Intercept Time Method Research Articles

Bedrock identification using refraction seismic method in establishment location of Hospital Education Sebelas Maret University

<span>A research of refraction methods in estabilishment location Hospital Education Sebelas Maret <span>University has been done on October 31 2012 with P.A.S.I Seismograph Mod. 16S24 seismic <span>refraction instruments with 24 geophone. This research in order to determine the depth of bedrock <span>in the estabilishment location hospital education Sebelas Maret University. In this data <span>acquisition, the travel time data was a distance function. Processing and interpretation data used <span>Intercept Time Method. Intercept Time Method used value of intercept time concept from travel <span>time curve. This interpretation which resulted P wave velocities and layer rocks thickness in three <span>line which scattered in area of survey. The result was that we were able to obtain bedrock having <span>the depth of 4 meters until 9 meters and heaving spreading velocity 1079 m/s – 1182 m/s is <span>interpreted as talus deposits, sandstone, clay, loam, loess, sand and gravel.</span></span></span></span></span></span></span></span></span><br /></span>

Seismic Investigation of Highway Pavement Failures in Parts of Southeastern Nigeria

P-wave and S-wave seismic refraction methods were used to investigate highway pavement failures at various locations on some highways in southeastern Nigeria. The locations are: 1) Ochon, on Obubra–Ikom Highway, 2) a bridge approach and Ikot-Abasi Eyop, both on Ikot-Ekpene–Umuahia Road, and 3) four locations on Calabar–Itu Highway. These highways traverse different geologic units, including sand, shale, marl, and claystone. The intercept-time method and Hagedoorn's Plus-Minus method were used in the analysis of the seismic data. Both the P- and S-wave methods generally delineated two layers, and in some locations a third layer. In combination, both types of surveys were employed to delineate and evaluate the quality and state of the subgrade at the failed locations, and at stable segments adopted as controls for the investigations. The qualities of the subgrades were evaluated based on elastic constants of Young's modulus, bulk modulus, and compressibility. Young's modulus values obtained for failed areas are less than that for stable segments of the highway adopted as controls for the study. Compressibility values follow the same trend, with lower values at the control locations and higher values at failed locations. Generally, the first layer stiffness parameters obtained for the failed segments had lower values than those for the stable segments.

Modeling crustal structure of the south-eastern Fennoscandia

We present new seismic velocity models of the crust and uppermost mantle along two refraction and wide-angle reflection profiles in the southern Fennoscandia: the Pribalt and 1-EB profiles. Some new results obtained along the Coast and the Baltic Sea profiles are also presented. The intercept time method and ray tracing are used for the modeling. The study shows that the lateral variations are small in the velocity structure of’ the crust up to the depth of 20–25 km. The most significant lateral variations are observed in the Moho discontinuity topography and in the seismic velocities in the lower crust. In Paleoproterosoic Svekofennian domain, besides the well-known Moho depression in southern Finland, another Moho depression is revealed in the region from the Gotland Island to the Gulf of Riga. We suggest that this depression can correspond to the unknown crustal unit (we call it the Gotland-Riga belt). The Moho depth increases from the average of 40–45 km to 55 km in this belt. The Moho depression is filled by the matter with velocities of 6.8–7.1 km/s. Deep faults inclined to the north and strong variations of the mantle velocities are typical for the uppermost mantle of the Gotland-Riga belt.

A review of shallow seismic methods

Shallow seismic methods have historical roots dating to the 1930s, when limited shallow refraction work was performed using the Intercept-Time (IT) method. Because of high costs and the general lack of appropriate equipment - particularly data-processing equipment and software - the shallow-reflection and surface-wave techniques did not catch on as quickly as the refraction techniques. However, since 1980 substantial progress has been made in the development of all of the shallow seismic approaches. The seismic-reflection method has been used increasingly in applications at depths of less than 30 m, incorporating both the standard Common-Midpoint (CMP) method of the petroleum industry and the Common-Offset (CO) method, which was developed specifically as a low-cost technique for use in shallow surveying. In refraction studies, the Generalized Reciprocal Method (GRM) largely has replaced the classical intercept-time method, and tomographic approaches are rapidly gaining popularity. The Spectral Analysis of Surface Waves (SASW) has been developed by civil engineers, and surface-wave analysis involving many seismograph channels (MASW) recently has shown promise. With any of the shallow seismic methods, however, selecting the appropriate seismic recording equipment, energy sources, and data-acquisition parameters, along with processing and interpretation strategies, often is critical to the success of a project.

