REFLECTED WAVE TECHNIQUE – MINI CDP

Of all the methods used in shallow-depth seismics, the method of reflected waves in the modification of the common depth point (CDP) is provided the greatest depth of the upper part of geological section investigation. The acquiring system provides a sufficiently high density of the points of elastic waves excitation and signal reception on a profile unit in order to provide multiple overlapping tracking of the reflecting boundaries.

The technology of work is quite labor-intensive, but provides the most reliable information about the geometry of physical properties boundaries in the studied space, associated not only with stratification, but also with possible violations of rock continuity due to tectonic faults and fractures . In many cases, the use of simple and environmentally safe surface sources of elastic waves such as “drop weight” allows to obtain sufficiently strong reflections from the boundaries at a depth of up to 400-500 m.

Seismic time CDP section obtained by source 40 kg “drop weight”. The seismic station IS128-01, the upper-Kama Deposit of potashe salts, (Materials submitted by the division of the active seismic of Mining Institute of UB RAS, Perm)

Buried palaeovalleys

Usually aim for engineering surveys is to study the properties of the soil massif and the bedrock at a depth of 40-50 m. But sometimes there are specific tasks that require depth of study of 150-200 m. An example of such problems in the North-West of the Russian platform is the mapping and study of the structure of the buried pre-glacial valleys filled with moraine deposits. Similar problems arise in the study of erosion cuts in Paleozoic halogen-carbonate strata filled with low-velocity Mesozoic or Quaternary deposits. In some cases it is possible to obtain section with depth up to 200 m when sledge hammer weighing 6-8 kg is used as a source of elastic waves. Below an example of a time section across the palaeovalley edge buried in the area downstream of Plavinas hydroelectric power station is shown on the site survey for the projected renovation of drainage wells.

Fragment of CDP time section on a profile across the buried palaeovalley edge. Area of the drainage wells location in the lower pool of Plavinas HPP

The upper part of the section is alluvial deposits. In the initial part of the profile, on the valley edge – Salaspils carbonates . Thickness of the dolomites is small, practically whole incision locates into the Upper Devonian Amata sandstone sediments. The sides of the lower part of the incision are composed of weak sandstones of the Amata sediments. The velocity of longitudinal waves propagation in them differs little from the moraine loam filling the valley, so the sides of the incision can be distinguished only conditionally, by changing the nature of the stratification.

Mini-CDP section, inverted into depth scale, along the profile through the secondary sleeve-duct of buried palaeovalley. The area of Plavinas HPP upstream

This profile clearly traces the roof of bedrock dolomites at a depth of 20-23 m under a thick layer of moraine loams. In the side parts of the valley reflecting boundary can be traced, associated, presumably, with the coarse deluvial sediments in the cut off parts of the strips. In this example, differences in depth of investigation of different methods clearly manifest. Thus, a boundary at a depth of about 150 m, presumably associated with the bed of loose Amata-Gauja sandstones, is traced on the CDP section, while the depth on the velocity section obtained by method of refracted waves is about 30 m with the length of the receiving geophone arrangement base of 126 m.

These two sections are obtained using a sledge hammer weighing 6 kg with the accumulation of three blows. Receivers step 2 m, points of excitation in the first case-4 m, in the second-2 m. the length of the flank arrangement 126 m.

Mini-CDP section, inverted into depth scale, along the profile via buried pre-glacial palaeovalley. Area of water intake wells location near Vangazi settlement, Latvia

The section shows the complex structure of moraine deposits, filling the buried glacial valley, the formation of which occurred when glacier plowing, evidently during for several glaciers.

Excitation of elastic waves by sledge hammer blows with accumulation of 3 impacts with the step of excitation points 4 m along the profile. Reflecting boundaries are traced to a depth of 200-220 m, which is displayed by the presence of energy sprouts on vertical velocity spectra.

The use of a compact source – hammer is most appropriate in hard-to-reach areas. The weight of the hammer, depending on the planned detail of the study of the cut, may vary at different sites in a wide range – from 1 to 10 kg. However, due to the relatively high frequency of excited oscillations, they quickly fade in the upper loose layers of the soil, so the depth of energy penetration into the array and, especially, its distribution along the profile line on the surface are limited.

CDP cross-section in depth scale on the area with shallow dolomite bedrock. Lower pool of Plavinas HPP dam, Latvia

In the most favorable cases of high-velocity carbonate section and low thickness of loose Quaternary deposits, the depth of study of the section by reflected waves using a sledge hammer with the accumulation of several impacts can be 150-200 m. The reduction of the tracking area of deep reflecting horizons in the initial and final parts of the profile at this section is due to the need to cut (muting) the zone of intense surface waves – interference registration on the initial seismograms using the observation system without sufficient removal of excitation point from the beginning of receiving base.