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Multicomponent high-resolution seismic reflection profiling

Geological Survey of Canada

View larger version (137K): [in this window] [in a new window]   Figure 1. Minivib minibuggy towing a landstreamer consisting of three-component geophones mounted on 48 sleds at 0.75-m spacing.

View larger version (89K): [in this window] [in a new window]   Figure 2. High-resolution seismic profile obtained over a buried valley beneath Holocene marine sediments. (top) SV-wave seismic section (in time). (center) Contoured plot of shear-wave interval velocity picked using semblance analysis. (bottom) Interpreted SV section (in elevation) after topographic corrections. Bedrock = Precambrian Shield; Gr = gravel; Sd = sand; Md = 1–4 marine mud units separated by unconformities.

View larger version (127K): [in this window] [in a new window]   Figure 3. Two-way traveltime P-wave (top) and SV-wave (bottom) seismic sections from Kelowna, south central British Columbia, recorded with an inline vibrating source. While the P-wave section gives very little information on the subsurface, the SV-wave inline receiver section is able to delineate parts of the valley infill.

View larger version (64K): [in this window] [in a new window]   Figure 4. Display of a 2-C raw record from the line shown in Figure 3. The vertical receivers (V) do not display useable P-wave reflection energy. The horizontal receivers (H1) display very coherent SV reflections.

View larger version (54K): [in this window] [in a new window]   Figure 5. 9-C records acquired over shallow bedrock (20 m below ground surface) under Champlain Sea sediments (marine clays). Each row displays the raw field records acquired with one source orientation (V = vertical, H1 = inline horizontal, and H2 = transverse or crossline horizontal); each column displays the records acquired with one receiver orientation. P, PS, and S phases can be clearly identified in the records as shown in the upper left. Circled in green are P-wave phases present on the vertical receivers with all three source orientations; in blue are very polarized, high-velocity phase (refraction?) at the sediment-bedrock interface; in red are frequency differences in the shear-wave bedrock reflection that are observed depending on the receiver orientation. Here the highest frequencies are seen on the vertical receivers and the lowest on the crossline receivers.

View larger version (150K): [in this window] [in a new window]   Figure 6. Multicomponent stacked sections acquired on the edge of a buried valley beneath Champlain Sea sediments. The upper three rows are processed shear-wave sections; in each case, the source and receiver orientations are indicated in the brackets. The strong bedrock reflection corresponds to a depth of 20 m. The highest-frequency shear-wave data and the shallowest reflections are obtained with an inline source and a vertical geophone (center of upper row). A P-wave section can be processed using a source vibrating in any direction (bottom row).

View larger version (49K): [in this window] [in a new window]   Figure 7. Spectral energy plots of the bedrock reflection event as a function of frequency and horizontal source orientation. The vibrating source is crossline at 0° and 180° and inline at 90°. Data were acquired at angle increments of 15°. SH-wave energy with a frequency band of 30–100 Hz is clearly present in the crossline receiver (bottom plot); SH plus a distinct SV phase with a frequency band of 100–200 Hz appears in the inline receiver (middle plot); only SV is present in the vertical receiver (top plot).

View larger version (10K): [in this window] [in a new window]   Figure 8. Hodogram showing particle motion of a refracted(?) phase which is almost linear (upper row), and the bedrock reflection which is more elliptic in the H1-V projection (lower row).

View larger version (13K): [in this window] [in a new window]   Figure 9. Principle of a polarized wave rotation. Polarized phases are rotated to a new reference system defined by the major axis (a) and the minor axis (b) of the polarization ellipsoid.

View larger version (19K): [in this window] [in a new window]   Figure 10. Demonstration of polarization and axis rotation as a function of time for four receiver positions in the (H1,V) plane. (left) V and H1 components at four receiver locations. (center) Angles of polarization determined as a function of time over a moving window of 0.2 s. (right) The two rotated components corresponding to the long (a) and short (b) axes of the polarization ellipsoid.

View larger version (166K): [in this window] [in a new window]   Figure 11. Stacked 2-C shear wave sections showing the effects of polarization rotation. The label in the bottom left corner of each section specifies the receiver component that was processed to produce the section (V = vertical, H1 = inline, the results of the rotation operation a and b are represented in the third and fourth panels). To the right of each section, we show an expanded plot of 150 m of the line. The b-axis stack shows remarkable continuity in the shallow reflections observed above the bedrock valley. The lower section shows the P-wave section stacked from the vertical component. The wavelength of the P-wave is approximately eight times longer than the S-wave wavelength.