Plus�minus method to solve large�amplitude near�surface static corrections

ABSTRACTThe surface conditions in Saudi Arabia demonstrate significant variability ranging from flat gravel plains to sand�dunes, sabkhas (salt flats) and desert wadis (washouts). Saudi Aramco has used a single�layer velocity model from surface to datum for most of the prospective areas for both 2D and 3D seismic data. Source gathers have been interpreted by picking the first arrivals to obtain a linear fit for the direct and refractor arrivals using the intercept�time method. This creates depth and velocity control input for a multiple‐layer model of the near‐surface, from which static corrections to datum are calculated. On 2D lines with very large amplitude near‐surface anomalies, the plusminus technique has been applied to solve local areas. The implementation of the plus‐minus technique utilizes a variable near‐surface velocity derived from either upholes or the direct‐arrival velocity determined from source gathers. With this method, multiple source‐pairs build up effective spreads. Then by compiling high fold of delay‐times at a location, a statistical average is taken where the high fold reduces the significance of picking errors. The model is built utilizing multiple refraction layers and is referenced to the datum, thus tying it to the regional model.The example 2D seismic line has a static model calculated from the single‐layer velocity model. The intercept‐time method was applied locally over a near‐surface anomaly caused by unconsoli‐dated material at the foot of a cliff. Further analysis of the near‐surface involved the plus‐minus method to solve the high amplitude time shifts.

Shallow seismic refraction and magnetic studies at Lake Ngami, The Okavango Delta, Northwest Botswana

A seismic refraction analysis was carried out on data acquired on an 8.5 km profile at Lake Ngami, NW Botswana, to determine the structure and stratigraphy underlying the lake. The seismic spread comprised a 48-channel seismograph with a 9.5 m receiver and source spacing. The seismic source comprised 10 hammer blows on an aluminum plate, from which a vertical stack was recorded. The data were processed with WinSism™ version 10 program using the intercept time method. A total field magnetic survey was also carried out along the same line. The seismic refraction profile shows a low velocity layer (400–1600 m/s) extending from the surface at the southern end of the line to a maximum depth of 30 m in the middle of the profile, returning to a depth of 20 m from 4.5 km to the northern end of the line. The low velocity layer consists of the unconsolidated sediments and is underlain by more compact and saturated material with seismic velocities of 1600–3600 m/s. This material is interpreted to comprise semi- to fully consolidated sandstone. The base of these sandstones, however, could only be mapped near the southern end of the line out to 2 km. High seismic velocity rocks (3600–5000 m/s), interpreted to be weathered to fresh basement, were mapped below the sandstones at the southern end of the line. The magnetic profile shows a high at the beginning of the line which progressively declines to a low at 3 km. The field strength increases rapidly over the next 1.5 km to reach a plateau level at 4.5 km. The magnetic high at the beginning of the line corresponds to shallow basement which is mapped in the refraction profile. The magnetic low at 3 km is indicative of a thickened sediment section at this location. The southern end of the line with high seismic velocities comprises basement rocks located in the footwall block of the Kunyere Border Fault of the Okavango Rift. The step-wise downthrown hanging wall block consists of the low to intermediate velocity basin deposits in the northern part of the profile.

