INTRODUCTION

Seismic reflection is the most important geophysical method used to find and exploit oil and gas fields. Huge sums are spent each year to decide where to drill and how to sequence extraction of these precious commodities. The method is also used routinely on deep-sea research vessels and in special projects to probe the continental crust. Earthquakes are analyzed for reflections from deep in the mantle and even from the core. On a small scale, seismic reflection profiling is done in connection with engineering projects such as submarine tunnels (Boston Harbor) and to find bedrock pockets holding placer deposits (South African diamonds, Malaysian cassiterite).

SOURCES

Besides earthquakes, many artificial seismic sources are used. In the early days various solid explosives were employed. For marine work explosive solids, explosive gases and high-current electric sparks have been used but nowadays air guns and water guns are favored from the point of view of safety, repeatability and ease of using several sources simultaneously. On land, explosive sources have largely been replaced by various vibrating sources. With suitable processing these confusing records can be made to look like those from impulsive (explosive) sources.

High frequency sources are used for high resolution records at shallow depths whereas low frequency sources are used for deep penetration.

RECEIVERS

At sea the usual receivers are hydrophones (pressure sensors) that typically are towed in long, multi-hydrophone arrays. Arrays are used to reduce the effect of horizontal rays such as ship noise and to allow common-mid-point processing. Geophones sensitive to vertical ground velocity have dominated land surveys for decades. Nowadays many surveys use three-component geophones so the S waves can be recorded.

RESOLUTION

We have seen that travel times though layers less than a quarter wavelength thick cannot be determined. The edges of reflectors narrower than the Fresnel zone width cannot be resolved. To improve resolution we can use higher frequency sources, place the sources and receivers closer to the reflectors (hard to do on land!), or use S waves (these have shorter wavelengths than P waves of the same frequency).

GEOMETRICAL AND VELOCITY DISTORTIONS

Multiple echoes, side echoes and diffractions can confuse and mislead us, especially in conventional two-dimensional surveys. Depths, thicknesses and slopes can easily be misinterpreted when velocity varies laterally and vertically (velocity "pull-ups" and "pull-downs").

WHERE DO ECHOES COME FROM?

From a physical point of view we know that reflections and diffractions are caused by changing of acoustic impedance over distances short compared to the dominant wavelength. But where do these impedance changes occur in nature? What are the geological sources of echoes? Remember that acoustic impedance is the product of velocity times density. We値l answer these questions in terms of a survey of the continental shelf of New Hampshire (Birch, 1984).

One obvious reflector is the sea-floor. Echoes are strong over bare bedrock (huge impedance contrast) and almost imperceptible over watery muds (very low impedance contrast). As we discussed before, this means that it is difficult or impossible to "see" echoes from below the bedrock but easy below the muds.

Many echoes come from bedding planes or, to use a trendier term, "strata surfaces" (Payton, 1977). These surfaces are very important, as they are isochrones. That is, they represent the sea-floor as it was at some instant in the past. Stratification is caused by changes in water depth, climate, location of river mouths and other factors that influence grain size, composition and water content of the sediments. Generally speaking echoes don稚 represent individual bedding planes but rather are interference composites of echoes from many nearby planes.

Facies boundaries generally do not produce echoes because lateral changes are too gradual in the horizontal direction. As our profile moves down a delta front we値l see weaker and weaker echoes as we move from gravel to sand to mud but the correlatable echoes will be from the foreset beds (bedding planes), not from the fuzzy facies boundaries (for example sand versus gravel).

Unconformities are frequently good reflectors. Unconformities are surfaces that represent gaps or time lost in the local geological record. The younger sediments above an unconformity typically have lower impedances than the older, more compact sediments below. In our example the bedrock surface is an unconformity with Quaternary sediments above Paleozoic or Precambrian crystalline rock below. Unconformities are usually recognized by truncation of echoes.

A final source of echoes here is gas. Strong horizontal echoes are found associated with drowned peat layers where the rotting vegetation produces methane. "Smears" from rising gas bubbles are found in deeper water here. In many places the continental margins are underlain by "bottom simulating reflectors" caused by huge deposits of methane hydrate.

SEISMIC UNITS AND STRATIGRAPHY

These terms are used for a systematic approach to interpreting reflection records in terms of lithology and geologic history. There is a huge new jargon used in this field but in this course we値l limit ourselves to the most basic terminology.

The first systematic step is to determine the horizontal scale, vertical scale, and vertical exaggeration. To do this we need details on geophone and shot location as well as ship speed for a marine survey such as our example. We値l also need to know the speed of sound waves in the water.

The next step is to estimate the limits of vertical and horizontal resolution. For this we need to know the dominant frequency of the source and the seismic wave speeds underground. Of course we may have to use some initial guesses that we値l modify in an iterative manner throughout the interpretation. We use these resolution limits to avoid over interpreting patterns with no real meaning.

Having done these technical chores we move on to the third step. This entails dividing the record up into units bounded above and below by unconformities (or their lateral continuations as conformities). These "units", "zones" or "architectural elements" represent some definite time interval. Thus the sequence of units is, in some sense, a record of geologic history. You may want to give each unit a neutral name such as "red unit" or "blue unit". It may lead to trouble to prematurely number the units or give them geological names ("deltaic unit", "bedrock").

Usually the pattern of echoes within a given unit will be more or less uniform. Thus our fourth step is to describe the pattern or "character" of the echoes within each unit. Are the echoes strong or weak? Horizontal or inclines? Continuous or discontinuous? Planar or wavy? Close together or widely spaced?

We now describe the external form of each unit in two dimensions or, if we have enough data, in three dimensions. Does the unit form a sheet? A ramp or wedge? A uniform drape? What is the shape in map view? Is it an extensive sheet? A linear strip? Is it straight or meandering?

The next step is to arrange the units in chronological order using available age dates and, of course, the principle of superposition.

Finally (step eight) we interpret the units in terms of materials, processes, origin, and history or any other aspects of interest. Here we draw on our geologic knowledge. Recall all the outcrops you have seen in the field or as pictures. Remember the cross-sections and maps you studied in other courses. Try to make an interpretation that fits into known (?) history of the area. Try to harmonize all aspects of the seismic and geologic information. In our example the units correspond to crystalline bedrock, glacial till, outwash muds, low stand delta sands and Holocene sands. We might want to pick locations to raise cores to test our interpretations.

Be sure to highlight novel aspects of your interpretation. What features do not fit conventional wisdom? Do not bury your novel ideas.

REFERENCES

Birch, 1984, Northeastern Geology, 6, 207-221.

Payton C. E. (editor), 1977, "Seismic Stratigraphy-applications to hydrocarbon exploration"; American Association of Petroleum Geologists, Tulsa, 516 pp.