Offset Shot Research Articles

Mapping Spontaneous Ground Fissures using Seismic Refraction, Reflection, and Ground Penetrating Radar in the Letlhakeng Area, Kweneng District, Botswana

Infrastructure in the Letlhakeng area is currently being damaged by ground fissures. Geophysical profiles of seismic refraction and reflection, and ground penetrating radar (GPR) were recorded at the areas affected by the ground fissures to map their depth extent and subsurface configuration. The GPR survey used 50 MHz antennas with an inline transmitter and receiver 2 m apart and a sample spacing of 30 cm. A reversed seismic refraction profile with two offset shots was recorded. Twenty four geophones were placed 2 m apart to make a total profile length of 46 m. The seismic reflection data were recorded with an equal source and receiver interval of 1 m and a total spread length of 23 m across a ground fissure. The GPR profile shows that the ground fissures are sub-vertical and extend to a vertical depth of ~5.0 m. The seismic refraction profile shows a two layer velocity model comprising overburden with a seismic velocity of 400-600 m/s lying over a substratum with a seismic velocity of 1200-1400 m/s. The top layer consists of the loose sand which is imaged to a depth of 5 m. The seismic reflection profile shows that the ground consists of subhorizontal layers from 0-15 m depth, which have all been broken by the ground fissure observed at the surface. The geophysical results show that the ground fissures persist from the surface to depths of 15 m.

Integration of Seismic Refraction and 2D Electrical Resistivity in Locating Geological Contact

The aim of this research is to locate the geological contact for engineering purpose applying seismic refraction and 2D electrical resistivity method. Resistivity and seismic refraction method was conducted on four survey lines with 3 lines running from NW to SE which about parallel to each other and 40 m apart while the fourth line was running from SW to NE. The 2D resistivity survey used minimum electrode spacing of 5 m and the survey used pole-dipole array with minimum current was 2 mA and maximum was 20 mA. The seismic refraction survey used 5 m geophone spacing with offset shot was + 30 m and - 30 m. Resistivity results generally show the area was divided into two main zones, alluvium with resistivity value of 10 - 800 ohm-m, and granite bedrock with resistivity value of > 2500 ohm-m. There is a geological contact between granite and alluvium. The seismic results show the area consists of two layers. The first layer (top layer) with velocity of 460 - 900 m/s which was alluvium mixed with boulders. The second layer with velocity of 2060 - 3140 m/s with depth 71 - 90 MSL. The thickness of the overburden is 5 - 15 m.

Multi-wave forward modeling of a reservoir and AVO analysis

In the process of accurate interpretation of multi-wave seismic data, we wanted to solve the problem of multi-wave information recognition. Based on techniques of elastic wave forwarding, targeting the geological model of a reservoir of an oil field exploration area, we used a high-order staggered-grid difference technology to simulate many shots of seismic records of nonzero offset shots, implemented multi-wave seismic data processing to acquire the CMP of P waves and converted waves, NMO traces of CCP pre stacks, including AVA information and superposition profiles. Based on the AVA calculation of the model, the layer parameters of the model and the forwarding wave field relations of the P-S wave, we also compared and studied the correspondence between P waves and converted waves. The results of our analysis show that the results from simulation and from the AVO analysis are consistent. Significant wave field differences between P waves and converted waves in the same reservoir were found, which are helpful in recognizing and interpreting the multi-wave information in this area. We made use of the multi-wave data to provide the important guidelines for reservoir prediction.

Offset and linear spread geometry in the MASW method

This paper evaluates the effect of near offset and geophone configuration on dispersion image for linear spreads in the multichannel analysis of surface waves (MASW) method. In order to determine the effect of near offset, MASW records were collected for a variety of offsets and the corresponding calculated dispersion images were compared for a fixed layout. To reveal the spectral resolution of dispersion images, MASW records were collected using nine different types of linear spread configuration, then dispersion images for each MASW record were calculated using the phase velocity analysis technique and were compared to the present preferable linear spread type for a MASW survey. The results show that at least one pair of near offset shots is required for distinguishing the fundamental mode from the higher modes without ambiguity, and for picking a dispersion curve in a wide frequency range. The first shot distance should not be longer than four times the geophone interval, and the second shot distance should be equal to or longer than the one-third spread length, depending on the power of the seismic source. A better dispersion image resolution and dispersion curve in the wide frequency range can be obtained by using the SL-type geophone configuration. Besides this, in a survey in which higher frequencies are more important, the SII-type configuration may be preferable. With the SII-type spread configuration, the fundamental mode can easily be distinguished from the higher modes.