Detection of shallow objects using refracted and diffracted seismic waves

The detection of targets such as tunnels, karst, mines and other local heterogeneities is an important but difficult task in subsurface studies. In this paper a new methodology for determining a near-surface velocity–depth model is described. Common shot gathers and first breaks of refracted seismic waves are used as input data. Low-frequency components of the model are constructed by the intercept time method and coherency inversion. Detection of high-frequency velocity variations is carried out by refraction tomography using the low-frequency model as background. Diffracted waves contain valuable information regarding both the structure and composition of seismic media, especially in cases where target size is comparable to seismic wavelength. A diffraction stack serves as an additional tool for delineation of local scattering objects. The methodology was applied to define tunnel position and to search for the karst formations.

Can the aeromagnetic method yield more information in the offshore oil and gas exploration?

The detection of targets such as tunnels, karst, mines and other local heterogeneities is an important but difficult task in subsurface studies. In this paper a new methodology for determining a near-surface velocity–depth model is described. Common shot gathers and first breaks of refracted seismic waves are used as input data. Low-frequency components of the model are constructed by the intercept time method and coherency inversion. Detection of high-frequency velocity variations is carried out by refraction tomography using the low-frequency model as background. Diffracted waves contain valuable information regarding both the structure and composition of seismic media, especially in cases where target size is comparable to seismic wavelength. A diffraction stack serves as an additional tool for delineation of local scattering objects. The methodology was applied to define tunnel position and to search for the karst formations.

Application of the intercept time method to full wave form acoustic data

Formation slowness is commonly estimated using first motion detection of the compressional headwave recorded with an acoustic tooI. This slowness estimation, in its simplest form, involves the difference in arrival time between two or more receivers. However, additional information is embodied in the traveltimes themselves. The measured traveltime at each receiver is actually a function of the path of the signal between the source and the receiver. In the presence of near-borehole changes in formation slowness, such as in altered or damaged zones (implying a reduction in the formation velocity in the vicinity of the borehole), refracted borehole arrivals may pass some inches to some feet from the borehole wall.

The Generalized Reciprocal Method — An Integrated Approach to Shallow Refraction Seismology

The generalized reciprocal method (GRM) and two special cases, the conventional reciprocal method (CRM) and the intercept time method (ITM), constitute an integrated approach which is powerful and convenient for the great majority of exploration refraction problems. The ITM, the CRM and the GRM form a logical sequence of increasing depth of application, increasing performance and increasing complexity. They can be used as reconnaissance methods with widely spaced detectors or for detailed definition using closely spaced detectors. These methods are well suited to good quality control practices because the data, the processed data and the final depths can be readily presented. A most important benefit of these presentations, especially for geotechnical applications, is that they help resolve whether lateral changes in the seismic velocity associated with a major change in depth to a refractor, are in fact genuine, or simply an artefact of the interpretation algorithm. Two case histories illustrate the recognition of both fictitious changes in the seismic velocity associated with changes in the depth of the refractor, as well as genuine changes in the seismic velocity associated with a known fault.

Crustal structure of the Barents Sea from seismic data

Abstract In 1976, the Institute of Physics of the Earth and the Institute of Oceanology, the U.S.S.R. Academy of Sciences, carried out deep seismic soundings in the Barents Sea along a profile 700 km long northeast of Murmansk. A system of reversed and overlapping traveltime curves from 200 to 400 km long has been obtained. The wave correlation was effected by several independent approaches, which identified on the records the refracted and reflected waves from boundaries in the Earth's crust and the upper mantle. Different methods were applied for the solution of the inverse problem: the isochrone method, the intercept-time method, and the iteration method. The use of these different methods gives an indication of the general applicability of the interpretation and of the most reliable elements in the seismic model. All the interpretations and representations of the section positively establish an essentially horizontal inhomogeneity of the Earth's crust in the Barents Sea. On the whole the structure is similar to that of deep sedimentary basins of the East European platform. The thickness of the sedimentary layer varies from 8 to 17 km, the average crustal thickness is about 35–40 km; the velocities in the upper part of the consolidated crust are 5.8–6.4 km/s; in the lower crust they are 6.8–7.0 km/s and higher.