SUSPENSION P-S VELOCITY LOGGING METHOD Overview Suspension P-S velocity logging is a relatively new method of measuring seismic wave velocity profiles. Developed in the mid-1970s to answer the need for a technique that could measure seismic shear-wave velocities in deep, uncased boreholes, it was originally used by researchers at the OYO Corporation of Japan. The method gained acceptance in Japan in the mid-1980s and was used with other velocity measurement methods to characterize earthquake site response. Since the early 1990s it has gained acceptance in the U.S., especially among earthquake engineering researchers. GEOVision personnel have logged over 300 boreholes using this technique since 1991.
Procedure
7-Conductor cable
The OYO P-S Logging System uses a 7-meter probe, containing a source and two receivers spaced 1 meter apart, suspended by a cable. The armored 4- or 7-conductor cable serves both to support the probe and to convey data to and from a recording/control device on the surface. The probe is lowered into the borehole to a specified depth (a rotary encoder on the winch measures probe depth), where the source generates a pressure wave in the borehole fluid. The pressure wave is converted to seismic waves (P and S) at the borehole wall. Along the wall at each receiver location, the P and S waves are converted back to pressure waves in the fluid and received by the geophones, which send the data to the recorder on the surface.
OYO PS-160 Logger/Recorder
Cable Head
Diskette with Data
Head Reducer Winch
Upper Geophone
Lower Geophone
Filter Tube
The elapsed time between arrivals of the waves at the receivers is used to determine the average velocity of a 1-meter-high column of soil around the borehole. Source to receiver analysis is also performed for quality assurance.
Source Source Driver Weight Overall Length ~ 25 ft
Oyo PS Suspension Logger Setup
Waveform Data for a Single Measurement Oyo PS Suspension Logging System
“a bold new vision in geophysical services”
Applications Typical applications of suspension P- and S-wave velocity logging include: • •
Dam safety investigations Seismic site response studies for bridge abutments, dams, buildings, etc. Foundation studies Measurement of soil/rock properties (i.e. shear modulus, bulk modulus, compressibility, and Poisson’s ratio) Characterization of strong motion sites Velocity control for seismic reflection surveys
• • • •
Key Benefits Suspension P- and S-wave velocity profiling using the OYO Suspension Logger has become the method of choice for obtaining high resolution borehole velocity measurements. The reasons are many:
• • • •
Depth Sequential Waveform Arrivals
TREASURE ISLAND BOREHOLE USN-1 SUSPENSION LOGGING P- AND S-WAVE VELOCITIES; DATA COLLECTED OCTOBER 13, 1994 0
R1-R2 S-WAVE R1-R2 P-WAVE
50
100
150
GOLDEN GATE BRIDGE HOLE B6 SUSPENSION LOGGING P and S Wave Velocities; Data collected December 14-15, 1991
0 1800
4800 2800
9300
3400
5800
100
S-Wave P-Wave Suggested layer velocities
7200
DEPTH, FEET
•
Only method that obtains both P- and S-wave velocity data reliably in a single hole at depths greater than 200 ft. Can be used in either uncased or cased (PVC) boreholes, although results are always better in uncased holes. Can be used in boreholes drilled from barges. Offers very high resolution (typically 1 meter) for resolving thin layers that can have a dramatic effect on surface response. Requires only 1 hole, as opposed to crosshole methods that require at least 2. Has been used to depths of 2,000 ft.
200
250
12400
300 6600
12000
350
200 7400
Depth, ft
•
13000
6300
11000
400
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
VELOCITY, FEET/SEC
300
8200
•
15000
400
• 500
0
5000
10000
Velocity, ft/sec
15000
20000
•
Specifically adapted to soils, whereas tools developed for oil exploration are optimized for rock. Permits measurement of soil and rock properties such as shear modulus, bulk modulus, compressibility, and Poisson’s ratio. Not hindered by fast layers and lack of depth penetration like surface methods such as downhole.
1151 Pomona Road, Unit P, Corona, California 92882, ph. 909-549-1234, fx. 909-549-1236, www.geovision.com