Duplicate Measurements After Nine Years Support a Non-tectonic Origin for Displacement of Shot Holes in West Texas

Meinardus et al. (1993) conducted a geologic evaluation of the Downie roadcut on U.S. Highway 285, 14.6 km (9 miles) north of Sanderson in Terrell County, TX. The roadcut was constructed in 1965. The exposures in the roadcut exhibited offset shot holes with an updip component on the massive limestone of the Santa Elena Formation exposed on the roadcut walls. The total displacement across three bedding planes in the limestone along which the shot holes were offset was approximately 73 cm. The offset shot holes have been interpreted by Bolden (1991) as evidence of active faulting in west Texas. The interpretation by Bolden (1991) implied an annualized total displacement of about 2.5 cm/year. The purpose of the geologic evaluation by Meinardus et al. (1993) was to determine whether active faulting was occurring and whether the structural style invoked by Bolden (1991) as the cause of the active faulting was present. Evidence of active or recent faulting is important in the environmental and regulatory context because state and federal location standards for many types of waste management facilities explicitly address faults and standoff distances from active faults (e.g., 30 TAC 335.204; 40 CFR 264.18). Meinardus et al. (1993) concluded that active faulting was unlikely, that the offset shot holes were caused by blasting effects, and that the structural style and fracture patterns exposed in the roadcut did not support the Reidel shear system postulated by Bolden (1991). Meinardus et al. (1993) made two sets of displacement measurements over 9 months in 1992 and concluded that there was no measurable displacement across the planes of interest during this time frame. The permanent benchmarks established in the rock offer the opportunity for subsequent displacement measurements over a much longer time period. The offset measurements reported in this Technical Note were made in …

Lateral Variations in Seismic Velocities in the Central Australian Region from Both Conventional and Experimental Seismic Reflection Profiling

In 1985, the Australian Bureau of Mineral Resources recorded 500 km of deep seismic reflection data across the Arunta Block and the Ngalia and Amadeus Basins in central Australia. Large offset shots and expanding reflection spreads were also recorded coincident with the near-vertical incidence reflection data in the Arunta Block and the northern part of the Amadeus Basin to better define seismic velocity variations throughout the crust. In the northern part of the Amadeus Basin lateral and vertical variations in seismic velocity or variable dip in the reflectors can be detected throughout the 10 km thick sedimentary sequence, thus providing useful constraints on lithological or facies variations. A new method of mapping lateral velocity variations below weathering has been developed for interpreting first break data from conventional reflection shots, and has enabled the presence of a northerly-dipping fault in crystalline rocks to be detected in a part of the Arunta Block with few outcrops. The detailed velocity variations measured in selected localities is assisting with the proper migration and interpretation of both shallow and deep reflections.

THE LAW OF PARALLELISM IN REFRACTION SHOOTING*

AbstractThe half‐intercept‐time for in‐line reversed profiles is highly adaptable to multi‐layer cases, but the deep evidence of the intercept time on the depths and the velocities at the shotpoint and detector position poses a problem. The law of parallelism provides a reliable and simple means to solve this problem. The interpretation procedure implies that from a travel‐time curve the refractor segment is projected back to the shotpoint parallel to a refractor curve from an offset shot. The intercept time thus obtained refers only to the geological conditions in the immediate vicinity of the shotpoint, and varying conditions outside this region are automatically climinated. The assumptions involved in the interpretation technique are confined to the region where, in relation to the shotpoint, the rays leave and strike the refractor surface.The use of the “ABC method” to establish true intercept times at the shotpoint is also discussed briefly as well as the applicability of the method to eliminate or minimize arrival time disturbances in order to improve velocity determinations.

A seismic refraction technique used for subsurface investigations at Meteor Crater, Arizona

A seismic refraction technique for interpreting the subsurface shape and velocity distribution of an anomalous surface feature such as an impact crater is described. The method requires the existence of a relatively deep refracting horizon and combines data obtained from both standard shallow refraction spreads and distant offset shots by using the deep refractor as a source of initial arrivals. Results obtained from applying the technique to Meteor crater generally agree with the known structure of the crater deduced by other investigators and provide new data on an extensive fractured zone surrounding the crater. The breccia lens is computed to extend roughly 190 m below the crater floor, about 30 m less than the value deduced from early drilling data. Rocks around the crater are fractured as distant as 900 m from the rim crest and to a depth of at least 800 m beneath the crater floor.