates this phenomenon is Andre Pugin’s example from some Canadian Geological Survey work in Ontario. Pugin acquired a land 2D land seismic survey with three component geophones over fully-saturated marine clay in a graben, the Ottawa Graben. His example of a 2D line, posted in late May 2016 and shown below, shows the comparison between the p-wave cross-section in p-wave two-way time and the s-wave cross-section in s-wave two-way time; the sections are displayed to be equivalent in the vertical scale.
2D seismic lines from the Ottawa Bonnechere Graben – marine clay fill
The s-wave travel paths are unaffected by fluid as there is no transport path for s-waves in fluids. These shear wave velocities are a good guide, your first estimate of p-wave sediment velocity should be at twice the s-wave velocity and subsequent tests should be around that mark. A quick QC check is to calculate Poisson’s Ratio using the p-wave and s-wave velocities – if you derive a value in the 0.45 – 0.5 range you’ve picked the fluid p-wave velocity.
The most effective method for accurately determining seabed sediment velocities is by a transmission survey – refraction, P-S logger, VSP or seismic cone. These methods allow the interpreter to see the two legs of p-wave transmission and compare them with the s-wave velocity. The arrival of the second, slower sediment frame waveform allows determination of the two-way reflection time of the core sample or in-situ test on the reflection record. The most graphic conversion of depth to time is to convert a VSP to a two-way time record so that all reflections are mappable to exact depth datums.
Illustrative of the fluid transport effect is a set of velocity surveys Bruce Redpath shot at the Hanford, WA nuclear site.Borehole C-4996.
This borehole has both p-wave data and s-wave velocity data, look at the resulting profiles – note the fine detail on the p-wave survey above surface-280 feet, which is the level of the water table. Below the water table the p-wave results are homogenous, almost a straight line. Now compare the much more detailed s-wave results and look at the very large velocity transition from 2,170 fps to 7300fps at surface -304 feet level. This is reported in the geological report as a compaction discontinuity. Indeed the s-wave results show the Ringold Unit A to be highly compacted whereas the Reworked Ringold overlying this unit is much less compacted. Now look for this transition on the p-wave section – it is absent. An s-wave VSP showed a prominent horizon at this level which is absent on the p-wave VSP, as it would be on a p-wave reflection seismic section.
Takeway: Marine site investigators beware, you may not be able to resolve near-surface compaction changes with p-wave reflection seismic – if you are trying to install a suction anchor this would give you a serious headache!