In this paper results are presented from a seismic refraction experiment (CANOBE) carried out in southern Norway. Ten explosions, fired at sea, were recorded on land by shifting 13 recording instruments along a profile with an average station spacing of 5 km. The main line runs in a northeasterly direction from the south coast at Lista along the western margin of the Oslo Graben into the NORSAR array, with a total length of 5 15 km. A separate leg runs across the Graben just north of Oslo, for the first time allowing a direct comparison of seismic records in the Graben with those in the adjacent Precambrian shield area. The laterally varying crustal structure along the profile is examined by modelling of travel times and amplitudes of P-wave arrivals. The Moho, which is the major discontinuity in the lowermost crust, appears to 'sink' beneath the coast from 27 km on the seaward side to 34 km onshore. The two-dimensional modelling procedure adopted proves invaluable in explaining the characteristic amplitude pattern observed in this area. Beyond the coastal area our picture of the crust is that of a relatively homogeneous one, as expected for a shield area. There are no indications of significant discontinuities in the crust along the first 300 km of the profile, although from the large Pg amplitudes within this distance range we infer a strong velocity gradient in the lower crust. Two structural models are proposed for the Oslo Graben where the Moho appears to be elevated to between 25 and 29 km.

An effective algorithm for computing synthetic seismograms in laterally inhomogeneous media has been developed. The method, based on zero-order asymptotic ray theory, is primarily intended for use in refraction and reflection studies and provides an economical means of seismic modelling.A given smoothed velocity-depth-distance model is divided into small squares with constant seismic parameters and first-order interfaces are represented by an arbitrary number of dipping linear segments. The computation of ray propagation and amplitudes through such a model does not involve complicated analytic expressions and therefore minimizes computer time.Amplitudes are determined by geometrical spreading of spherical wavefronts and energy partitioning at interfaces. Synthetic seismograms calculated for laterally homogeneous models are in good agreement with those obtained by the Reflectivity Method.

