- Home
- A-Z Publications
- First Break
- Previous Issues
- Volume 4, Issue 12, 1986
First Break - Volume 4, Issue 12, 1986
Volume 4, Issue 12, 1986
-
-
The geophone and front-end fidelity
By P.J. StanleySeismic acquisition on land is expensive. Even small improvements in data quality can be costly in terms of increased field effort. Instrument performance should never be allowed to become a limitation, and for this reason digital systems have been designed to record to an accuracy which more than matches the potential of processing techniques. However, the system is no better than its weakest link and it is therefore desirabie that we identify this and avoid practices, however standard these may have become, which place unnecessary restrictions on recording fidelity. When specifying a seismic system, we need to give particular attention to the performance of the analogue elements between ground motion (or water pressure ) and the analogue-to-digital converter. Digital specifications are robust and easily checked, but specifications at the analogue 'front end' can be ambiguous or misleading and can be difficult to verify, especially in the field. This part of the system calls for careful consideration, especially of its performance at low frequencies. One of the characteristics of land seismie data is the high content of source-generated and other noise which travels in various surface modes. This noise is usually of lower frequency than most of the signal and may, at the points of detection, be stronger than the signal by orders of magnitude. This is particularly the case where surface sourees are employed. The industry-wide popularity of geophones with natural frequencies of 8 to 10 Hz suggests a lack of adequate consideration for the problems which surround this analogue interface. The following offers three separate arguments which favour natural frequencies approximately an octave higher than those popularly in use. These relate to: (1) Low-frequency performance of the geophone. (2) Low-frequency performance of the analogue 'front end' electronics. (3) High-frequency spurious modes within the geophone. Each of these arguments is, of itself, convincing. Taken together the case for higher frequency geophones appears to be unanswerable. The popularity of the lO-Hz geophone remains unchallenged through much of the seismic contracting industry because its disadvantages are rarely acknowledged.
-
-
-
Some shallow seismic reflections
Authors J.W. Bredewout and N.R. GoultyThe seismic reflection method has been applied mainly in the exploration for oil and gas, with targets in the depth range 500-10000 m. For shallow depths between, say, 10 and 200 m there are several possible applications for seismic reflection surveys on land: groundwater exploration, site investigation for civil engineering construction, identification of shallow sources of geothermal energy, delineation of aquifers for seasonal storage of hot water, and exploration for coal and other minerals at strippable depths. However, the use of the seismic reflection method for these applications has been limited. A fundamental difficulty with shallow seismic reflection surveying on land is that for receivers close to the source there can be substantial interference from refracted arrivals and surface waves. This is illustrated in Fig. 1 where these other wave types generated by the shot mask any reflectcd energy which may be present at travel times less than 100 ms. For most purposes economic considerations preclude the obvious way out of the difficulty. The shallower the target, the cheaper is the cast of drilling boreholes, so it is not usually worthwhile to drill deep shotholes to improve seismic reflection data quality, Either a surface or near-surtace source must be used, or the seismic reflection method might as weil be abandoned in favour of drilling boreholes only. Many companies and institutions must have acquired shallow seismic reflection data in recent years, yet little of it has appeared in the published literature. Consequently it is frustratingly difficult to assess the relationship between shallow geology and seismie reflection data quality. Engineers responsible for shallow exploration projects need same guidance on this relationship if they are to give serious consideration to the use of the seismic reflection method. In this article we combine some surprisingly good data obtained using a surface source in the eastern Netherlands with some more typical data obtained using explosive detonators in northern England, make a comment on the use of the explosive sources in shallow reflection work, and review the geological factors which influence data quality.
-
Volumes & issues
-
Volume 42 (2024)
-
Volume 41 (2023)
-
Volume 40 (2022)
-
Volume 39 (2021)
-
Volume 38 (2020)
-
Volume 37 (2019)
-
Volume 36 (2018)
-
Volume 35 (2017)
-
Volume 34 (2016)
-
Volume 33 (2015)
-
Volume 32 (2014)
-
Volume 31 (2013)
-
Volume 30 (2012)
-
Volume 29 (2011)
-
Volume 28 (2010)
-
Volume 27 (2009)
-
Volume 26 (2008)
-
Volume 25 (2007)
-
Volume 24 (2006)
-
Volume 23 (2005)
-
Volume 22 (2004)
-
Volume 21 (2003)
-
Volume 20 (2002)
-
Volume 19 (2001)
-
Volume 18 (2000)
-
Volume 17 (1999)
-
Volume 16 (1998)
-
Volume 15 (1997)
-
Volume 14 (1996)
-
Volume 13 (1995)
-
Volume 12 (1994)
-
Volume 11 (1993)
-
Volume 10 (1992)
-
Volume 9 (1991)
-
Volume 8 (1990)
-
Volume 7 (1989)
-
Volume 6 (1988)
-
Volume 5 (1987)
-
Volume 4 (1986)
-
Volume 3 (1985)
-
Volume 2 (1984)
-
Volume 1 (1983)