Summary Microseismic events or acoustic emissions associated with hydraulicfracturing are recorded with a borehole seismic tool in a deviated well duringmultirate injection, shut-in, and flowback. The event locations indicate thatfracture orientation, length, and height are compatible with regional stressdirections and estimates of the fracture size that are based on pressuredecline. Introduction Fractures, either natural or induced, can be the most important featurecontrolling the productivity of a hydrocarbon reservoir. Knowledge of theirorientation, length, and height can affect all stages of planning and isespecially important for horizontal drilling, stimulation of low-permeabilityreservoirs, in-fill drilling, and EOR programs. Many methods have beendeveloped to model or measure the characteristics of hydraulically inducedfractures. These include insitu or regional stress studies, inference ofmaterial-strength properties from wireline logs, pressure-decline analysis, properties from wireline logs, pressure-decline analysis, laboratoryexperiments on cores, and comparison of such pre- and postfracture logs astemperature, borehole images, gamma ray, and postfracture logs as temperature, borehole images, gamma ray, and sonic. Most of these techniques investigate thefracture in the near-wellbore region and are successful only in open holescontained in the fracture plane. The acoustic-emission-mapping technique wasdeveloped for applications in fracturing crystalline rock and has a history ofsuccess in such hot-dry-rock geothermal fields as Fenton Hill, Camborne, and Kakkonda. Acoustic emissions have been used with varying degrees of success tomap hydraulic fractures in sedimentary formations. The high attenuation ofseismic waves in sedimentary rocks imposes limitations in the instrumentationand acquisition configurations that can "listen" to fractures. Acousticemissions may be thought of as microearthquakes, or microseismic events causedby brittle fracture in a region surrounding a hydraulic fracture. They arebelieved to be caused by a combination of stress release in the zone of highpore pressure surrounding the fracture and the presence of inhomogeneities inthe formation. Laboratory presence of inhomogeneities in the formation. Laboratory experiments show that these events arise from discrete ruptures onthe fracture plane. 10 If these microseismic events can be located, the strikeand dip of the fracture can be determined. This provides a characterization ofthe fracture far from the borehole provides a characterization of the fracturefar from the borehole that can be implemented in cased or nonvertical wells, even for nonvertical fractures. As in earthquake location, acoustic-emissionlocation is more accurate when multiple receivers are used. The ideal situationis to deploy several vertical arrays of three-component sensors near theexpected fracture to get accurate locations of many events by triangulation. Unfortunately, few hydrocarbon-reservoir sites present this opportunity. Inprinciple, however, locations can be present this opportunity. In principle, however, locations can be determined with a single calibrated three-componentsensor, either in the injection well itself or in a nearby observation well. Assuming that the first motion created by the source mechanism is radiallycompressional (or dilatational), the polarization angle of the event's firstarrival compression al (P) wave defines the direction from receiver to event. This is a simple problem to solve in a homogeneous medium but is slightly moredifficult in a layered medium because the seismic rays bend at interfaces. Thedifference in travel times between the P wave and the secondary, or shear (S)wave is related to the distance to the event. This location method requiresknowledge of the P- and S-wave velocity structure near the fracture and carefulplacement of the sensor to ensure that all events occur above or below it. Thelatter requirement arises because the source mechanism is unknown: i.e., a Pwave coming from above has the same signature as a dilatational wave frombelow. The first requirement, knowledge of the velocities, is often satisfiedonly in an approximate fashion: average velocities from prestimulation soniclogging, check shots, or vertical seismic profiles (VSP's) can give reasonableestimates, and poststimulation profiles (VSP's) can give reasonable estimates, and poststimulation velocities may be slightly different. Green and Bariareported a decrease of less than 1 % in P-wave velocities after injectiontreatment in crystalline rock. Deriving the polarization information fromseismic records requires tool calibration because the polarization calculationdepends on the relative amplitudes recorded on the three axes. Calibration canbe done in situ by clamping the tool at the level where the fracture recordingwill occur and locating a known energy source, such as an airgun shot, a buriedcharge, or a downhole source, in a nearby well. Fracture-Mapping Survey Objective. The objective of the experiment was to test the feasibility of an EOR program that would entail water injection above fracturing pressure if wecould confirm that the fracture would not grow toward the pressure if we couldconfirm that the fracture would not grow toward the producing well. Bymonitoring the seismic activity generated by high producing well. By monitoringthe seismic activity generated by high rate injection, the fracture orientationand propagation length can be determined. Data Acquisition. The downhole equipment consisted of a modified slimholeseismic tool with a 2.125-in. OD and a 20,000-psi pressure rating. The lockingdevice was a hydraulic arm. The modification involved replacing the triaxialgeophone assembly with accelerometers so that we could record signals in therange of thousands of cycles/sec. The laboratory-calibrated accelerometers hada sensitivity of 1,000 X 10 coulomb per unit gravitational acceleration with alinear response of more than 2,000 cycles/sec over a temperature range up to482F. These sensors were arranged orthogonally with the z axis tangential tothe tool axis and the x axis in the same plane as the clamping arm, effectivelythe radial component. The relative hearing of the tool is given by the anglebetween the x axis and the vertical plane along the deviated borehole. Thisangle was measured by a circular potmeter attached to the tool housing, Thewiper of the potmeter always points vertically because of an attached weight. During the survey, the × axis was at an angle of 27 to the vertical (Fig. 1, insert). Surface airgun sources were deployed to obtain an independentmeasurement of the tool orientation, effectively calibrating the relativeresponse of the tool's three axes at the frequencies contained in the airgunsignature. The surface shots, however, were not detected by the downholeaccelerometers, probably because of the low-frequency content of the signalspropagating from the source through soft sedimentary material. Because thesurrounding area had a heavy surface buildup, it was not feasible to attemptother dangerous or destructive calibration techniques so we base ourcalibration on the laboratory accelerometer response. Surface equipment wassupplied by a subcontractor and consisted of amplifiers, a videocassetterecorder, and a power supply for the downhole tool. Data were recorded inanalog form on four 240-minute videocassette tapes, and the downhole signalswere monitored on an oscilloscope during the survey. SPEFE P. 139

No abstract

No abstract